Boost.Asio翻译文档

实战出精华

在具体的C++网络编程中提升你的逼格

John Torjo

Boost.Asio C++ 网络编程

关于作者

做为一名权威的C++专家，John Torjo 的编程生涯已经超过了15年，在这15年中，除了偶尔用 C# 和 Java 写程序，他大部分时间都在研究 C++。

他还很喜欢在 C++ Users Journa l和其他杂志上写一些编程相关的文章。

闲暇的时候，他喜欢玩扑克、开快车。他有很多自由职业，其中一个就把他玩扑克和编程的爱好结合在了一起，如果你想联系他，可以发邮件到[email protected]。

我要感谢我的朋友 Alexandru Chis, Aurelian Hale, Bela Tibor Bartha, Cristian Fatu, Horia Uifaleanu, Nicolae Ghimbovschi 以及 Ovidiu Deac。感谢他们对本书提出的反馈和建议。同时我也要感谢 Packt 公司各位对我频繁错过截稿日期行为的包容。然后最需要感谢的是 Chris Kohlhoff，Boost.Asio 的作者，是他写出了如此伟大的库。

把这本书献给我最好的朋友 Darius。

关于评审员

Béla Tibor Bartha

一个使用多种技术和语言进行开发的专业软件工程师。尽管在过去的4年里，他做的是 iOS 和 OSX 应用开发，但是 C++ 陪伴他度过了早期个人游戏项目开发的激情岁月。

我要感谢 John，因为他我才能做这本书的评审

Nicolae Ghimbovschi

一个参加各类 C++ 项目超过5年的天才个人开发者。他主要参与一些企业通信工程的项目。作为一个狂热的 Linux 爱好者，他喜欢利用不同的操作系统、脚本工具和编程语言进行测试和实验。除了编程，他还喜欢骑自行车、瑜伽和冥想。

我要感谢 John 让我来评审这本书

关于译者

画渣程序猿mmoaay，技术很烂，喜欢平面设计、鼠绘、交友、运动和翻译，但是确作为一只程序猿混迹在IT行业。热爱开源，技术烂就只好做做设计和翻译的工作。

微博：http://weibo.com/smmoaay

关于avplayer

http://avplayer.org 中国第一技术社区。

网络 API
Boost.Asio 命名空间
IP 地址
端点
Sockets
    同步错误代码
    Socket 成员函数
    其他注意事项
read/write/connect自由函数
    connect 函数
    read/write 函数
异步编程
    为什么要异步？
    异步 run(),run_one(),poll(),poll_one()
        持续运行
        run_one(),poll(),poll_one() 函数
    异步工作
    异步 post() VS dispatch() VS wrap()
保持运行
总结

第三章：回显服务端/客户端

TCP 回显服务端/客户端
    TCP 同步客户端
    TCP 同步服务端
    TCP 异步客户端
    TCP 同步服务端
    代码
UDP 回显服务端/客户端
    UDP 同步回显客户端
    UDP 同步回显服务端
总结

第四章：客户端和服务端

同步客户端/服务端
    同步客户端
    同步服务端
异步客户端/服务端
    异步客户端
    异步服务端
总结

第五章：同步VS异步

同步异步混合编程
客户端和服务端之间消息的互相传递
客户端软件中的同步 I/O
服务端软件中的同步 I/O
    同步服务端中的线程
客户端软件中的异步 I/O
服务端软件中的异步 I/O
    异步服务端中的线程
异步操作
代理实现
总结

第六章：Boost.Asio-其他特性

std streams 和 std buffer I/O
Boost.Asio 和 STL流
streambuf 类
处理 streambuf 对象的自由函数
协程
总结

第七章：Boost.Asio-进阶

Asio VS Boost.Asio
调试
    处理程序跟踪信息
    例子
    处理程序跟踪文件
SSL
Boost.Asio 的 Windows特性
    流处理
    随机存储处理
    对象处理
Boost.Asio 的 POSIX 特性
    本地 sockects
    连接本地 sockets
    POSIX 文件描述符
    Fork
    总结

索引

前言

网络编程由来已久，并且是一个极富挑战性的任务。Boost.Asio 对网络编程做了一个极好的抽象，从而保证只需要少量的编程就可以实现一个优雅的客户端/服务端软件。在实现的过程中，它能让你体会到极大的乐趣。而且更为有益的是：Boost.Asio 包含了一些非网络的特性，用 Boost.Asio 写出来的代码紧凑、易读，而且如果按照我在书中所讲的来做，你的代码会无懈可击。

这本书涵盖了什么？

第一章：Boost.Asio入门将告诉你 Boost.Asio 是什么？怎么编译它？顺带着会有一些例子。你会发现 Boost.Asio 不仅仅是一个网络库。同时你也会接触到 Boost.Asio 中最核心的类 io_service。

第二章：Boost.Asio基本原理包含了你必须了解的内容：什么时候使用 Boost.Asio？我们将深入了解异步编程——一种比同步更需要技巧，且更有乐趣的编程方式。这一章也是在开发你自己的网络应用时可以作为参考的一章。

第三章：回显服务端/客户端将会告诉你如何实现一个小的客户端/服务端应用；也许这会是你写过的最简单的客户端/服务端应用。回显应用就是把客户端发过来的消息发送回去然后关闭客户端连接的服务。我们会先实现一个同步的版本，然后再实现一个异步的版本，这样就可以非常容易地看到它们之间的不同。

第四章：客户端和服务端会深入讨论如何用 Boost.Asio 创建一个简单的客户端/服务端应用。我们将讨论如何避免诸如内存泄漏和死锁的缺陷。所有的程序都只是实现一个简单的框架，从而使你能更方便地对它们进行扩展以满足你的需求。

第五章：同步 VS 异步会带你了解在同步和异步方式之间做选择时需要考虑的事情。首要的事情就是不要混淆它们。在这一章，我们会发现实现、测试和调试每个类型应用是非常容易的。

第六章：Boost.Asio 的其他特性将带你了解 Boost.Asio 一些不为人知的特性。你会发现，虽然 std streams 和 streambufs 有一点点难用，但是却表现出了它们得天独厚的优势。最后，是姗姗来迟的 Boost.Asio 协程，它可以让你用一种更易读的方式来写异步代码。（就好像写同步代码一样）

第七章：Boost.Asio 进阶包含了一些 Boost.Asio 进阶问题的处理。虽然在日常编程中不需要深入研究它们，但是了解它们对你有益无害（Boost.Asio 高级调试，SSL，Windows 特性，POSIX 特性等）。

读这本书之前你需要准备什么？

如果要编译 Boost.Asio 以及运行本书中的例子，你需要一个现代编译器。例如，Visual Studio 2008 及其以上版本或者 g++ 4.4 及其以上版本

这本书是为谁写的？

这本书对于那些需要进行网络编程却不想深入研究复杂的原始网络 API 的开发者来说是一个福音。所有你需要的只是 Boost.Asio 提供的一套 API 。作为著名 Boost C++ 库的一部分，你只需要额外添加几个 #include 文件即可转换到 Boost.Asio。

在读这本书之前，你需要熟悉 Boost 核心库的一些知识，例如 Boost 智能指针、boost::noncopyable、Boost Functors、Boost Bind、shared_ from_this/enabled_shared_from_this 和 Boost 线程（线程和互斥量）。同时还需要了解 Boost 的 Date/Time。读者还需要知道阻塞的概念以及“非阻塞”操作。

约定

本书使用不同样式的文字来区分不同种类的信息。这里给出这些样式的例子以及它们的解释。

文本中的代码会这样显示：“通常一个 io_service 的例子就足够了”。

代码是下面这样的：

<code class="hljs erlang has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-title" style="box-sizing: border-box;">read</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, buffer [, extra options])</span>

<span class="hljs-title" style="box-sizing: border-box;">async_read</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, buffer [, extra options], handler)</span>

<span class="hljs-title" style="box-sizing: border-box;">write</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, buffer [, extra options])</span>

<span class="hljs-title" style="box-sizing: border-box;">async_write</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, buffer [, extra options], handler)</span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

专业词汇和重要的单词用黑体显示

[！警告或者重要的注释在这样的一个框里面]

[？技巧在这样的一个框里面]

读者反馈

我们欢迎来自读者的反馈。告诉我们你对这本书的看法——你喜欢哪部分，不喜欢哪部分。读者的反馈对我们非常重要，它能让我们写出对读者更有帮助的书。

你只需要发送一封邮件到 [email protected] 即可进行反馈，注意在邮件的主题中注明书名。

如果你有一个擅长的专题，想撰写一本书或者为某本书做贡献。请阅读我们在 www.packtpub.com/authors 上的作者指引。

用户支持

现在你已经是 Packt 书籍的拥有者，我们将告诉你一些事项，让你购买本书得到的收益最大化。

下载示例代码

你可以在 http://www.packtpub.com 登录你的帐号，然后下载你所购买的书籍的全部示例代码。同时，你也可以通过访问http://www.packtpub.com/support 进行注册，然后这些示例代码文件将直接发送到你的邮箱。

纠错

尽管我们已经尽最大的努力去保证书中内容的准确性，但是错误还是不可避免的。如果你在我们的书籍中发现了错误——也许是文字，也许是代码——如果你能将它们报告给我们，我们将不胜感激。这样的话，你不仅能帮助其他读者，同时也能帮助我们改进这本书的下一个版本。如果你发现任何需要纠正的地方，访问 http://www.packtpub.com/submit-errata，选择你的书籍，点击errata submission form链接，然后输入详细的纠错信息来将错误报告给我们。一经确定，你的提交就会通过，然后这个纠错就会被上传到我们的网站，或者添加到那本书的纠错信息区域的纠错列表中。所有已发现的纠错都可以访问 http://www.packtpub.com/support，然后通过选择书名的方式来查看。

答疑

如果你有关于本书任何方面的问题，你可以通过 [email protected] 联系我们。我们将尽我们最大的努力进行解答

Boost.Asio入门

首先，让我们先来了解一下什么是 Boost.Asio？怎么编译它？了解的过程中我们会给出一些例子。然后在发现 Boost.Asio 不仅仅是一个网络库的同时你也会接触到 Boost.Asio 中最核心的类——io_service。

什么是Boost.Asio

简单来说，Boost.Asio是一个跨平台的、主要用于网络和其他一些底层输入/输出编程的 C++ 库。

网络 API 的设计方式有很多种，但是 Boost.Asio 的的方式远远优于其它的设计方式。它在 2005 年就被引入到 Boost，然后被大量 Boost 用户测试并在很多项目中使用，比如 Remobo(http://www.remobo.com)，可以让你创建你自己的即时私有网络(IPN)的应用，libtorrent(http://www.rasterbar.com/products/libtorrent) 一个实现了比特流客户端的库，PokerTH (http://www.pokerth.net) 一个支持 LAN 和互联网对战的纸牌游戏。

Boost.Asio 在网络通信、COM 串行端口和文件上成功地抽象了输入输出的概念。你可以基于这些进行同步或者异步的输入输出编程。

<code class="hljs erlang has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-title" style="box-sizing: border-box;">read</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, buffer [, extra options])</span>
<span class="hljs-title" style="box-sizing: border-box;">async_read</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, buffer [, extra options], handler)</span>
<span class="hljs-title" style="box-sizing: border-box;">write</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, buffer [, extra options])</span>
<span class="hljs-title" style="box-sizing: border-box;">async_write</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, buffer [, extra options], handler)</span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

从前面的代码片段可以看出，这些函数支持传入包含任意内容（不仅仅是一个socket，我们可以对它进行读写）的流实例。

作为一个跨平台的库，Boost.Asio可以在大多数操作系统上使用。能同时支持数千个并发的连接。其网络部分的灵感来源于伯克利软件分发(BSD)socket，它提供了一套可以支持传输控制协议(TCP)**socket、用户数据报协议(UDP)**socket和**Internet控制消息协议(IMCP)**socket的API，而且如果有需要，你可以对其进行扩展以支持你自己的协议。

历史

Boost.Asio在2003被开发出来，然后于2005年的12月引入到Boost 1.35版本中。原作者是Christopher M. Kohlhoff，你可以通过[email protected]联系他。

这个库在以下的平台和编译器上测试通过：

32-bit和64-bit Windows，使用Visual C++ 7.1及以上
Windows下使用MinGW
Windows下使用Cygwin(确保已经定义 __USE_232_SOCKETS)
基于2.4和2.6内核的Linux，使用g++ 3.3及以上
Solaris下使用g++ 3.3及以上
MAC OS X 10.4以上下使用g++ 3.3及以上

它也可能能在诸如AIX 5.3，HP-UX 11i v3，QNX Neutrino 6.3，Solaris下使用Sun Studio 11以上，True64 v5.1，Windows下使用Borland C++ 5.9.2以上等平台上使用。（更多细节请咨询www.boost.org）

依赖

Boost.Asio依赖于如下的库：

Boost.System：这个库为Boost库提供操作系统支持(http://www.boost.org/doc/libs/1_51_0/doc/html/boost_system/index.html)
Boost.Regex：使用这个库（可选的）以便你重载read_until()或者async_read_until()时使用boost::regex参数。
Boost.DateTime：使用这个库（可选的）以便你使用Boost.Asio中的计时器
OpenSSL：使用这个库（可选的）以便你使用Boost.Asio提供的SSL支持。

编译Boost.Asio

Boost.Asio是一个只需要引入头文件就可以使用的库。然而，考虑到你的编译器和程序的大小，你可以选择用源文件的方式来编译Boost.Asio。如果你想要这么做以减少编译时间，有如下几种方式：

在某个源文件中，添加#include “boost/asio/impl/src.hpp”（如果你在使用SSL，添加#include “boost/asio/ssl/impl/src.hpp”）
在所有的源文件中，添加#define BOOST_ASIO_SEPARATE_COMPILATION

注意Boost.Asio依赖于Boost.System，必要的时候还依赖于Boost.Regex，所以你需要用如下的指令先编译Boost：

bjam –with-system –with-regex stage

如果你还想同时编译tests，你需要使用如下的指令：

bjam –with-system –with-thread –with-date_time –with-regex –with-serialization stage

这个库有大量的例子，你可以连同本书中的例子一块看看。

重要的宏

如果设置了BOOST_ASIO_DISABLE_THREADS；不管你是否在编译Boost的过程中使用了线程支持，Boost.Asio中的线程支持都会失效。

同步VS异步

首先，异步编程和同步编程是非常不同的。在同步编程中，所有的操作都是顺序执行的，比如从socket中读取（请求），然后写入（回应）到socket中。每一个操作都是阻塞的。因为操作是阻塞的，所以为了不影响主程序，当在socket上读写时，通常会创建一个或多个线程来处理socket的输入/输出。因此，同步的服务端/客户端通常是多线程的。

相反的，异步编程是事件驱动的。虽然启动了一个操作，但是你不知道它何时会结束；它只是提供一个回调给你，当操作结束时，它会调用这个API，并返回操作结果。对于有着丰富经验的QT（诺基亚用来创建跨平台图形用户界面应用程序的库）程序员来说，这就是他们的第二天性。因此，在异步编程中，你只需要一个线程。

因为中途做改变会非常困难而且容易出错，所以你在项目初期（最好是一开始）就得决定用同步还是异步的方式实现网络通信。不仅API有极大的不同，你程序的语意也会完全改变（异步网络通信通常比同步网络通信更加难以测试和调试）。你需要考虑是采用阻塞调用和多线程的方式（同步，通常比较简单），或者是更少的线程和事件驱动（异步，通常更复杂）。

下面是一个基础的同步客户端例子：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">using <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:asio</span>;
io_service service;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2001</span>);
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service);
sock.connect(ep);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

首先，你的程序至少需要一个io_service实例。Boost.Asio使用io_service同操作系统的输入/输出服务进行交互。通常一个io_service的实例就足够了。然后，创建你想要连接的地址和端口，再建立socket。把socket连接到你创建的地址和端口。

下面是一个简单的使用Boost.Asio的服务端：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">typedef <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:shared_ptr<ip</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket></span> socket_ptr;
io_service service;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:v4</span>(), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2001</span>)); <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> listen on <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2001</span>
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:acceptor</span> acc(service, ep);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {
    socket_ptr sock(new <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span>(service));
    acc.accept(*sock);
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:thread</span>( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:bind</span>(client_session, sock));
}
void client_session(socket_ptr sock) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {
        char data[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
        size_t len = sock->read_some(buffer(data));
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( len > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>)
            write(*sock, buffer(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ok"</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>));
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

首先，同样是至少需要一个io_service实例。然后你指定你想要监听的端口，再创建一个接收器——一个用来接收客户端连接的对象。在接下来的循环中，你创建一个虚拟的socket来等待客户端的连接。然后当一个连接被建立时，你创建一个线程来处理这个连接。

在client_session线程中来读取一个客户端的请求，进行解析，然后返回结果。

而创建一个异步的客户端，你需要做如下的事情：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">using boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::asio</span>;
io_service service;
ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endpoint</span> ep( ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::address</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2001</span>);
ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::socket</span> sock(service);
sock<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>async_connect(ep, connect_handler);
service<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>run();
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> connect_handler(const boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> ec) {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 如果ec返回成功我们就可以知道连接成功了</span>
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

在程序中你需要创建至少一个io_service实例。你需要指定连接的地址以及创建socket。

当连接完成时（其完成处理程序）你就异步地连接到了指定的地址和端口，也就是说，connect_handler被调用了。

当connect_handler被调用时，检查错误代码（ec），如果成功，你就可以向服务端进行异步的写入。

注意：只要还有待处理的异步操作，servece.run()循环就会一直运行。在上述例子中，只执行了一个这样的操作，就是socket的async_connect。在这之后，service.run()就退出了。

每一个异步操作都有一个完成处理程序——一个操作完成之后被调用的函数。下面的代码是一个基本的异步服务端

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">using boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::asio</span>;
typedef boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::shared_ptr</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::socket</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> socket_ptr;
io_service service;
ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endpoint</span> ep( ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::v4</span>(), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2001</span>)); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 监听端口2001</span>
ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::acceptor</span> acc(service, ep);
socket_ptr sock(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::socket</span>(service));
start_accept(sock);
service<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>run();
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> start_accept(socket_ptr sock) {
    acc<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>async_accept(<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">*</span>sock, boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>( handle_accept, sock, _1) );
}
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> handle_accept(socket_ptr sock, const boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>
err) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( err) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 从这里开始, 你可以从socket读取或者写入</span>
    socket_ptr sock(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::socket</span>(service));
    start_accept(sock);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

在上述代码片段中，首先，你创建一个io_service实例，指定监听的端口。然后，你创建接收器acc——一个接受客户端连接，创建虚拟的socket，异步等待客户端连接的对象。

最后，运行异步service.run()循环。当接收到客户端连接时，handle_accept被调用（调用async_accept的完成处理程序）。如果没有错误，这个socket就可以用来做读写操作。

在使用这个socket之后，你创建了一个新的socket，然后再次调用start_accept()，用来创建另外一个“等待客户端连接”的异步操作，从而使service.run()循环一直保持忙碌状态。

异常处理VS错误代码

Boost.Asio允许同时使用异常处理或者错误代码，所有的异步函数都有抛出错误和返回错误码两种方式的重载。当函数抛出错误时，它通常抛出boost::system::system_error的错误。

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">using <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:asio</span>;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service);
sock.connect(ep); <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> 第一行
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:system</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:error_code</span> err;
sock.connect(ep, err); <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> 第二行</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

在前面的代码中，sock.connect(ep)会抛出错误，sock.connect(ep, err)则会返回一个错误码。

看一下下面的代码片段：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
    sock.connect(ep);
} <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span>(boost::system::system_error e) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << e.code() << <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::endl;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

下面的代码片段和前面的是一样的：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:system</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:error_code</span> err;
sock.connect(ep, err);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( err)
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:cout</span> << err << <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endl</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

当使用异步函数时，你可以在你的回调函数里面检查其返回的错误码。异步函数从来不抛出异常，因为这样做毫无意义。那谁会捕获到它呢？

在你的异步函数中，你可以使用异常处理或者错误码（随心所欲），但要保持一致性。同时使用这两种方式会导致问题，大部分时候是崩溃（当你不小心出错，忘记去处理一个抛出来的异常时）。如果你的代码很复杂（调用很多socket读写函数），你最好选择异常处理的方式，把你的读写包含在一个函数try {} catch块里面。

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">void client_session(socket_ptr sock) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>
    } catch ( boost::system::system_error e) {
        // 处理错误
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

如果使用错误码，你可以使用下面的代码片段很好地检测连接是何时关闭的：

<code class="hljs mel has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">char data[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
boost::<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">system</span>::error_code <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error</span>;
size_t length = sock.read_some(buffer(data), <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error</span>);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error</span> == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error</span>::eof)
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 连接关闭</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

Boost.Asio的所有错误码都包含在ˆ的命名空间中（以便你创造一个大型的switch来检查错误的原因）。如果想要了解更多的细节，请参照boost/asio/error.hpp头文件

Boost.Asio中的线程

当说到Boost.Asio的线程时，我们经常在讨论：

io_service:io_service是线程安全的。几个线程可以同时调用io_service::run()。大多数情况下你可能在一个单线程函数中调用io_service::run()，这个函数必须等待所有异步操作完成之后才能继续执行。然而，事实上你可以在多个线程中调用io_service::run()。这会阻塞所有调用io_service::run()的线程。只要当中任何一个线程调用了io_service::run()，所有的回调都会同时被调用；这也就意味着，当你在一个线程中调用io_service::run()时，所有的回调都被调用了。
socket:socket类不是线程安全的。所以，你要避免在某个线程里读一个socket时，同时在另外一个线程里面对其进行写入操作。（通常来说这种操作都是不推荐的，更别说Boost.Asio）。
utility:就utility来说，因为它不是线程安全的，所以通常也不提倡在多个线程里面同时使用。里面的方法经常只是在很短的时间里面使用一下，然后就释放了。

除了你自己创建的线程，Boost.Asio本身也包含几个线程。但是可以保证的是那些线程不会调用你的代码。这也意味着，只有调用了io_service::run()方法的线程才会调用回调函数。

不仅仅是网络通信

除了网络通信，Boost.Asio还包含了其他的I/O功能。

Boost.Asio支持信号量，比如SIGTERM（软件终止）、SIGINT（中断信号）、SIGSEGV（段错误）等等。你可以创建一个signal_set实例，指定异步等待的信号量，然后当这些信号量产生时，就会调用你的异步处理程序：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> signal_handler(const boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err, int signal)
{
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 纪录日志，然后退出应用</span>
}
boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::asio</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::signal_set</span> sig(service, SIGINT, SIGTERM);
sig<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>async_wait(signal_handler);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

如果SIGINT产生，你就能在你的signal_handler回调中捕获到它。

你可以使用Boost.Asio轻松地连接到一个串行端口。在Windows上端口名称是COM7，在POSIX平台上是/dev/ttyS0。

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
serial_port <span class="hljs-function" style="box-sizing: border-box;">sp(service, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"COM7"</span>)</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

打开端口后，你就可以使用下面的代码设置一些端口选项，比如端口的波特率、奇偶校验和停止位。

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-label" style="box-sizing: border-box;">serial_port:</span>:baud_rate rate(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9600</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
sp<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.set</span>_option(rate)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

打开端口后，你可以把这个串行端口看做一个流，然后基于它使用自由函数对串行端口进行读/写操作。比如async_read(), write, async_write(), 就像下面的代码片段：

<code class="hljs haskell has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-title" style="box-sizing: border-box;">char</span> <span class="hljs-typedef" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">data</span>[512];</span>
<span class="hljs-title" style="box-sizing: border-box;">read</span>(sp, buffer(<span class="hljs-typedef" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">data</span>, 512));</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

Boost.Asio也可以连接到Windows的文件，然后同样使用自由函数，比如read(), asyn_read()等等，就像下面的代码片段：

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">HANDLE h = ::OpenFile(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>);
windows::stream_handle sh(service, h);
char data[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
read(h, buffer(data, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

对于POXIS文件描述符，比如管道，标准I/O和各种设备（但不包括普通文件）你也可以这样做，就像下面的代码所做的一样：

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">posix<span class="hljs-value" style="box-sizing: border-box;">::stream_descriptor sd_in(service, ::dup(STDIN_FILENO));</span>
<span class="hljs-value" style="box-sizing: border-box;">char</span> data<span class="hljs-attr_selector" style="color: rgb(0, 136, 0); box-sizing: border-box;">[512]</span>;
<span class="hljs-function" style="box-sizing: border-box;">read(sd_in, <span class="hljs-function" style="box-sizing: border-box;">buffer(data, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>)</span>)</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

计时器

一些I/O操作需要一个超时时间。这只能应用在异步操作上（同步意味着阻塞，因此没有超时时间）。例如，下一条信息必须在100毫秒内从你的同伴那传递给你。

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">bool read <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>;
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> deadline_handler(const boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>) {
    std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::cout</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> (read <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">?</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"read successfully"</span> : <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"read failed"</span>) <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endl</span>;
}
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> read_handler(const boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>) {
    read <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;
}
ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::socket</span> sock(service);
…
read <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>;
char <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">data</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">[</span><span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">]</span><span class="hljs-markup" style="box-sizing: border-box;">;
sock.async_read_some(buffer(data, 512));
deadline_timer t(service, boost::posix_time::milliseconds(100));
t.async_wait(&deadline_handler);
service.run();</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

在上述代码片段中，如果你在超时之前读完了数据，read则被设置成true，这样我们的伙伴就及时地通知了我们。否则，当deadline_handler被调用时，read还是false，也就意味着我们的操作超时了。

Boost.Asio也支持同步计时器，但是它们通常和一个简单的sleep操作是一样的。boost::this_thread::sleep(500);这段代码和下面的代码片段完成了同一件事情：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">deadline_timer t(service, <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:posix_time</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:milliseconds</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">500</span>));
t.wait();</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

io_service类

你应该已经发现大部分使用Boost.Asio编写的代码都会使用几个io_service的实例。io_service是这个库里面最重要的类；它负责和操作系统打交道，等待所有异步操作的结束，然后为每一个异步操作调用其完成处理程序。

如果你选择用同步的方式来创建你的应用，你则不需要考虑我将在这一节向你展示的东西。
你有多种不同的方式来使用io_service。在下面的例子中，我们有3个异步操作，2个socket连接操作和一个计时器等待操作：
* 有一个io_service实例和一个处理线程的单线程例子：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service; <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> 所有socket操作都由service来处理 
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock1(service); <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> all the socket operations are handled by service 
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock2(service); sock1.asyncconnect( ep, connect_handler); 
sock2.async_connect( ep, connect_handler); 
deadline_timer t(service, <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:posixtime</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:seconds</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>));
t.async_wait(timeout_handler); 
service.run();</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

有一个io_service实例和多个处理线程的多线程例子：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock1(service);
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock2(service);
sock1.asyncconnect( ep, connect_handler);
sock2.async_connect( ep, connect_handler);
deadline_timer t(service, <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:posixtime</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:seconds</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>));
t.async_wait(timeout_handler);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( int i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>; ++i)
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:thread</span>( run_service);
void run_service()
{
    service.run();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

有多个io_service实例和多个处理线程的多线程例子：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>];
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock1(service[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]);
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock2(service[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>]);
sock1.asyncconnect( ep, connect_handler);
sock2.async_connect( ep, connect_handler);
deadline_timer t(service[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>], <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:posixtime</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:seconds</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>));
t.async_wait(timeout_handler);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( int i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>; ++i)
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:thread</span>( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:bind</span>(run_service, i));
void run_service(int idx)
{
    service[idx].run();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

首先，要注意你不能拥有多个io_service实例却只有一个线程。下面的代码片段没有任何意义：

<code class="hljs matlab has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( int <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">i</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">i</span> < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>; ++<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">i</span>)
    service<span class="hljs-matrix" style="box-sizing: border-box;">[i].</span>run();</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

上面的代码片段没有意义是因为service[1].run()需要service[0].run()先结束。因此，所有由service[1]处理的异步操作都需要等待，这显然不是一个好主意。

在前面的3个方案中，我们在等待3个异步操作结束。为了解释它们之间的不同点，我们假设：过一会操作1完成，然后接着操作2完成。同时我们假设每一个完成处理程序需要1秒钟来完成执行。

在第一个例子中，我们在一个线程中等待三个操作全部完成，第1个操作一完成，我们就调用它的完成处理程序。尽管操作2紧接着完成了，但是操作2的完成处理程序需要在1秒钟后，也就是操作1的完成处理程序完成时才会被调用。

第二个例子，我们在两个线程中等待3个异步操作结束。当操作1完成时，我们在第1个线程中调用它的完成处理程序。当操作2完成时，紧接着，我们就在第2个线程中调用它的完成处理程序（当线程1在忙着响应操作1的处理程序时，线程2空闲着并且可以回应任何新进来的操作）。

在第三个例子中，因为操作1是sock1的connect，操作2是sock2的connect，所以应用程序会表现得像第二个例子一样。线程1会处理sock1 connect操作的完成处理程序，线程2会处理sock2的connect操作的完成处理程序。然而，如果sock1的connect操作是操作1，deadline_timer t的超时操作是操作2，线程1会结束正在处理的sock1 connect操作的完成处理程序。因而，deadline_timer t的超时操作必须等sock1 connect操作的完成处理程序结束（等待1秒钟），因为线程1要处理sock1的连接处理程序和t的超时处理程序。

下面是你需要从前面的例子中学到的：
* 第一种情况是非常基础的应用程序。因为是串行的方式，所以当几个处理程序需要被同时调用时，你通常会遇到瓶颈。如果一个处理程序需要花费很长的时间来执行，所有随后的处理程序都不得不等待。
* 第二种情况是比较适用的应用程序。他是非常强壮的——如果几个处理程序被同时调用了（这是有可能的），它们会在各自的线程里面被调用。唯一的瓶颈就是所有的处理线程都很忙的同时又有新的处理程序被调用。然而，这是有快速的解决方式的，增加处理线程的数目即可。
* 第三种情况是最复杂和最难理解的。你只有在第二种情况不能满足需求时才使用它。这种情况一般就是当你有成千上万实时（socket）连接时。你可以认为每一个处理线程（运行io_service::run()的线程）有它自己的select/epoll循环；它等待任意一个socket连接，然后等待一个读写操作，当它发现这种操作时，就执行。大部分情况下，你不需要担心什么，唯一你需要担心的就是当你监控的socket数目以指数级的方式增长时（超过1000个的socket）。在那种情况下，有多个select/epoll循环会增加应用的响应时间。

如果你觉得你的应用程序可能需要转换到第三种模式，请确保监听操作的这段代码（调用io_service::run()的代码）和应用程序其他部分是隔离的，这样你就可以很轻松地对其进行更改。

最后，需要一直记住的是如果没有其他需要监控的操作，.run()就会结束，就像下面的代码片段：

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">; </span>
<span class="hljs-label" style="box-sizing: border-box;">tcp:</span>:socket sock(service)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">; </span>
sock<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.async</span>_connect( ep, connect_handler)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">; </span>
service<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.run</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

在上面的例子中，只要sock建立了一个连接，connect_handler就会被调用，然后接着service.run()就会完成执行。

如果你想要service.run()接着执行，你需要分配更多的工作给它。这里有两个方式来完成这个目标。一种方式是在connect_handler中启动另外一个异步操作来分配更多的工作。另一种方式会模拟一些工作给它，用下面的代码片段：

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">typedef boost<span class="hljs-value" style="box-sizing: border-box;">::shared_ptr work_ptr;</span>
work_ptr <span class="hljs-function" style="box-sizing: border-box;">dummy_work(new io_service::<span class="hljs-function" style="box-sizing: border-box;">work(service)</span>)</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

上面的代码可以保证service.run()一直运行直到你调用useservice.stop()或者 dummy_work.reset(0);// 销毁 dummy_work.

总结

做为一个复杂的库，Boost.Asio让网络编程变得异常简单。构建起来也简单。而且在避免使用宏这一点上也做得很好；它虽然定义了少部分的宏来做选项开关，但是你需要关心的很少。

Boost.Asio支持同步和异步编程。他们有很大不同；你需要在项目早期就选择其中的一种来实现，因为它们之间的转换是非常复杂而且易错的。

如果你选择同步，你可以选择异常处理或者错误码，从异常处理转到错误码；只需要在call函数中增加一个参数即可（错误码）。

Boost.Asio不仅仅可以用来做网络编程。它还有其他更多的特性，这让它显得更有价值，比如信号量，计时器等等。

下一章我们将深入研究大量Boost.Asio中用来做网络编程的函数和类。同时我们也会学一些异步编程的诀窍。

Boost.Asio基本原理

这一章涵盖了使用Boost.Asio时必须知道的一些事情。我们也将深入研究比同步编程更复杂、更有乐趣的异步编程。

网络API

这一部分包含了当使用Boost.Asio编写网络应用程序时必须知道的事情。

Boost.Asio命名空间

Boost.Asio的所有内容都包含在boost::asio命名空间或者其子命名空间内。

boost::asio：这是核心类和函数所在的地方。重要的类有io_service和streambuf。类似read, read_at, read_until方法，它们的异步方法，它们的写方法和异步写方法等自由函数也在这里。
boost::asio::ip：这是网络通信部分所在的地方。重要的类有*address, endpoint, tcp,
udp和icmp*，重要的自由函数有connect和async_connect。要注意的是在boost::asio::ip::tcp::socket中间，socket只是boost::asio::ip::tcp类中间的一个typedef关键字。
boost::asio::error：这个命名空间包含了调用I/O例程时返回的错误码
boost::asio::ssl：包含了SSL处理类的命名空间
boost::asio::local：这个命名空间包含了POSIX特性的类
boost::asio::windows：这个命名空间包含了Windows特性的类

IP地址

对于IP地址的处理，Boost.Asio提供了ip::address , ip::address_v4和ip::address_v6类。
它们提供了相当多的函数。下面列出了最重要的几个：

ip::address(v4_or_v6_address):这个函数把一个v4或者v6的地址转换成ip::address
ip::address:from_string(str)：这个函数根据一个IPv4地址（用.隔开的）或者一个IPv6地址（十六进制表示）创建一个地址。
ip::address::to_string() ：这个函数返回这个地址的字符串。
ip::address_v4::broadcast([addr, mask]):这个函数创建了一个广播地址
ip::address_v4::any()：这个函数返回一个能表示任意地址的地址。
ip::address_v4::loopback(), ip_address_v6::loopback()：这个函数返回环路地址（为v4/v6协议）
ip::host_name()：这个函数用string数据类型返回当前的主机名。

大多数情况你会选择用ip::address::from_string：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span> addr = <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

如果你想通过一个主机名进行连接，下面的代码片段是无用的：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 抛出异常</span>
ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::address</span> addr <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::address</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"www.yahoo.com"</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

端点

端点是使用某个端口连接到的一个地址。不同类型的socket有它自己的endpoint类，比如ip::tcp::endpoint、ip::udp::endpoint和ip::icmp::endpoint

如果想连接到本机的80端口，你可以这样做：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">80</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

有三种方式来让你建立一个端点：

endpoint()：这是默认构造函数，某些时候可以用来创建UDP/ICMP socket。
endpoint(protocol, port)：这个方法通常用来创建可以接受新连接的服务器端socket。
endpoint(addr, port):这个方法创建了一个连接到某个地址和端口的端点。

例子如下：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep1;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep2(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:v4</span>(), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">80</span>);
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep3( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1), 80);</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

如果你想连接到一个主机（不是IP地址），你需要这样做：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 输出 "87.248.122.122"</span>
io_service service;
ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::resolver</span> resolver(service);
ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::resolver</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::query</span> query(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"www.yahoo.com"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"80"</span>);
ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::resolver</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::iterator</span> iter <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> resolver<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>resolve( query);
ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endpoint</span> ep <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">*</span>iter;
std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::cout</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> ep<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>address()<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>to_string() <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endl</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

你可以用你需要的socket类型来替换tcp。首先，为你想要查询的名字创建一个查询器，然后用resolve()函数解析它。如果成功，它至少会返回一个入口。你可以利用返回的迭代器，使用第一个入口或者遍历整个列表来拿到全部的入口。

给定一个端点，可以获得他的地址，端口和IP协议（v4或者v6）：

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-label" style="box-sizing: border-box;">std:</span>:cout << ep<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.address</span>()<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.to</span>_string() << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">":"</span> << ep<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.port</span>()
<< <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"/"</span> << ep<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.protocol</span>() << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">std</span>::endl<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

套接字

Boost.Asio有三种类型的套接字类：ip::tcp, ip::udp和ip::icmp。当然它也是可扩展的，你可以创建自己的socket类，尽管这相当复杂。如果你选择这样做，参照一下boost/asio/ip/tcp.hpp, boost/asio/ip/udp.hpp和boost/asio/ip/icmp.hpp。它们都是含有内部typedef关键字的超小类。

你可以把ip::tcp, ip::udp, ip::icmp类当作占位符；它们可以让你便捷地访问其他类/函数，如下所示：

ip::tcp::socket, ip::tcp::acceptor, ip::tcp::endpoint,ip::tcp::resolver, ip::tcp::iostream
ip::udp::socket, ip::udp::endpoint, ip::udp::resolver
ip::icmp::socket, ip::icmp::endpoint, ip::icmp::resolver

socket类创建一个相应的socket。而且总是在构造的时候传入io_service实例：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service)
sock.set_option(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:reuse_address</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

每一个socket的名字都是一个typedef关键字

ip::tcp::socket = basic_stream_socket
ip::udp::socket = basic_datagram_socket
ip::icmp::socket = basic_raw_socket

同步错误码

所有的同步函数都有抛出异常或者返回错误码的重载，比如下面的代码片段：

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">sync_func( arg1, arg2 <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span> argN); // 抛出异常
boost::system::error_code ec;
sync_func( arg1 arg2, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>, argN, ec); // 返回错误码</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

在这一章剩下的部分，你会见到大量的同步函数。简单起见，我省略了有返回错误码的重载，但是不可否认它们确实是存在的。

socket成员方法

这些方法被分成了几组。并不是所有的方法都可以在各个类型的套接字里使用。这个部分的结尾将有一个列表来展示各个方法分别属于哪个socket类。

注意所有的异步方法都立刻返回，而它们相对的同步实现需要操作完成之后才能返回。

连接相关的函数

这些方法是用来连接或绑定socket、断开socket字连接以及查询连接是活动还是非活动的：

assign(protocol,socket)：这个函数分配了一个原生的socket给这个socket实例。当处理老（旧）程序时会使用它（也就是说，原生socket已经被建立了）
open(protocol)：这个函数用给定的IP协议（v4或者v6）打开一个socket。你主要在UDP/ICMP socket，或者服务端socket上使用。
bind(endpoint)：这个函数绑定到一个地址
connect(endpoint)：这个函数用同步的方式连接到一个地址
async_connect(endpoint)：这个函数用异步的方式连接到一个地址
is_open()：如果套接字已经打开，这个函数返回true
close()：这个函数用来关闭套接字。调用时这个套接字上任何的异步操作都会被立即关闭，同时返回error::operation_aborted错误码。
shutdown(type_of_shutdown)：这个函数立即使send或者receive操作失效，或者两者都失效。
cancel()：这个函数取消套接字上所有的异步操作。这个套接字上任何的异步操作都会立即结束，然后返回error::operation_aborted错误码。

例子如下：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">80</span>);
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service);
sock.open(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:v4</span>()); n
sock.connect(ep);
sock.write_some(buffer(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"GET /index.html\r\n"</span>));
char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>]; sock.read_some(buffer(buff,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>));
sock.shutdown(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:shutdown_receive</span>);
sock.close();</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

读写函数

这些是在套接字上执行I/O操作的函数。

对于异步函数来说，处理程序的格式void handler(const boost::system::error_code& e, size_t bytes)都是一样的

async_receive(buffer, [flags,] handler)：这个函数启动从套接字异步接收数据的操作。
async_read_some(buffer,handler)：这个函数和async_receive(buffer, handler)功能一样。
async_receive_from(buffer, endpoint[, flags], handler)：这个函数启动从一个指定端点异步接收数据的操作。
async_send(buffer [, flags], handler)：这个函数启动了一个异步发送缓冲区数据的操作。
async_write_some(buffer, handler)：这个函数和a*sync_send(buffer, handler)*功能一致。
async_send_to(buffer, endpoint, handler)：这个函数启动了一个异步send缓冲区数据到指定端点的操作。
receive(buffer [, flags])：这个函数异步地从所给的缓冲区读取数据。在读完所有数据或者错误出现之前，这个函数都是阻塞的。
read_some(buffer)：这个函数的功能和receive(buffer)是一致的。
- receive_from(buffer, endpoint [, flags])*：这个函数异步地从一个指定的端点获取数据并写入到给定的缓冲区。在读完所有数据或者错误出现之前，这个函数都是阻塞的。
send(buffer [, flags])：这个函数同步地发送缓冲区的数据。在所有数据发送成功或者出现错误之前，这个函数都是阻塞的。
write_some(buffer)：这个函数和send(buffer)的功能一致。
send_to(buffer, endpoint [, flags])：这个函数同步地把缓冲区数据发送到一个指定的端点。在所有数据发送成功或者出现错误之前，这个函数都是阻塞的。
available()：这个函数返回有多少字节的数据可以无阻塞地进行同步读取。

稍后我们将讨论缓冲区。让我们先来了解一下标记。标记的默认值是0，但是也可以是以下几种：

ip::socket_type::socket::message_peek：这个标记只监测并返回某个消息，但是下一次读消息的调用会重新读取这个消息。
ip::socket_type::socket::message_out_of_band：这个标记处理带外（OOB）数据，OOB数据是被标记为比正常数据更重要的数据。关于OOB的讨论在这本书的内容之外。
ip::socket_type::socket::message_do_not_route：这个标记指定数据不使用路由表来发送。
ip::socket_type::socket::message_end_of_record：这个标记指定的数据标识了记录的结束。在Windows下不支持。

你最常用的可能是message_peek，使用方法请参照下面的代码片段：

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-value" style="box-sizing: border-box;">char</span> buff<span class="hljs-attr_selector" style="color: rgb(0, 136, 0); box-sizing: border-box;">[1024]</span>;
sock<span class="hljs-class" style="box-sizing: border-box;">.receive</span>(<span class="hljs-function" style="box-sizing: border-box;">buffer(buff)</span>, ip<span class="hljs-value" style="box-sizing: border-box;">::tcp::socket::message_peek );</span>
<span class="hljs-function" style="box-sizing: border-box;">memset(buff,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>)</span>;
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 重新读取之前已经读取过的内容</span>
sock<span class="hljs-class" style="box-sizing: border-box;">.receive</span>(<span class="hljs-function" style="box-sizing: border-box;">buffer(buff)</span> );</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

下面的是一些教你如何同步或异步地从不同类型的套接字上读取数据的例子：

例1是在一个TCP套接字上进行同步读写：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">80</span>);
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service);
sock.connect(ep);
sock.write_some(buffer(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"GET /index.html\r\n"</span>));
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"bytes available "</span> << sock.available() << <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endl</span>;
char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
size_t read = sock.read_some(buffer(buff));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

例2是在一个UDP套接字上进行同步读写：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service);
sock.open(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:v4</span>());
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> receiver_ep(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"87.248.112.181"</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">80</span>);
sock.send_to(buffer(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"testing\n"</span>), receiver_ep);
char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> sender_ep;
sock.receive_from(buffer(buff), sender_ep);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

[？注意：就像上述代码片段所展示的那样，使用receive_from从一个UDP套接字读取数据时，你需要构造一个默认的端点]

例3是从一个UDP服务套接字中异步读取数据：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">using namespace <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:asio</span>;
io_service service;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service);
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:asio</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:ip</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> sender_ep;
char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
void on_read(const <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:system</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:error_code</span> & err, <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:size_t</span> read_bytes) {
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"read "</span> << read_bytes << <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endl</span>;
    sock.async_receive_from(buffer(buff), sender_ep, on_read);
}
int main(int argc, char* argv[]) {
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>),
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8001</span>);
    sock.open(ep.protocol());
    sock.set_option(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:asio</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:ip</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:reuse_address</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>));
    sock.bind(ep);
    sock.async_receive_from(buffer(buff,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>), sender_ep, on_read);
    service.run();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

套接字控制：

这些函数用来处理套接字的高级选项：

get_io_service()：这个函数返回构造函数中传入的io_service实例
get_option(option)：这个函数返回一个套接字的属性
set_option(option)：这个函数设置一个套接字的属性
io_control(cmd)：这个函数在套接字上执行一个I/O指令

这些是你可以获取/设置的套接字选项：

名字	描述	类型
broadcast	如果为true，允许广播消息	bool
debug	如果为true，启用套接字级别的调试	bool
do_not_route	如果为true，则阻止路由选择只使用本地接口	bool
enable_connection_aborted	如果为true，记录在accept()时中断的连接	bool
keep_alive	如果为true，会发送心跳	bool
linger	如果为true，套接字会在有未发送数据的情况下挂起close()	bool
receive_buffer_size	套接字接收缓冲区大小	int
receive_low_watemark	规定套接字输入处理的最小字节数	int
reuse_address	如果为true，套接字能绑定到一个已用的地址	bool
send_buffer_size	套接字发送缓冲区大小	int
send_low_watermark	规定套接字数据发送的最小字节数	int
ip::v6_only	如果为true，则只允许IPv6的连接	bool

每个名字代表了一个内部套接字typedef或者类。下面是对它们的使用：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">80</span>);
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service);
sock.connect(ep);
<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> <span class="hljs-constant" style="box-sizing: border-box;">TCP</span>套接字可以重用地址
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:reuse_address</span> ra(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>);
sock.set_option(ra);
<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> 获取套接字读取的数据
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:receive_buffer_size</span> rbs;
sock.get_option(rbs);
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:cout</span> << rbs.value() << <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endl</span>;
<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> 把套接字的缓冲区大小设置为<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8192</span>
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:send_buffer_size</span> sbs(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8192</span>);
sock.set_option(sbs);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

[?在上述特性工作之前，套接字要被打开。否则，会抛出异常]

TCP VS UDP VS ICMP

就像我之前所说，不是所有的成员方法在所有的套接字类中都可用。我做了一个包含成员函数不同点的列表。如果一个成员函数没有出现在这，说明它在所有的套接字类都是可用的。

名字	TCP	UDP	ICMP
async_read_some	是	-	-
async_receive_from	-	是	是
async_write_some	是	-	-
async_send_to	-	是	是
read_some	是	-	-
receive_from	-	是	是
write_some	是	-	-
send_to	-	是	是

其他方法

其他与连接和I/O无关的函数如下：

local_endpoint()：这个方法返回套接字本地连接的地址。
remote_endpoint()：这个方法返回套接字连接到的远程地址。
native_handle()：这个方法返回原始套接字的处理程序。你只有在调用一个Boost.Asio不支持的原始方法时才需要用到它。
non_blocking()：如果套接字是非阻塞的，这个方法返回true，否则false。
native_non_blocking()：如果套接字是非阻塞的，这个方法返回true，否则返回false。但是，它是基于原生的套接字来调用本地的api。所以通常来说，你不需要调用这个方法（non_blocking()已经缓存了这个结果）；你只有在直接调用native_handle()这个方法的时候才需要用到这个方法。
at_mark()：如果套接字要读的是一段OOB数据，这个方法返回true。这个方法你很少会用到。

其他需要考虑的事情

最后要注意的一点，套接字实例不能被拷贝，因为拷贝构造方法和＝操作符是不可访问的。

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> s1(service), s2(service);
s1 = s2; <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> 编译时报错
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> s3(s1); <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> 编译时报错</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

这是非常有意义的，因为每一个实例都拥有并管理着一个资源（原生套接字本身）。如果我们允许拷贝构造，结果是我们会有两个实例拥有同样的原生套接字；这样我们就需要去处理所有者的问题（让一个实例拥有所有权？或者使用引用计数？还是其他的方法）Boost.Asio选择不允许拷贝（如果你想要创建一个备份，请使用共享指针）

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">typedef <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:shared_ptr<ip</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket></span> socket_ptr;
socket_ptr sock1(new <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span>(service));
socket_ptr sock2(sock1); <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> ok
socket_ptr sock3;           
sock3 = sock1; <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> ok</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

套接字缓冲区

当从一个套接字读写内容时，你需要一个缓冲区，用来保存读取和写入的数据。缓冲区内存的有效时间必须比I/O操作的时间要长；你需要保证它们在I/O操作结束之前不被释放。
对于同步操作来说，这很容易；当然，这个缓冲区在receive和send时都存在。

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>
sock.receive(buffer(buff));
strcpy(buff, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ok\n"</span>);
sock.send(buffer(buff));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

但是在异步操作时就没这么简单了，看下面的代码片段：

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">// 非常差劲的代码 <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>
void on_read(const boost::system::error_code & err, std::size_t read_bytes)
{ <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span> }
void func() {
    char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
    sock.async_receive(buffer(buff), on_read);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

在我们调用async_receive()之后，buff就已经超出有效范围，它的内存当然会被释放。当我们开始从套接字接收一些数据时，我们会把它们拷贝到一片已经不属于我们的内存中；它可能会被释放，或者被其他代码重新开辟来存入其他的数据，结果就是：内存冲突。

对于上面的问题有几个解决方案：

使用全局缓冲区
创建一个缓冲区，然后在操作结束时释放它
使用一个集合对象管理这些套接字和其他的数据，比如缓冲区数组

第一个方法显然不是很好，因为我们都知道全局变量非常不好。此外，如果两个实例使用同一个缓冲区怎么办？

下面是第二种方式的实现：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_read(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> * ptr, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> boost::system::error_code & err, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::size_t read_bytes) {                       
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">delete</span>[] ptr;
}
....
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> * buff = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
sock.async_receive(buffer(buff, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>), boost::bind(on_read,buff,_1,_2))</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

或者，如果你想要缓冲区在操作结束后自动超出范围，使用共享指针

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">struct</span> shared_buffer {
    boost::shared_array<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>> buff;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> size;
    shared_buffer(size_t size) : buff(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>[size]), size(size) {
    }
    mutable_buffers_1 asio_buff() <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> buffer(buff.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span>(), size);
    }
};


<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 当on_read超出范围时, boost::bind对象被释放了,</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 同时也会释放共享指针</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_read(shared_buffer, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> boost::system::error_code & err, std::size_t read_bytes) {}
sock.async_receive(buff.asio_buff(), boost::bind(on_read,buff,_1,_2));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

shared_buffer类拥有实质的shared_array<>，shared_array<>存在的目的是用来保存shared_buffer实例的拷贝－当最后一个share_array<>元素超出范围时，shared_array<>就被自动销毁了，而这就是我们想要的结果。

因为Boost.Asio会给完成处理句柄保留一个拷贝，当操作完成时就会调用这个完成处理句柄，所以你的目的达到了。那个拷贝是一个boost::bind的仿函数，它拥有着实际的shared_buffer实例。这是非常优雅的！

第三个选择是使用一个连接对象来管理套接字和其他数据，比如缓冲区，通常来说这是正确的解决方案但是非常复杂。在这一章的末尾我们会对这种方法进行讨论。

缓冲区封装函数

纵观所有代码，你会发现：无论什么时候，当我们需要对一个buffer进行读写操作时，代码会把实际的缓冲区对象封装在一个buffer()方法中，然后再把它传递给方法调用：

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-value" style="box-sizing: border-box;">char</span> buff<span class="hljs-attr_selector" style="color: rgb(0, 136, 0); box-sizing: border-box;">[512]</span>;
sock<span class="hljs-class" style="box-sizing: border-box;">.async_receive</span>(<span class="hljs-function" style="box-sizing: border-box;">buffer(buff)</span>, on_read);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

基本上我们都会把缓冲区包含在一个类中以便Boost.Asio的方法能遍历这个缓冲区，比方说，使用下面的代码：

<code class="hljs bash has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">sock.async_receive(some_buffer, on_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">read</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

实例some_buffer需要满足一些需求，叫做ConstBufferSequence或者MutableBufferSequence（你可以在Boost.Asio的文档中查看它们）。创建你自己的类去处理这些需求的细节是非常复杂的，但是Boost.Asio已经提供了一些类用来处理这些需求。所以你不用直接访问这些缓冲区，而可以使用buffer()方法。

自信地讲，你可以把下面列出来的类型都包装到一个buffer()方法中：

一个char[] const 数组
一个字节大小的void *指针
一个std::string类型的字符串
一个POD const数组（POD代表纯数据，这意味着构造器和释放器不做任何操作）
一个pod数据的std::vector
一个包含pod数据的boost::array
一个包含pod数据的std::array

下面的代码都是有效的：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">struct</span> pod_sample { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> l; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> c; };
...
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> b1[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> * b2 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> b3; b3.resize(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">128</span>);
pod_sample b4[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>];
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-stl_container" style="box-sizing: border-box;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">vector</span><pod_sample></span> b5; b5.resize(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>);
boost::<span class="hljs-stl_container" style="box-sizing: border-box;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span><pod_sample,16></span> b6;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-stl_container" style="box-sizing: border-box;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span><pod_sample,16></span> b7;
sock.async_send(buffer(b1), on_read);
sock.async_send(buffer(b2,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>), on_read);
sock.async_send(buffer(b3), on_read);
sock.async_send(buffer(b4), on_read);
sock.async_send(buffer(b5), on_read);
sock.async_send(buffer(b6), on_read);
sock.async_send(buffer(b7), on_read);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>

总的来说就是：与其创建你自己的类来处理ConstBufferSequence或者MutableBufferSequence的需求，不如创建一个能在你需要的时候保留缓冲区，然后返回一个mutable_buffers_1实例的类，而我们早在shared_buffer类中就这样做了。

read/write/connect自由函数

Boost.Asio提供了处理I/O的自由函数，我们分四组来分析它们。

connect方法

这些方法把套接字连接到一个端点。

connect(socket, begin [, end] [, condition])：这个方法遍历队列中从start到end的端点来尝试同步连接。begin迭代器是调用socket_type::resolver::query的返回结果（你可能需要回顾一下端点这个章节）。特别提示end迭代器是可选的；你可以忽略它。你还可以提供一个condition的方法给每次连接尝试之后调用。用法是Iterator connect_condition(const boost::system::error_code & err,Iterator next);。你可以选择返回一个不是next的迭代器，这样你就可以跳过一些端点。
async_connect(socket, begin [, end] [, condition], handler)：这个方法异步地调用连接方法，在结束时，它会调用完成处理方法。用法是void handler(constboost::system::error_code & err, Iterator iterator);。传递给处理方法的第二个参数是连接成功端点的迭代器（或者end迭代器）。

它的例子如下：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">using namespace <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:asio</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:ip</span>;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">tcp:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:resolver</span> resolver(service);
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">tcp:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:resolver</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:iterator</span> iter = resolver.resolve(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">tcp:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:resolver</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:query</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"www.yahoo.com"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"80"</span>));
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">tcp:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service);
connect(sock, iter);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

一个主机名可以被解析成多个地址，而connect和async_connect能很好地把你从尝试每个地址然后找到一个可用地址的繁重工作中解放出来，因为它们已经帮你做了这些。

read/write方法

这些方法对一个流进行读写操作（可以是套接字，或者其他表现得像流的类）：

async_read(stream, buffer [, completion] ,handler)：这个方法异步地从一个流读取。结束时其处理方法被调用。处理方法的格式是：void handler(const boost::system::error_ code & err, size_t bytes);。你可以选择指定一个完成处理方法。完成处理方法会在每个read操作调用成功之后调用，然后告诉Boost.Asio async_read操作是否完成（如果没有完成，它会继续读取）。它的格式是：size_t completion(const boost::system::error_code& err, size_t bytes_transfered) 。当这个完成处理方法返回0时，我们认为read操作完成；如果它返回一个非0值，它表示了下一个async_read_some操作需要从流中读取的字节数。接下来会有一个例子来详细展示这些。
async_write(stream, buffer [, completion], handler)：这个方法异步地向一个流写入数据。参数的意义和async_read是一样的。
read(stream, buffer [, completion])：这个方法同步地从一个流中读取数据。参数的意义和async_read是一样的。
write(stream, buffer [, completion]): 这个方法同步地向一个流写入数据。参数的意义和async_read是一样的。

<code class="hljs erlang has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-title" style="box-sizing: border-box;">async_read</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, stream_buffer [, completion], handler)</span>
<span class="hljs-title" style="box-sizing: border-box;">async_write</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(strean, stream_buffer [, completion], handler)</span>
<span class="hljs-title" style="box-sizing: border-box;">write</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, stream_buffer [, completion])</span>
<span class="hljs-title" style="box-sizing: border-box;">read</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(stream, stream_buffer [, completion])</span> </span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

首先，要注意第一个参数变成了流，而不单是socket。这个参数包含了socket但不仅仅是socket。比如，你可以用一个Windows的文件句柄来替代socket。
当下面情况出现时，所有read和write操作都会结束：

可用的缓冲区满了（当读取时）或者所有的缓冲区已经被写入（当写入时）
完成处理方法返回0（如果你提供了这么一个方法）
错误发生时

下面的代码会异步地从一个socket中间读取数据直到读取到’\n’：

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
ip::tcp::socket sock(service);
char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
int offset = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
size_t up_to_enter(const boost::system::error_code &, size_t bytes) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( size_t i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < bytes; ++i)
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( buff[i + offset] == <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'\n'</span>) 
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>; 
 }
void on_read(const boost::system::error_code &, size_t) {}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>
async_read(sock, buffer(buff), up_to_enter, on_read); </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

Boost.Asio也提供了一些简单的完成处理仿函数：

transfer_at_least(n)
transfer_exactly(n)
transfer_all()

例子如下：

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-value" style="box-sizing: border-box;">char</span> buff<span class="hljs-attr_selector" style="color: rgb(0, 136, 0); box-sizing: border-box;">[512]</span>; 
void <span class="hljs-function" style="box-sizing: border-box;">on_read(const boost::system::error_code &, size_t)</span> {} 
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 读取32个字节 </span>
<span class="hljs-function" style="box-sizing: border-box;">async_read(sock, <span class="hljs-function" style="box-sizing: border-box;">buffer(buff)</span>, <span class="hljs-function" style="box-sizing: border-box;">transfer_exactly(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">32</span>)</span>, on_read)</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

上述的4个方法，不使用普通的缓冲区，而使用由Boost.Asio的std::streambuf类继承来的stream_buffer方法。stl流和流缓冲区非常复杂；下面是例子：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;  
void on_read(streambuf& buf, const <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:system</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:error_code</span> &, size_t) { 
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:istream</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span>(&buf);
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:string</span> line;
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:getline</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span>, line);
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"first line: "</span> << line << <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endl</span>; 
}
int main(int argc, char* argv[]) { 
    <span class="hljs-constant" style="box-sizing: border-box;">HANDLE</span> file = <span class="hljs-constant" style="box-sizing: border-box;">::CreateFile</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"readme.txt"</span>, <span class="hljs-constant" style="box-sizing: border-box;">GENERIC_READ</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-constant" style="box-sizing: border-box;">OPEN_ALWAYS</span>, <span class="hljs-constant" style="box-sizing: border-box;">FILE_ATTRIBUTE_NORMAL</span> | <span class="hljs-constant" style="box-sizing: border-box;">FILE_FLAG_OVERLAPPED</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">windows:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:stream_handle</span> h(service, file);
    streambuf buf;
    async_read(h, buf, transfer_exactly(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">256</span>), <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:bind</span>(on_read,<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:ref</span>(buf),_1,_2));
    service.run(); 
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

在这里，我向你们展示了如何在一个Windows文件句柄上调用async_read。读取前256个字符，然后把它们保存到缓冲区中，当操作结束时。on_read被调用，再创建std::istream用来传递缓冲区，读取第一行（std::getline），最后把它输出到命令行中。

read_until/async_read_until方法

这些方法在条件满足之前会一直读取：

async_read_until(stream, stream_buffer, delim, handler):这个方法启动一个异步read操作。read操作会在读取到某个分隔符时结束。分隔符可以是字符,std::string或者boost::regex。处理方法的格式为：void handler(const boost::system::error_code & err, size_t bytes);。
async_read_until(strem, stream_buffer, completion, handler)：这个方法和之前的方法是一样的，但是没有分隔符，而是一个完成处理方法。完成处理方法的格式为：pair< iterator,bool > completion(iterator begin, iterator end);，其中迭代器的类型为buffers_iterator< streambuf::const_buffers_type >。你需要记住的是这个迭代器是支持随机访问的。你扫描整个区间（begin，end），然后决定read操作是否应该结束。返回的结果是一个结果对，第一个成员是一个迭代器，它指向最后被这个方法访问的字符；第二个成员指定read操作是否需要结束，需要时返回true，否则返回false。
read_until(stream, stream_buffer, delim)：这个方法执行一个同步的read操作，参数的意义和async_read_until一样。
read_until(stream, stream_buffer, completion)：这个方法执行一个同步的read操作，参数的意义和async_read_until一样。

下面这个例子在读到一个指定的标点符号之前会一直读取：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">typedef buffers_iterator<<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">streambuf:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:const_buffers_type></span> iterator;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:pair<iterator</span>, bool> match_punct(iterator <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">begin</span>, iterator <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">begin</span> != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>)
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:ispunct</span>(*<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">begin</span>))
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:make_pair</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">begin</span>,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:make_pair</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>);
}
void on_read(const <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:system</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:error_code</span> &, size_t) {}
...
streambuf buf;
async_read_until(sock, buf, match_punct, on_read);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

如果我们想读到一个空格时就结束，我们需要把最后一行修改为：

<code class="hljs bash has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">async_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">read</span>_until(sock, buff, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">' '</span>, on_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">read</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

*_at方法

这些方法用来在一个流上面做随机存取操作。由你来指定read和write操作从什么地方开始（offset）：

async_read_at(stream, offset, buffer [, completion], handler)：这个方法在指定的流的offset处开始执行一个异步的read操作，当操作结束时，它会调用handler。handler的格式为：void handler(const boost::system::error_code& err, size_t bytes);。buffer可以是普通的wrapper()封装或者streambuf方法。如果你指定一个completion方法，它会在每次read操作成功之后调用，然后告诉Boost.Asio async_read_at操作已经完成（如果没有，则继续读取）。它的格式为：size_t completion(const boost::system::error_code& err, size_t bytes);。当completion方法返回0时，我们认为read操作完成了；如果返回一个非零值，它代表了下一次调用流的async_read_some_at方法的最大读取字节数。
async_write_at(stream, offset, buffer [, completion], handler)：这个方法执行一个异步的write操作。参数的意义和async_read_at是一样的
read_at(stream, offset, buffer [, completion])：这个方法在一个执行的流上，指定的offset处开始read。参数的意义和async_read_at是一样的
write_at(stream, offset, buffer [, completion])：这个方法在一个执行的流上，指定的offset处开始write。参数的意义和async_read_at是一样的

这些方法不支持套接字。它们用来处理流的随机访问；也就是说，流是可以随机访问的。套接字显然不是这样（套接字是不可回溯的）。

下面这个例子告诉你怎么从一个文件偏移为256的位置读取128个字节：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> argc, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>* argv[]) {
    HANDLE file = ::CreateFile(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"readme.txt"</span>, GENERIC_READ, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);
    windows::random_access_handle h(service, file);
    streambuf buf;
    read_at(h, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">256</span>, buf, transfer_exactly(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">128</span>));
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::istream in(&buf);
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> line;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::getline(in, line);
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"first line: "</span> << line << <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::endl;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

异步编程

这部分对异步编程时可能碰到的一些问题进行了深入的探究。我建议你先读一遍，然后在接下来读这本书的过程中，再经常回过头来看看，从而增强你对这部分的理解。

异步的需求

就像我之前所说的，同步编程比异步编程简单很多。这是因为，线性的思考是很简单的（调用A，调用A结束，调用B，调用B结束，然后继续，这是以事件处理的方式来思考）。后面你会碰到这种情况，比如：五件事情，你不知道它们执行的顺序，也不知道他们是否会执行！

尽管异步编程更难，但是你会更倾向于选择使用它，比如：写一个需要处理很多并发访问的服务端。并发访问越多，异步编程就比同步编程越简单。

假设：你有一个需要处理1000个并发访问的应用，从客户端发给服务端的每个信息都会再返回给客户端，以‘\n’结尾。

同步方式的代码，1个线程：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> boost::asio;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">struct</span> client {
    ip::tcp::socket sock;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>]; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 每个信息最多这么大</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> already_read; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 你已经读了多少</span>
};
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-stl_container" style="box-sizing: border-box;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">vector</span><client></span> clients;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> handle_clients() {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>)
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < clients.size(); ++i)
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( clients[i].sock.available() ) on_read(clients[i]);
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_read(client & c) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> to_read = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::min( <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span> - c.already_read, c.sock.available());
    c.sock.read_some( buffer(c.buff + c.already_read, to_read));
    c.already_read += to_read;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::find(c.buff, c.buff + c.already_read, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'\n'</span>) < c.buff + c.already_read) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> pos = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::find(c.buff, c.buff + c.already_read, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'\n'</span>) - c.buff;
        <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> msg(c.buff, c.buff + pos);
        <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::copy(c.buff + pos, c.buff + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>, c.buff);
        c.already_read -= pos;
        on_read_msg(c, msg);
    }
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_read_msg(client & c, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg) {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 分析消息，然后返回</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg == <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"request_login"</span>)
        c.sock.write( <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"request_ok\n"</span>);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ...
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li></ul>

有一种情况是在任何服务端（和任何基于网络的应用）都需要避免的，就是代码无响应的情况。在我们的例子里，我们需要handle_clients()方法尽可能少的阻塞。如果方法在某个点上阻塞，任何进来的信息都需要等待方法解除阻塞才能被处理。

为了保持响应，只在一个套接字有数据的时候我们才读，也就是说，if ( clients[i].sock.available() ) on_read(clients[i])。在on_read时，我们只读当前可用的；调用read_until(c.sock, buffer(…), ‘\n’)会是一个非常糟糕的选择，因为直到我们从一个指定的客户端读取了完整的消息之前，它都是阻塞的（我们永远不知道它什么时候会读取到完整的消息）

这里的瓶颈就是on_read_msg()方法；当它执行时，所有进来的消息都在等待。一个良好的on_read_msg()方法实现会保证这种情况基本不会发生，但是它还是会发生（有时候向一个套接字写入数据，缓冲区满了时，它会被阻塞）
同步方式的代码，10个线程

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> boost::asio;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">struct</span> client {
　  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// ... 和之前一样</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">bool</span> set_reading() {
        boost::mutex::scoped_lock lk(cs_);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( is_reading_) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 已经在读取</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> { is_reading_ = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>; }
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> unset_reading() {
        boost::mutex::scoped_lock lk(cs_);
        is_reading_ = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;
    }
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    boost::mutex cs_;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">bool</span> is_reading_;
};
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-stl_container" style="box-sizing: border-box;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">vector</span><client></span> clients;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> handle_clients() {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>; ++i)
        boost::thread( handle_clients_thread);
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> handle_clients_thread() {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>)
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < clients.size(); ++i)
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( clients[i].sock.available() )
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( clients[i].set_reading()) {
                    on_read(clients[i]);
                    clients[i].unset_reading();
                }
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_read(client & c) {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 和之前一样</span>
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_read_msg(client & c, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg) {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 和之前一样</span>
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li></ul>

为了使用多线程，我们需要对线程进行同步，这就是set_reading()和set_unreading()所做的。set_reading()方法非常重要，比如你想要一步实现“判断是否在读取然后标记为读取中”。但这是有两步的（“判断是否在读取”和“标记为读取中”），你可能会有两个线程同时为一个客户端判断是否在读取，然后你会有两个线程同时为一个客户端调用on_read，结果就是数据冲突甚至导致应用崩溃。

你会发现代码变得极其复杂。

同步编程有第三个选择，就是为每个连接开辟一个线程。但是当并发的线程增加时，这就成了一种灾难性的情况。

然后，让我们来看异步编程。我们不断地异步读取。当一个客户端请求某些东西时，on_read被调用，然后回应，然后等待下一个请求（然后开始另外一个异步的read操作）。

异步方式的代码，10个线程

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> boost::asio;
io_service service;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">struct</span> client {
    ip::tcp::socket sock;
    streambuf buff; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 从客户端取回结果</span>
}
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-stl_container" style="box-sizing: border-box;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">vector</span><client></span> clients;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> handle_clients() {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < clients.size(); ++i)
        async_read_until(clients[i].sock, clients[i].buff, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'\n'</span>, boost::bind(on_read, clients[i], _1, _2));
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>; ++i)
        boost::thread(handle_clients_thread);
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> handle_clients_thread() {
    service.run();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_read(client & c, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> error_code & err, size_t read_bytes) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::istream in(&c.buff);
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> msg;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::getline(in, msg);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg == <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"request_login"</span>)
        c.sock.async_write( <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"request_ok\n"</span>, on_write);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ...
    ...
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 等待同一个客户端下一个读取操作</span>
    async_read_until(c.sock, c.buff, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'\n'</span>, boost::bind(on_read, c, _1, _2));
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul>

发现代码变得有多简单了吧？client结构里面只有两个成员，handle_clients()仅仅调用了async_read_until，然后它创建了10个线程，每个线程都调用service.run()。这些线程会处理所有来自客户端的异步read操作，然后分发所有向客户端的异步write操作。另外需要注意的一件事情是：on_read()一直在为下一次异步read操作做准备（看最后一行代码）。

异步run(), run_one(), poll(), poll_ one()

为了实现监听循环，io_service类提供了4个方法，比如：run(), run_one(), poll()和poll_one()。虽然大多数时候使用service.run()就可以，但是你还是需要在这里学习其他方法实现的功能。

持续运行

再一次说明，如果有等待执行的操作，run()会一直执行，直到你手动调用io_service::stop()。为了保证io_service一直执行，通常你添加一个或者多个异步操作，然后在它们被执行时，你继续一直不停地添加异步操作，比如下面代码：

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">using namespace boost::asio;
io_service service;
ip::tcp::socket sock(service);
char buff_read[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>], buff_write[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>] = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ok"</span>;
void on_read(const boost::system::error_code &err, std::size_t bytes);
void on_write(const boost::system::error_code &err, std::size_t bytes)
{
    sock.async_read_some(buffer(buff_read), on_read);
}
void on_read(const boost::system::error_code &err, std::size_t bytes)
{
    // <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span> 处理读取操作 <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>
    sock.async_write_some(buffer(buff_write,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>), on_write);
}
void on_connect(const boost::system::error_code &err) {
    sock.async_read_some(buffer(buff_read), on_read);
}
int main(int argc, char* argv[]) {
    ip::tcp::endpoint ep( ip::address::from_string(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2001</span>);
    sock.async_connect(ep, on_connect);
    service.run();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li></ul>

当service.run()被调用时，有一个异步操作在等待。
当socket连接到服务端时，on_connect被调用了，它会添加一个异步操作。
当on_connect结束时，我们会留下一个等待的操作（read）。
当on_read被调用时，我们写入一个回应，这又添加了另外一个等待的操作。
当on_read结束时，我们会留下一个等待的操作（write）。
当on_write操作被调用时，我们从服务端读取另外一个消息，这也添加了另外一个等待的操作。
当on_write结束时，我们有一个等待的操作（read）。
然后一直继续循环下去，直到我们关闭这个应用。

run_one(), poll(), poll_one() 方法

我在之前说过异步方法的handler是在调用了io_service::run的线程里被调用的。因为在至少90%～95%的时候，这是你唯一要用到的方法，所以我就把它说得简单了。对于调用了run_one(), poll()或者poll_one()的线程这一点也是适用的。

run_one()方法最多执行和分发一个异步操作：
* 如果没有等待的操作，方法立即返回0
* 如果有等待操作，方法在第一个操作执行之前处于阻塞状态，然后返回1

你可以认为下面两段代码是等效的：

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
service<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.run</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">; // 或者</span>
while ( !service<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.stopped</span>()) service<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.run</span>_once()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

你可以使用run_once()启动一个异步操作，然后等待它执行完成。

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
bool write_complete <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>;
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_write(const boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err, size_t <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>)
{ write_complete <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>; }
 …
std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">data</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"login ok”;
write_complete = false;
async_write(sock, buffer(data), on_write);
do service.run_once() while (!write_complete);</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

还有一些使用run_one()方法的例子，包含在Boost.Asio诸如blocking_tcp_client.cpp和blocking_udp_client.cpp的文件中。

poll_one方法以非阻塞的方式最多运行一个准备好的等待操作：

如果至少有一个等待的操作，而且准备好以非阻塞的方式运行，poll_one方法会运行它并且返回1
否则，方法立即返回0

操作正在等待并准备以非阻塞方式运行，通常意味着如下的情况：

一个计时器过期了，然后它的async_wait处理方法需要被调用
一个I/O操作完成了（比如async_read），然后它的hanlder需要被调用
之前被加入io_services实例队列中的自定义handler（这会在之后的章节中详解）

你可以使用poll_one去保证所有I/O操作的handler完成运行，同时做一些其他的工作

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( true) {
    // 运行所有完成了IO操作的handler
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( service.poll_one()) ;
    // <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span> 在这里做其他的事情 …
} </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

poll()方法会以非阻塞的方式运行所有等待的操作。下面两段代码是等效的：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
service.poll(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 或者</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( service.poll_one()) ;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

所有上述方法都会在失败的时候抛出boost::system::system_error异常。这是我们所不希望发生的事情；这里抛出的异常通常都是致命的，也许是资源耗尽，或者是你handler的其中一个抛出了异常。另外，每个方法都有一个不抛出异常，而是返回一个boost::system::error_code的重载：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:system</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:error_code</span> err = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
service.run(err);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( err) <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Error "</span> << err << <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endl</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

异步工作

异步工作不仅仅指用异步地方式接受客户端到服务端的连接、异步地从一个socket读取或者写入到socket。它包含了所有可以异步执行的操作。

默认情况下，你是不知道每个异步handler的调用顺序的。除了通常的异步调用（来自异步socket的读取/写入/接收）。你可以使用service.post()来使你的自定义方法被异步地调用。例如：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <boost/thread.hpp></span>
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <boost/bind.hpp></span>
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <boost/asio.hpp></span>
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <iostream></span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> boost::asio;
io_service service;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> func(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"func called, i= "</span> << i << <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::endl;
}

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> worker_thread() {
    service.run();
}

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> argc, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>* argv[]) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>; ++i)
        service.post(boost::bind(func, i));
    boost::thread_group threads;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>; ++i)
        threads.create_thread(worker_thread);
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 等待所有线程被创建完</span>
    boost::this_thread::sleep( boost::posix_time::millisec(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">500</span>));
    threads.join_all();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li></ul>

在上面的例子中，service.post(some_function)添加了一个异步方法调用。

这个方法在某一个调用了service.run()的线程中请求io_service实例，然后调用给定的some_funtion之后立即返回。在我们的例子中，这个线程是我们之前创建的三个线程中的一个。你不能确定异步方法调用的顺序。你不要期待它们会以我们调用post()方法的顺序来调用。下面是运行之前代码可能得到的结果：

<code class="hljs mizar has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 0
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 2
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 1
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 4
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 3
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 6
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 7
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 8
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 5
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 9</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

有时候你会想让一些异步处理方法顺序执行。比如，你去一个餐馆（go_to_restaurant），下单（order），然后吃（eat）。你需要先去餐馆，然后下单，最后吃。这样的话，你需要用到io_service::strand，这个方法会让你的异步方法被顺序调用。看下面的例子：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> boost::asio;
io_service service;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> func(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"func called, i= "</span> << i << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"/"</span> << boost::this_thread::get_id() << <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::endl;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> worker_thread() {
    service.run();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> argc, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>* argv[])
{
    io_service::strand strand_one(service), strand_two(service);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>; ++i)
        service.post( strand_one.wrap( boost::bind(func, i)));
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>; ++i)
        service.post( strand_two.wrap( boost::bind(func, i)));
    boost::thread_group threads;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>; ++i)
        threads.create_thread(worker_thread);
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 等待所有线程被创建完</span>
    boost::this_thread::sleep( boost::posix_time::millisec(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">500</span>));
    threads.join_all();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li></ul>

在上述代码中，我们保证前面的5个线程和后面的5个线程是顺序执行的。func called, i = 0在func called, i = 1之前被调用，然后调用func called, i = 2……同样func called, i = 5在func called, i = 6之前，func called, i = 6在func called, i = 7被调用……你需要注意的是尽管方法是顺序调用的，但是不意味着它们都在同一个线程执行。运行这个程序可能得到的一个结果如下：

<code class="hljs mizar has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 0/002A60C8
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 5/002A6138
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 6/002A6530
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 1/002A6138
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 7/002A6530
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 2/002A6138
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 8/002A6530
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 3/002A6138
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 9/002A6530
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 4/002A6138</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

异步post() VS dispatch() VS wrap()

Boost.Asio提供了三种让你把处理方法添加为异步调用的方式：

service.post(handler)：这个方法能确保其在请求io_service实例，然后调用指定的处理方法之后立即返回。handler稍后会在某个调用了service.run()的线程中被调用。
service.dispatch(handler)：这个方法请求io_service实例去调用给定的处理方法，但是另外一点，如果当前的线程调用了service.run()，它可以在方法中直接调用handler。
service.wrap(handler)：这个方法创建了一个封装方法，当被调用时它会调用service.dispatch(handler)，这个会让人有点困惑，接下来我会简单地解释它是什么意思。

在之前的章节中你会看到关于service.post()的一个例子，以及运行这个例子可能得到的一种结果。我们对它做一些修改，然后看看service.dispatch()是怎么影响输出的结果的：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> boost::asio;
io_service service;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> func(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"func called, i= "</span> << i << <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::endl;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> run_dispatch_and_post() {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>; i += <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>) {
    service.dispatch(boost::bind(func, i));
    service.post(boost::bind(func, i + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>));
    }
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> argc, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>* argv[]) {
    service.post(run_dispatch_and_post);
    service.run();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

在解释发生了什么之前，我们先运行程序，观察结果：

<code class="hljs mizar has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 0
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 2
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 4
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 6
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 8
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 1
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 3
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 5
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 7
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">func</span> called, i= 9</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

偶数先输出，然后是奇数。这是因为我用dispatch()输出偶数，然后用post()输出奇数。dispatch()会在返回之前调用hanlder，因为当前的线程调用了service.run()，而post()每次都立即返回了。

现在，让我们讲讲service.wrap(handler)。wrap()返回了一个仿函数，它可以用来做另外一个方法的参数：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> boost::asio;
io_service service;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> dispatched_func_1() {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"dispatched 1"</span> << <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::endl;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> dispatched_func_2() {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"dispatched 2"</span> << <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::endl;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> test(boost::function<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span>()> func) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"test"</span> << <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::endl;
    service.dispatch(dispatched_func_1);
    func();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> service_run() {
    service.run();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> argc, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>* argv[]) {
    test( service.wrap(dispatched_func_2));
    boost::thread th(service_run);
    boost::this_thread::sleep( boost::posix_time::millisec(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">500</span>));
    th.join();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li></ul>

test(service.wrap(dispatched_func_2));会把dispatched_ func_2包装起来创建一个仿函数，然后传递给test当作一个参数。当test()被调用时，它会分发调用方法1，然后调用func()。这时，你会发现调用func()和service.dispatch(dispatched_func_2)是等价的，因为它们是连续调用的。程序的输出证明了这一点：

<code class="hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">test
dispatched 1
dispatched 2</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

io_service::strand 类（用来序列化异步调用）也包含了poll(), dispatch()和 wrap()等成员函数。它们的作用和io_service的poll(), dispatch()和wrap()是一样的。然而，大多数情况下你只需要把io_service::strand::wrap()方法做为io_service::poll()或者io_service::dispatch()方法的参数即可。

保持活动

假设你需要做下面的操作：

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
ip::tcp::socket sock(service);
char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>
read(sock, buffer(buff));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

在这个例子中，sock和buff的存在时间都必须比read()调用的时间要长。也就是说，在调用read()返回之前，它们都必须有效。这就是你所期望的；你传给一个方法的所有参数在方法内部都必须有效。当我们采用异步方式时，事情会变得比较复杂。

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
ip::tcp::socket sock(service);
char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">512</span>];
void on_read(const boost::system::error_code &, size_t) {}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>
async_read(sock, buffer(buff), on_read);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

在这个例子中，sock和buff的存在时间都必须比read()操作本身时间要长，但是read操作持续的时间我们是不知道的，因为它是异步的。

当使用socket缓冲区的时候，你会有一个buffer实例在异步调用时一直存在（使用boost::shared_array<>）。在这里，我们可以使用同样的方式，通过创建一个类并在其内部管理socket和它的读写缓冲区。然后，对于所有的异步操作，传递一个包含智能指针的boost::bind仿函数给它：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> boost::asio;
io_service service;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">struct</span> connection : boost::enable_shared_from_this<connection> {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">typedef</span> boost::system::error_code error_code;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">typedef</span> boost::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">shared_ptr</span><connection> ptr;
    connection() : sock_(service), started_(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {}
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> start(ip::tcp::endpoint ep) {
        sock_.async_connect(ep, boost::bind(&connection::on_connect, shared_from_this(), _1));
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> stop() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( !started_) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
        started_ = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;
        sock_.close();
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">bool</span> started() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> started_; }
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_connect(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> error_code & err) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 这里你决定用这个连接做什么: 读取或者写入</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( !err) do_read();
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> stop();
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_read(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> error_code & err, size_t bytes) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( !started() ) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
        <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> msg(read_buffer_, bytes);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg == <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"can_login"</span>) do_write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"access_data"</span>);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"data "</span>) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) process_data(msg);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg == <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"login_fail"</span>) stop();
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_write(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> error_code & err, size_t bytes) {
        do_read(); 
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> do_read() {
        sock_.async_read_some(buffer(read_buffer_), boost::bind(&connection::on_read, shared_from_this(),   _1, _2)); 
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> do_write(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( !started() ) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 注意: 因为在做另外一个async_read操作之前你想要发送多个消息, </span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 所以你需要多个写入buffer</span>
        <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::copy(msg.begin(), msg.end(), write_buffer_);
        sock_.async_write_some(buffer(write_buffer_, msg.size()), boost::bind(&connection::on_write, shared_from_this(), _1, _2)); 
    }

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> process_data(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 处理服务端来的内容，然后启动另外一个写入操作</span>
    }
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    ip::tcp::socket sock_;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">enum</span> { max_msg = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span> };
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> read_buffer_[max_msg];
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> write_buffer_[max_msg];
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">bool</span> started_;
};

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> argc, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>* argv[]) {
    ip::tcp::endpoint ep( ip::address::from_string(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8001</span>);
    connection::ptr(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> connection)->start(ep);
} </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li></ul>

在所有异步调用中，我们传递一个boost::bind仿函数当作参数。这个仿函数内部包含了一个智能指针，指向connection实例。只要有一个异步操作等待时，Boost.Asio就会保存boost::bind仿函数的拷贝，这个拷贝保存了指向连接实例的一个智能指针，从而保证connection实例保持活动。问题解决！

当然，connection类仅仅是一个框架类；你需要根据你的需求对它进行调整（它看起来会和当前服务端例子的情况相当不同）。

你需要注意的是创建一个新的连接是相当简单的：connection::ptr(new connection)- >start(ep)。这个方法启动了到服务端的（异步）连接。当你需要关闭这个连接时，调用stop()。

当实例被启动时（start()），它会等待客户端的连接。当连接发生时。on_connect()被调用。如果没有错误发生，它启动一个read操作（do_read()）。当read操作结束时，你就可以解析这个消息；当然你应用的on_read()看起来会各种各样。而当你写回一个消息时，你需要把它拷贝到缓冲区，然后像我在do_write()方法中所做的一样将其发送出去，因为这个缓冲区同样需要在这个异步写操作中一直存活。最后需要注意的一点——当写回时，你需要指定写入的数量，否则，整个缓冲区都会被发送出去。

总结

网络api实际上要繁杂得多，这个章节只是做为一个参考，当你在实现自己的网络应用时可以回过头来看看。

Boost.Asio实现了端点的概念，你可以认为是IP和端口。如果你不知道准确的IP，你可以使用resolver对象将主机名，例如www.yahoo.com转换为一个或多个IP地址。

我们也可以看到API的核心——socket类。Boost.Asio提供了TCP、UDP和 ICMP的实现。而且你还可以用你自己的协议来对它进行扩展；当然，这个工作不适合缺乏勇气的人。

异步编程是刚需。你应该已经明白为什么有时候需要用到它，尤其在写服务端的时候。调用service.run()来实现异步循环就已经可以让你很满足，但是有时候你需要更进一步，尝试使用run_one()、poll()或者poll_one()。

当实现异步时，你可以异步执行你自己的方法；使用service.post()或者service.dispatch()。

最后，为了使socket和缓冲区（read或者write）在整个异步操作的生命周期中一直活动，我们需要采取特殊的防护措施。你的连接类需要继承自enabled_shared_from_this，然后在内部保存它需要的缓冲区，而且每次异步调用都要传递一个智能指针给this操作。

下一章会进行实战操作；在实现回显客户端/服务端应用时会有大量的编程实践。

回显服务端/客户端

在这一章，我们将会实现一个小的客户端/服务端应用，这可能会是你写过的最简单的客户端/服务端应用。回显应用就是一个把客户端发过来的任何内容回显给其本身，然后关闭连接的的服务端。这个服务端可以处理任何数量的客户端。每个客户端连接之后发送一个消息，服务端接收到完成消息后把它发送回去。在那之后，服务端关闭连接。

因此，每个回显客户端连接到服务端，发送一个消息，然后读取服务端返回的结果，确保这是它发送给服务端的消息就结束和服务端的会话。

我们首先实现一个同步应用，然后实现一个异步应用，以便你可以很容易对比他们：

为了节省空间，下面的代码有一些被裁剪掉了。你可以在附加在这本书的代码中看到全部的代码。

TCP回显服务端/客户端

对于TCP而言，我们需要一个额外的保证；每一个消息以换行符结束(‘\n’)。编写一个同步回显服务端/客户端非常简单。

我们会展示编码内容，比如同步客户端，同步服务端，异步客户端和异步服务端。

TCP同步客户端

在大多数有价值的例子中，客户端通常比服务端编码要简单（因为服务端需要处理多个客户端请求）。

下面的代码展示了不符合这条规则的一个例外：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">size_t read_complete(char <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">*</span> buf, const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err, size_t <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>)
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( err) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
    bool found <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::find</span>(buf, buf <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'\n'</span>) <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span> buf <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 我们一个一个读取直到读到回车，不缓存</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> found <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">?</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> : <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
}
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> sync_echo(std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> msg) {
    msg <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+=</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"\n”;
    ip::tcp::socket sock(service);
    sock.connect(ep);
    sock.write_some(buffer(msg));
    char buf[1024];
    int bytes = read(sock, buffer(buf), boost::bind(read_complete,buf,_1,_2));
    std::string copy(buf, bytes - 1);
    msg = msg.substr(0, msg.size() - 1);
    std::cout << "</span>server echoed our <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" << msg << "</span>: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<< (copy == msg ? "</span>OK<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" : "</span>FAIL<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">") << std::endl;
    sock.close();
}
int main(int argc, char* argv[]) {
    char* messages[] = { "</span>John says hi<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">", "</span>so does James<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">", "</span>Lucy just got home<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">", "</span>Boost<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>Asio is Fun<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">", 0 };
    boost::thread_group threads;
    for ( char ** message = messages; *message; ++message) {
        threads.create_thread( boost::bind(sync_echo, *message));
        boost::this_thread::sleep( boost::posix_time::millisec(100));
    }
    threads.join_all();
}</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul>

核心功能sync_echo。它包含了连接到服务端，发送信息然后等待回显的所有逻辑。

你会发现，在读取时，我使用了自由函数read()，因为我想要读’\n’之前的所有内容。sock.read_some()方法满足不了这个要求，因为它只会读可用的，而不是全部的消息。

read()方法的第三个参数是完成处理句柄。当读取到完整消息时，它返回0。否则，它会返回我下一步（直到读取结束）能都到的最大的缓冲区大小。在我们的例子中，返回结果始终是1，因为我永远不想读的消息比我们需要的更多。

在main()中，我们创建了几个线程；每个线程负责把消息发送到客户端，然后等待操作结束。如果你运行这个程序，你会看到下面的输出：

<code class="hljs vbscript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">server</span> echoed our John says hi: OK
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">server</span> echoed our so does James: OK
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">server</span> echoed our Lucy just got home: OK
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">server</span> echoed our Boost.Asio <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> Fun!: OK</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

注意：因为我们是同步的，所以不需要调用service.run()。

TCP同步服务端

回显同步服务端的编写非常容易，参考如下的代码片段：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
size_t read_complete(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> * buff, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> error_code & err, size_t bytes) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( err) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">bool</span> found = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::find(buff, buff + bytes, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'\n'</span>) < buff + bytes;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 我们一个一个读取直到读到回车，不缓存</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> found ? <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> : <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> handle_connections() {
    ip::tcp::acceptor acceptor(service, ip::tcp::endpoint(ip::tcp::v4(),<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8001</span>));
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>];
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {
        ip::tcp::socket sock(service);
        acceptor.accept(sock);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> bytes = read(sock, buffer(buff), boost::bind(read_complete,buff,_1,_2));
        <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> msg(buff, bytes);
        sock.write_some(buffer(msg));
        sock.close();
    }
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> argc, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>* argv[]) {
    handle_connections();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li></ul>

服务端的逻辑主要在handle_connections()。因为是单线程，我们接受一个客户端请求，读取它发送给我们的消息，然后回显，然后等待下一个连接。可以确定，当两个客户端同时连接时，第二个客户端需要等待服务端处理完第一个客户端的请求。

还是要注意因为我们是同步的，所以不需要调用service.run()。

TCP异步客户端

当我们开始异步时，编码会变得稍微有点复杂。我们会构建在第二章保持活动中展示的connection类。

观察这个章节中接下来的代码，你会发现每个异步操作启动了新的异步操作，以保持service.run()一直工作。

首先，核心功能如下：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">#define</span> MEM_FN(x)       boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>(<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>self_type<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::x</span>, shared_from_this())
<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">#define</span> MEM_FN1(x,y)    boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>(<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>self_type<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::x</span>, shared_from_this(),y)
<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">#define</span> MEM_FN2(x,y,z)  boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>(<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>self_type<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::x</span>, shared_from_this(),y,z)
class talk_to_svr : <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::enable_shared_from_this</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>talk_to_svr<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> , boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::noncopyable</span> {
    typedef talk_to_svr self_type;
    talk_to_svr(const std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> message) : sock_(service), started_(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>), message_(message) {}
    <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> start(ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endpoint</span> ep) {
        sock_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>async_connect(ep, MEM_FN1(on_connect,_1));
    }
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    typedef boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::error_code</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span>;
    typedef boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::shared_ptr</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>talk_to_svr<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> ptr;
    static ptr start(ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endpoint</span> ep, const std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>message) {
        ptr new_(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> talk_to_svr(message));
        new_<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>start(ep);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> new_;
    }
    <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> stop() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>started_) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
        started_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>;
        sock_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>close();
    }
    bool started() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> started_; }
    <span class="hljs-attribute" style="box-sizing: border-box;">...</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::socket</span> sock_;
    enum { max_msg <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span> };
    char read_buffer_<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">[</span>max_msg<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">]</span><span class="hljs-markup" style="box-sizing: border-box;">;
    char write_buffer_</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">[</span>max_msg<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">]</span><span class="hljs-markup" style="box-sizing: border-box;">;
    bool started_;
    std::string message_; 
}; </span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li></ul>

我们需要一直使用指向talk_to_svr的智能指针，这样的话当在tack_to_svr的实例上有异步操作时，那个实例是一直活动的。为了避免错误，比如在栈上构建一个talk_to_svr对象的实例时，我把构造方法设置成了私有而且不允许拷贝构造（继承自boost::noncopyable）。

我们有了核心方法，比如start(),stop()和started()，它们所做的事情也正如它们名字表达的一样。如果需要建立连接，调用talk_to_svr::start(endpoint, message)即可。我们同时还有一个read缓冲区和一个write缓冲区。（read_buufer_和write_buffer_）。

MEM_FN 是一个方便使用的宏，它们通过*shared_ptr_from_this()方法强制使用一个指向* this 的智能指针。

下面的几行代码和之前的解释非常不同：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//等同于 "sock_.async_connect(ep, MEM_FN1(on_connect,_1));"</span>
sock_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>async_connect(ep,boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>(<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>talk_to_svr<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::on_connect</span>,shared_ptr_from_this(),_1));
sock_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>async_connect(ep, boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>(<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>talk_to_svr<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::on_connect</span>,this,_1));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

在上述例子中，我们正确的创建了async_connect的完成处理句柄；在调用完成处理句柄之前它会保留一个指向talk_to_server实例的智能指针，从而保证当其发生时talk_to_server实例还是保持活动的。

在接下来的例子中，我们错误地创建了完成处理句柄，当它被调用时，talk_to_server实例很可能已经被释放了。

从socket读取或写入时，你使用如下的代码片段：

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">void <span class="hljs-function" style="box-sizing: border-box;">do_read()</span> {
    <span class="hljs-function" style="box-sizing: border-box;">async_read(sock_, <span class="hljs-function" style="box-sizing: border-box;">buffer(read_buffer_)</span>, <span class="hljs-function" style="box-sizing: border-box;">MEM_FN2(read_complete,_1,_2)</span>, <span class="hljs-function" style="box-sizing: border-box;">MEM_FN2(on_read,_1,_2)</span>)</span>;
}
void <span class="hljs-function" style="box-sizing: border-box;">do_write(const std::string & msg)</span> {
    if ( !<span class="hljs-function" style="box-sizing: border-box;">started()</span> ) return;
    std<span class="hljs-value" style="box-sizing: border-box;">::copy(msg.begin(), msg.end(), write_buffer_);</span>
    sock_<span class="hljs-class" style="box-sizing: border-box;">.async_write_some</span>( <span class="hljs-function" style="box-sizing: border-box;">buffer(write_buffer_, msg.<span class="hljs-function" style="box-sizing: border-box;">size()</span>)</span>, <span class="hljs-function" style="box-sizing: border-box;">MEM_FN2(on_write,_1,_2)</span>);
}
size_t <span class="hljs-function" style="box-sizing: border-box;">read_complete(const boost::system::error_code & err, size_t bytes)</span> {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 和TCP客户端中的类似</span>
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

do_read()方法会保证当on_read()被调用的时候，我们从服务端读取一行。do_write()方法会先把信息拷贝到缓冲区（考虑到当async_write发生时msg可能已经超出范围被释放），然后保证实际的写入操作发生时on_write()被调用。

然后是最重要的方法，这个方法包含了类的主要逻辑：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_connect(const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>err)      do_write(message_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"\n"</span>);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span>            stop();
}
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_read(const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err, size_t <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>err) {
        std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> copy(read_buffer_, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>);
        std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::cout</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"server echoed our "</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> message_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">": "</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> (copy <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> message_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">?</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"OK"</span> : <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"FAIL"</span>) <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endl</span>; 
    }
    stop(); 
}
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_write(const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err, size_t <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>) {
    do_read();
} </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

当连接成功之后，我们发送消息到服务端,do_write()。当write操作结束时，on_write()被调用，它初始化了一个do_read()方法，当do_read()完成时。on_read()被调用；这里，我们简单的检查一下返回的信息是否是服务端的回显，然后退出服务。

我们会发送三个消息到服务端让它变得更有趣一点：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">int main(int argc, char* argv[]) {
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8001</span>);
    char* messages[] = { <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"John says hi"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"so does James"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Lucy got home"</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> };
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( char ** message = messages; *message; ++message) {
        <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">talk_to_svr:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:start</span>( ep, *message);
        <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:this_thread</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:sleep</span>( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:posix_time</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:millisec</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>));
    }
    service.run();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

上述的代码会生成如下的输出：

<code class="hljs axapta has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">server</span> echoed our John says hi: OK
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">server</span> echoed our so does James: OK
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">server</span> echoed our Lucy just got home: OK</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

TCP异步服务端

核心功能和同步服务端的功能类似，如下：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">class talk_to_client : <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::enable_shared_from_this</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>talk_to_
   client<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span>, boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::noncopyable</span> {
    typedef talk_to_client self_type;
    talk_to_client() : sock_(service), started_(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>) {}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    typedef boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::error_code</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span>;
    typedef boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::shared_ptr</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>talk_to_client<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> ptr;
    <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> start() {
        started_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;
        do_read(); 
    }

    static ptr new_() {
        ptr new_(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> talk_to_client);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> new_;
    }
    <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> stop() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>started_) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
        started_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>;
        sock_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>close();
    }
    ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::socket</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> sock() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> sock_;}
    <span class="hljs-attribute" style="box-sizing: border-box;">...</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::socket</span> sock_;
    enum { max_msg <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span> };
    char read_buffer_<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">[</span>max_msg<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">]</span><span class="hljs-markup" style="box-sizing: border-box;">;
    char write_buffer_</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">[</span>max_msg<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">]</span><span class="hljs-markup" style="box-sizing: border-box;">;
    bool started_;
};</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li></ul>

因为我们是非常简单的回显服务，这里不需要is_started()方法。对每个客户端，仅仅读取它的消息，回显，然后关闭它。

do_read()，do_write()和read_complete()方法和TCP同步服务端的完全一致。

主要的逻辑同样是在on_read()和on_write()方法中：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_read(const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err, size_t <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>err) {
        std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> msg(read_buffer_, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>);
        do_write(msg <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"\n"</span>);
    }
    stop(); 
}
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_write(const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err, size_t <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>) {
    do_read();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

对客户端的处理如下：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::acceptor</span> acceptor(service, ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endpoint</span>(ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::v4</span>(),<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8001</span>));
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> handle_accept(talk_to_client<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::ptr</span> client, const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err)
{
    client<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>start();
    talk_to_client<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::ptr</span> new_client <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> talk_to_client<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::new_</span>();
    acceptor<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>async_accept(new_client<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>sock(), boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>(handle_accept,new_client,_1));
}
int main(int argc, char<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">*</span> argv<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">[</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">]</span><span class="hljs-markup" style="box-sizing: border-box;">) {
    talk_to_client::ptr client = talk_to_client::new_();
    acceptor.async_accept(client->sock(), boost::bind(handle_accept,client,_1));
    service.run();
} </span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

每一次客户端连接到服务时，handle_accept被调用，它会异步地从客户端读取，然后同样异步地等待一个新的客户端。

代码

你会在这本书相应的代码中得到所有4个应用（TCP回显同步客户端，TCP回显同步服务端，TCP回显异步客户端，TCP回显异步服务端）。当测试时，你可以使用任意客户端/服务端组合（比如，一个异步客户端和一个同步服务端）。

UDP回显服务端/客户端

因为UDP不能保证所有信息都抵达接收者，我们不能保证“信息以回车结尾”。
没收到消息，我们只是回显，但是没有socket去关闭（在服务端），因为我们是UDP。

UDP同步回显客户端

UDP回显客户端比TCP回显客户端要简单：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> ep( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:address</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8001</span>);
void sync_echo(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:string</span> msg) {
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service, <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span>(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:v4</span>(), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>));
    sock.send_to(buffer(msg), ep);
    char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>];
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> sender_ep;
    int bytes = sock.receive_from(buffer(buff), sender_ep);
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:string</span> copy(buff, bytes);
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"server echoed our "</span> << msg << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">": "</span> << (copy == msg ? <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"OK"</span> <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"FAIL"</span>) << <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endl</span>;
    sock.close();
}
int main(int argc, char* argv[]) {
    char* messages[] = { <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"John says hi"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"so does James"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Lucy got home"</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> };
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:thread_group</span> threads;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> ( char ** message = messages; *message; ++message) {
        threads.create_thread( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:bind</span>(sync_echo, *message));
        <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:this_thread</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:sleep</span>( <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:posix_time</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:millisec</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>));
    }
    threads.join_all();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

所有的逻辑都在synch_echo()中；连接到服务端，发送消息，接收服务端的回显，然后关闭连接。

UDP同步回显服务端

UDP回显服务端会是你写过的最简单的服务端：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">io_service service;
void handle_connections() {
    char buff[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>];
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock(service, <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span>(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:v4</span>(), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8001</span>));
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {
        <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:udp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span> sender_ep;
        int bytes = sock.receive_from(buffer(buff), sender_ep);
        <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:string</span> msg(buff, bytes);
        sock.send_to(buffer(msg), sender_ep);
    } 
}
int main(int argc, char* argv[]) {
    handle_connections();
} </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

它非常简单，而且能很好的自释。

我把异步UDP客户端和服务端留给读者当作一个练习。

总结

我们已经写了完整的应用，最终让Boost.Asio得以工作。回显应用是开始学习一个库时非常好的工具。你可以经常学习和运行这个章节所展示的代码，这样你就可以非常容易地记住这个库的基础。
在下一章，我们会建立更复杂的客户端/服务端应用，我们要确保避免低级错误，比如内存泄漏，死锁等等。

客户端和服务端

在这一章节，我们会深入学习怎样使用Boost.Asio建立非凡的客户端和服务端应用。你可以运行并测试它们，而且在理解之后，你可以把它们做为框架来构造自己的应用。

在接下来的例子中：

客户端使用一个用户名（无密码）登录到服务端
所有的连接由客户端建立，当客户端请求时服务端回应
所有的请求和回复都以换行符结尾（’\n’）
对于5秒钟没有ping操作的客户端，服务端会自动断开其连接

客户端可以发送如下请求：

获得所有已连接客户端的列表
客户端可以ping，当它ping时，服务端返回ping ok或者ping client_list_chaned（在接下来的例子中，客户端重新请求已连接的客户端列表）

为了更有趣一点，我们增加了一些难度：

每个客户端登录6个用户连接，比如Johon,James,Lucy,Tracy,Frank和Abby
每个客户端连接随机地ping服务端（随机7秒；这样的话，服务端会时不时关闭一个连接）

同步客户端/服务端

首先，我们会实现同步应用。你会发现它的代码很直接而且易读的。而且因为所有的网络调用都是阻塞的，所以它不需要独立的线程。

同步客户端

同步客户端会以你所期望的串行方式运行；连接到服务端，登录服务器，然后执行连接循环，比如休眠一下，发起一个请求，读取服务端返回，然后再休眠一会，然后一直循环下去……

因为我们是同步的，所以我们让事情变得简单一点。首先，连接到服务器，然后再循环，如下：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endpoint</span> ep( ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::address</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::from_string</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"127.0.0.1"</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8001</span>);
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> run_client(const std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> client_name) {
    talk_to_svr client(client_name);
    try {
        client<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>connect(ep);
        client<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">loop</span>();
    } catch(boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system_error</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err) {
        std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::cout</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"client terminated "</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><<</span> std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endl</span>;
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

下面的代码片段展示了talk_to_svr类：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">struct</span> talk_to_svr {
    talk_to_svr(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & username) : sock_(service), started_(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>), username_(username) {}
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> connect(ip::tcp::endpoint ep) {
        sock_.connect(ep);
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> loop() {
        write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"login "</span> + username_ + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"\n"</span>);
        read_answer();
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( started_) {
            write_request();
            read_answer();
            boost::this_thread::sleep(millisec(rand() % <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7000</span>));
        }
    }

    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> username() <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> username_; }
    ... 
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    ip::tcp::socket sock_;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">enum</span> { max_msg = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span> };
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> already_read_;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> buff_[max_msg];
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">bool</span> started_;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> username_;
}; </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li></ul>

在这个循环中，我们仅仅填充1个比特，做一个ping操作之后就进入睡眠状态，之后再读取服务端的返回。我们的睡眠是随机的（有时候超过5秒），这样服务端就有可能在某个时间点断开我们的连接：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> write_request() {
    write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping\n"</span>);
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> read_answer() {
    already_read_ = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
    read(sock_, buffer(buff_), boost::bind(&talk_to_svr::read_complete, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, _1, _2));
    process_msg();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> process_msg() {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> msg(buff_, already_read_);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"login "</span>) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_login();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping"</span>) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_ping(msg);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"clients "</span>) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_clients(msg);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cerr</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"invalid msg "</span> << msg << <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::endl;
} </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

对于读取结果，我们使用在之前章节就有说到的read_complete来保证我们能读到换行符（’\n’）。这段逻辑在process_msg()中，在这里我们读取服务端的返回，然后分发到正确的方法去处理：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_login() { do_ask_clients(); }
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_ping(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">istringstream</span> in(msg);
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> answer;
    in >> answer >> answer;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( answer == <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"client_list_changed"</span>)
        do_ask_clients();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_clients(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> clients = msg.substr(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>);
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << username_ << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">", new client list:"</span> << clients;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> do_ask_clients() {
    write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ask_clients\n"</span>);
    read_answer();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> write(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg) { sock_.write_some(buffer(msg)); }
size_t read_complete(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> boost::system::error_code & err, size_t bytes) {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// ... 和之前一样</span>
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

在读取服务端对我们ping操作的返回时，如果得到的消息是client_list_changed，我们就需要重新请求客户端列表。

同步服务端

同步服务端也是相当简单的。它只需要两个线程，一个负责接收新的客户端连接，另外一个负责处理已经存在的客户端请求。它不能使用单线程，因为等待新的客户端连接是一个阻塞操作，所以我们需要另外一个线程来处理已经存在的客户端请求。

正常来说服务端都比客户端要难实现。一方面，它要管理所有已经连接的客户端。因为我们是同步的，所以我们需要至少两个线程，一个负责接受新的客户端连接（因为accept()是阻塞的）而另一个负责回复已经存在的客户端。

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">void accept_thread() {
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:acceptor</span> acceptor(service,<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:endpoint</span>(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:v4</span>(), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8001</span>));
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {
        client_ptr new<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>( new talk_to_client);
        acceptor.accept(new<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>->sock());
        <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:recursive_mutex</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:scoped_lock</span> lk(cs);
        clients.push_back(new<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>);
    }
}

void handle_clients_thread() {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> ( <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {
        <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:this_thread</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:sleep</span>( millisec(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>));
        <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:recursive_mutex</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:scoped_lock</span> lk(cs);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span>(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">array:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:iterator</span> b = clients.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">begin</span>(), e = clients.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>(); b!= e; ++b)
            (*b)->answer_to_client();
        <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> 删除已经超时的客户端
        clients.erase(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:remove_if</span>(clients.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">begin</span>(), clients.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>(), <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:bind</span>(&<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">talk_to_client:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:timed_out</span>,_1)), clients.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>());
    }
}
int main(int argc, char* argv[]) {
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:thread_group</span> threads;
    threads.create_thread(accept_thread);
    threads.create_thread(handle_clients_thread);
    threads.join_all();
} </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li></ul>

为了分辨客户端发送过来的请求我们需要保存一个客户端的列表。

每个talk_to_client实例都拥有一个socket，socket类是不支持拷贝构造的，所以如果你想要把它们保存在一个std::vector对象中，你需要一个指向它的智能指针。这里有两种实现的方式：在talk_to_client内部保存一个指向socket的智能指针然后创建一个talk_to_client实例的数组，或者让talk_to_client实例用变量的方式保存socket，然后创建一个指向talk_to_client智能指针的数组。我选择后者，但是你也可以选前面的方式：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">typedef</span> boost::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">shared_ptr</span><talk_to_client> client_ptr;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">typedef</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-stl_container" style="box-sizing: border-box;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">vector</span><client_ptr></span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span> clients;
boost::recursive_mutex cs; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 用线程安全的方式访问客户端数组</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

talk_to_client的主要代码如下：

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">struct talk_to_client : boost::enable_shared_from_this<talk_to_client>
{
    talk_to_client() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span> }
    std::string username() const { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> username_; }
    void answer_to_client() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
            read_request();
            process_request();
        } catch ( boost::system::system_error&) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stop</span>(); }
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( timed_out())
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stop</span>();
    }
    void set_clients_changed() { clients_changed_ = true; }
    ip::tcp::socket & sock() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> sock_; }
    bool timed_out() const {
        ptime now = microsec_clock::local_time();
        long long ms = (now - last_ping).total_milliseconds();
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> ms > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5000</span> ;
    }
    void <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stop</span>() {
        boost::system::error_code err; sock_.close(err);
    }
    void read_request() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( sock_.available())
            already_read_ += sock_.read_some(buffer(buff_ + already_read_, max_msg - already_read_));
    }
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span> 
private:
    // <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>  和同步客户端中的一样
    bool clients_changed_;
    ptime last_ping;
}; </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li></ul>

上述代码拥有非常好的自释能力。其中最重要的方法是read_request()。它只在存在有效数据的情况才读取，这样的话，服务端永远都不会阻塞：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> process_request() {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">bool</span> found_enter = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::find(buff_, buff_ + already_read_, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'\n'</span>) < buff_ + already_read_;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( !found_enter)
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 消息不完整</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 处理消息</span>
    last_ping = microsec_clock::local_time();
    size_t pos = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::find(buff_, buff_ + already_read_, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'\n'</span>) - buff_;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> msg(buff_, pos);
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::copy(buff_ + already_read_, buff_ + max_msg, buff_);
    already_read_ -= pos + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"login "</span>) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_login(msg);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping"</span>) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_ping();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ask_clients"</span>) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_clients();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cerr</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"invalid msg "</span> << msg << <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::endl;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_login(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">istringstream</span> in(msg);
    in >> username_ >> username_;
    write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"login ok\n"</span>);
    update_clients_changed();
} 
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_ping() {
    write(clients_changed_ ? <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping client_list_changed\n"</span> : <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping ok\n"</span>);
    clients_changed_ = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_clients() {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> msg;
    { boost::recursive_mutex::scoped_lock lk(cs);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span>( <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>::const_iterator b = clients.begin(), e = clients.end() ; b != e; ++b)
            msg += (*b)->username() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" "</span>;
    }
    write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"clients "</span> + msg + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"\n"</span>);
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> write(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg){sock_.write_some(buffer(msg)); }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li></ul>

观察process_request()。当我们读取到足够多有效的数据时，我们需要知道我们是否已经读取到整个消息(如果found_enter为真)。这样做的话，我们可以使我们避免一次读多个消息的可能（’\n’之后的消息也被保存到缓冲区中），然后我们解析读取到的整个消息。剩下的代码都是很容易读懂的。

异步客户端/服务端

现在，是比较有趣（也比较难）的异步实现！
当查看示意图时，你需要知道Boost.Asio代表由Boost.Asio执行的一个异步调用。例如do_read()，Boost.Asio和on_read()代表了从do_read()到on_read()的逻辑流程，但是你永远不知道什么时候轮到on_read()被调用，你只是知道你最终会调用它。

异步客户端

到这里事情会变得有点复杂，但是仍然是可控的。当然你也会拥有一个不会阻塞的应用。

下面的代码你应该已经很熟悉：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">#define</span> MEM_FN(x)       boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>(<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>self_type<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::x</span>, shared_from_this())
<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">#define</span> MEM_FN1(x,y)    boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>(<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>self_type<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::x</span>, shared_from_
this(),y)
<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">#define</span> MEM_FN2(x,y,z)  boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>(<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>self_type<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::x</span>, shared_from_
this(),y,z)
class talk_to_svr : <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::enable_shared_from_this</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>talk_to_svr<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span>, boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::noncopyable</span> {
    typedef talk_to_svr self_type;
    talk_to_svr(const std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> username) : sock_(service), started_(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>), username_(username), timer_
(service) {}
    <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> start(ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endpoint</span> ep) {
        sock_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>async_connect(ep, MEM_FN1(on_connect,_1));
} 
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    typedef boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::error_code</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span>;
    typedef boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::shared_ptr</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>talk_to_svr<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> ptr;
    static ptr start(ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endpoint</span> ep, const std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> username) {
        ptr new_(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> talk_to_svr(username));
        new_<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>start(ep);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> new_;
    }
    <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> stop() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>started_) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
        started_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>;
        sock_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>close();
    }
    bool started() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> started_; }
    <span class="hljs-attribute" style="box-sizing: border-box;">...</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    size_t read_complete(const boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::system</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span>err, size_t <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( err) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
        bool found <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::find</span>(read_buffer_, read_buffer_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'\n'</span>) <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span> read_buffer_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> found <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">?</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> : <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
    }
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::socket</span> sock_;
    enum { max_msg <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span> };
    char read_buffer_<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">[</span>max_msg<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">]</span><span class="hljs-markup" style="box-sizing: border-box;">;
    char write_buffer_</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">[</span>max_msg<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">]</span><span class="hljs-markup" style="box-sizing: border-box;">;
    bool started_;
    std::string username_;
    deadline_timer timer_;
};</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li></ul>

你会看到额外还有一个叫deadline_timer timer_的方法用来ping服务端；而且ping操作同样是随机的。

下面是类的逻辑：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_connect(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> error_code & err) {
       <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( !err)      do_write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"login "</span> + username_ + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"\n"</span>);
       <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span>            stop();
   }
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_read(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> error_code & err, size_t bytes) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( err) stop();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( !started() ) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 处理消息</span>
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> msg(read_buffer_, bytes);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"login "</span>) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_login();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping"</span>) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_ping(msg);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"clients "</span>) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_clients(msg);
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_login() {
    do_ask_clients();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_ping(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">istringstream</span> in(msg);
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> answer;
    in >> answer >> answer;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( answer == <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"client_list_changed"</span>) do_ask_clients();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> postpone_ping();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> on_clients(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> & msg) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> clients = msg.substr(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>);
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << username_ << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">", new client list:"</span> << clients ;
    postpone_ping();
} </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul>

在on_read()中，首先的两行代码是亮点。在第一行，如果出现错误，我们就停止。而第二行，如果我们已经停止了（之前就停止了或者刚好停止），我们就返回。反之如果所有都是OK，我们就对收到的消息进行处理。

最后是do_*方法，实现如下：

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">void <span class="hljs-function" style="box-sizing: border-box;">do_ping()</span> { <span class="hljs-function" style="box-sizing: border-box;">do_write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping\n"</span>)</span>; }
void <span class="hljs-function" style="box-sizing: border-box;">postpone_ping()</span> {
    timer_<span class="hljs-class" style="box-sizing: border-box;">.expires_from_now</span>(boost<span class="hljs-value" style="box-sizing: border-box;">::posix_time::millisec(rand() % <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7000</span>));</span>
    timer_<span class="hljs-class" style="box-sizing: border-box;">.async_wait</span>( <span class="hljs-function" style="box-sizing: border-box;">MEM_FN(do_ping)</span>);
}
void <span class="hljs-function" style="box-sizing: border-box;">do_ask_clients()</span> { <span class="hljs-function" style="box-sizing: border-box;">do_write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ask_clients\n"</span>)</span>; }
void <span class="hljs-function" style="box-sizing: border-box;">on_write(const error_code & err, size_t bytes)</span> { <span class="hljs-function" style="box-sizing: border-box;">do_read()</span>; }
void <span class="hljs-function" style="box-sizing: border-box;">do_read()</span> {
    <span class="hljs-function" style="box-sizing: border-box;">async_read(sock_, <span class="hljs-function" style="box-sizing: border-box;">buffer(read_buffer_)</span>, <span class="hljs-function" style="box-sizing: border-box;">MEM_FN2(read_complete,_1,_2)</span>, <span class="hljs-function" style="box-sizing: border-box;">MEM_FN2(on_read,_1,_2)</span>)</span>;
}
void <span class="hljs-function" style="box-sizing: border-box;">do_write(const std::string & msg)</span> {
    if ( !<span class="hljs-function" style="box-sizing: border-box;">started()</span> ) return;
    std<span class="hljs-value" style="box-sizing: border-box;">::copy(msg.begin(), msg.end(), write_buffer_);</span>
    sock_<span class="hljs-class" style="box-sizing: border-box;">.async_write_some</span>( <span class="hljs-function" style="box-sizing: border-box;">buffer(write_buffer_, msg.<span class="hljs-function" style="box-sizing: border-box;">size()</span>)</span>, <span class="hljs-function" style="box-sizing: border-box;">MEM_FN2(on_write,_1,_2)</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

注意每一个read操作都会触发一个ping操作

当read操作结束时，on_read()被调用
on_read()调用on_login()，on_ping()或者on_clients()
每一个方法要么发出一个ping，要么请求客户端列表
如果我们请求客户端列表，当read操作接收到它们时，它会发出一个ping操作。

异步服务端

这个示意图是相当复杂的；从Boost.Asio出来你可以看到4个箭头指向on_accept，on_read，on_write和on_check_ping。这也就意味着你永远不知道哪个异步调用是下一个完成的调用，但是你可以确定的是它是这4个操作中的一个。

现在，我们是异步的了；我们可以继续保持单线程。接受客户端连接是最简单的部分，如下所示：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::acceptor</span> acceptor(service, ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::endpoint</span>(ip<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::tcp</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::v4</span>(), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8001</span>));
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> handle_accept(talk_to_client<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::ptr</span> client, const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err)
{
    client<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>start();
    talk_to_client<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::ptr</span> new_client <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> talk_to_client<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::new_</span>();
    acceptor<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>async_accept(new_client<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>sock(), boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::bind</span>(handle_accept,new_client,_1));
}
int main(int argc, char<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">*</span> argv<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">[</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">]</span><span class="hljs-markup" style="box-sizing: border-box;">) {
    talk_to_client::ptr client = talk_to_client::new_();
    acceptor.async_accept(client->sock(),boost::bind(handle_accept,client,_1));
    service.run();
}</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

上述代码会一直异步地等待一个新的客户端连接（每个新的客户端连接会触发另外一个异步等待操作）。

我们需要监控client list changed事件（一个新客户端连接或者一个客户端断开连接），然后当事件发生时通知所有的客户端。因此，我们需要保存一个客户端连接的数组，否则除非你不需要在某一时刻知道所有连接的客户端，你才不需要这样一个数组。

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> talk_to_client; 
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">typedef</span> boost::<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">shared_ptr</span><talk_to_client>client_ptr;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">typedef</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>::<span class="hljs-stl_container" style="box-sizing: border-box;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">vector</span><client_ptr></span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span> clients;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

connection类的框架如下：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">talk_to_client</span> : <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">public</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">boost::enable_shared_from_this</span><span class="hljs-inheritance" style="box-sizing: border-box;"><<span class="hljs-parent" style="box-sizing: border-box;">talk_to_client</span></span>> , <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">boost::noncopyable</span> {</span>
    talk_to_client() { ... }
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">public:</span>
    typedef <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:system</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:error_code</span> error_code;
    typedef <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:shared_ptr<talk_to_client></span> ptr;
    void start() {
        started<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>;
        clients.push_back( shared_from_this());
        last_ping = <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:posix_time</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:microsec_clock</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:local_time</span>();
        do_read(); <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span>首先，我们等待客户端连接
    }
    static ptr new<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>() { ptr new<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>(new talk_to_client); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> new<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>; }
    void stop() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( !started<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
        started<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;
        sock<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>.close();
        ptr <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span> = shared_from_this();
        <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">array:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:iterator</span> it = <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:find</span>(clients.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">begin</span>(), clients.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>(), <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span>);
        clients.erase(it);
        update_clients_changed();
    }
    bool started() const { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> started<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>; }
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> & sock() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> sock<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>;}
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:string</span> username() const { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> username<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>; }
    void set_clients_changed() { clients_changed<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>; }
    … 
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">private:</span>
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">ip:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:tcp</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:socket</span> sock<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>;
    enum { max_msg = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span> };
    char read_buffer<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>[max_msg];
    char write_buffer<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>[max_msg];
    bool started<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>;
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">std:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:string</span> username<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>;
    deadline_timer timer<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>;
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">boost:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:posix_time</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:ptime</span> last_ping;
    bool clients_changed<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span>;
};</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li></ul>

我会用talk_to_client或者talk_to_server来调用connection类，从而让你更明白我所说的内容。

现在你需要用到之前的代码了；它和我们在客户端应用中所用到的是一样的。我们还有另外一个stop()方法，这个方法用来从客户端数组中移除一个客户端连接。

服务端持续不断地等待异步的read操作：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_read(const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error_code</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> err, size_t <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( err) stop();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>started() ) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
    std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> msg(read_buffer_, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">bytes</span>);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"login "</span>) <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_login(msg);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping"</span>) <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_ping();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( msg<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ask_clients"</span>) <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) on_clients();
}
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_login(const std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&</span> msg) {
    std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::istringstream</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span>(msg);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">>></span> username_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">>></span> username_;
    do_write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"login ok\n"</span>);
    update_clients_changed();
}
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_ping() {
    do_write(clients_changed_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">?</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping client_list_changed\n"</span> : <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping ok\n"</span>);
    clients_changed_ <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>;
}
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_clients() {
    std<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::string</span> msg;
    for(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::const_iterator</span> b <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span>clients<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>begin(),e <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span>clients<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>end(); b <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!=</span> e; <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">++</span>b)
        msg <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+=</span> (<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">*</span>b)<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>username() <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" "</span>;
    do_write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"clients "</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+</span> msg <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">+</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"\n"</span>);
} </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li></ul>

这段代码是简单易懂的；需要注意的一点是：当一个新客户端登录，我们调用update_clients_changed()，这个方法为所有客户端将clients_changed_标志为true。

服务端每收到一个请求就用相应的方式进行回复，如下所示：

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">void <span class="hljs-function" style="box-sizing: border-box;">do_ping()</span> { <span class="hljs-function" style="box-sizing: border-box;">do_write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ping\n"</span>)</span>; }
void <span class="hljs-function" style="box-sizing: border-box;">do_ask_clients()</span> { <span class="hljs-function" style="box-sizing: border-box;">do_write(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ask_clients\n"</span>)</span>; }
void <span class="hljs-function" style="box-sizing: border-box;">on_write(const error_code & err, size_t bytes)</span> { <span class="hljs-function" style="box-sizing: border-box;">do_read()</span>; }
void <span class="hljs-function" style="box-sizing: border-box;">do_read()</span> {
    <span class="hljs-function" style="box-sizing: border-box;">async_read(sock_, <span class="hljs-function" style="box-sizing: border-box;">buffer(read_buffer_)</span>, <span class="hljs-function" style="box-sizing: border-box;">MEM_FN2(read_complete,_1,_2)</span>, <span class="hljs-function" style="box-sizing: border-box;">MEM_FN2(on_read,_1,_2)</span>)</span>;
    <span class="hljs-function" style="box-sizing: border-box;">post_check_ping()</span>;
}
void <span class="hljs-function" style="box-sizing: border-box;">do_write(const std::string & msg)</span> {
    if ( !<span class="hljs-function" style="box-sizing: border-box;">started()</span> ) return;
    std<span class="hljs-value" style="box-sizing: border-box;">::copy(msg.begin(), msg.end(), write_buffer_);</span>
    sock_<span class="hljs-class" style="box-sizing: border-box;">.async_write_some</span>( <span class="hljs-function" style="box-sizing: border-box;">buffer(write_buffer_, msg.<span class="hljs-function" style="box-sizing: border-box;">size()</span>)</span>, <span class="hljs-function" style="box-sizing: border-box;">MEM_FN2(on_write,_1,_2)</span>);
}
size_t <span class="hljs-function" style="box-sizing: border-box;">read_complete(const boost::system::error_code & err, size_t bytes)</span> {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// ... 就像之前</span>
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

在每个write操作的末尾，on_write()方法被调用，这个方法会触发另外一个异步读操作，这样的话“等待请求－回复请求”这个循环就会一直执行，直到客户端断开连接或者超时。

在每次读操作开始之前，我们异步等待5秒钟来观察客户端是否超时。如果超时，我们关闭它的连接：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> on_check_ping() {
    ptime now <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> microsec_clock<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::local_time</span>();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( (now <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-</span> last_ping)<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>total_milliseconds() <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5000</span>)
        stop();
    last_ping <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::posix_time</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::microsec_clock</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::local_time</span>();
}
<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> post_check_ping() {
    timer_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>expires_from_now(boost<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::posix_time</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::millisec</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5000</span>));
    timer_<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>async_wait( MEM_FN(on_check_ping));
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

这就是整个服务端的实现。你可以运行并让它工作起来！

在代码中，我向你们展示了这一章我们学到的东西，为了更容易理解，我把代码稍微精简了下；比如，大部分的控制台输出我都没有展示，尽管在这本书附赠的代码中它们是存在的。我建议你自己运行这些例子，因为从头到尾读一次代码能加强你对本章展示应用的理解。

总结

我们已经学到了怎么写一些基础的客户端/服务端应用。我们已经避免了一些诸如内存泄漏和死锁的低级错误。所有的编码都是框架式的，这样你就可以根据你自己的需求对它们进行扩展。

在接下来的章节中，我们会更加深入地了解使用Boost.Asio进行同步编程和异步编程的不同点，同时你也会学会如何嵌入你自己的异步操作。

原文：

http://blog.csdn.net/mmoaay/article/details/43759611