[C++] - C++11 多线程 - Thread

转载整理自：https://github.com/forhappy/Cplusplus-Concurrency-In-Practice/tree/master/zh/chapter3-Thread

1.与 C++11 多线程相关的头文件

C++11 新标准中引入了五个头文件来支持多线程编程，它们分别是 <atomic>, <thread>, <mutex>, <condition_variable> 和 <future>。

<atomic>：该头文主要声明了两个类, std::atomic 和 std::atomic_flag，另外还声明了一套 C 风格的原子类型和与 C 兼容的原子操作的函数。
<thread>：该头文件主要声明了 std::thread 类，另外 std::this_thread 命名空间也在该头文件中。
<mutex>：该头文件主要声明了与互斥量(Mutex)相关的类，包括 std::mutex_* 一系列类，std::lock_guard, std::unique_lock, 以及其他的类型和函数。
<condition_variable>：该头文件主要声明了与条件变量相关的类，包括 std::condition_variable 和 std::condition_variable_any。
<future>：该头文件主要声明了 std::promise, std::package_task 两个 Provider 类，以及 std::future 和 std::shared_future 两个 Future 类，另外还有一些与之相关的类型和函数，std::async() 函数就声明在此头文件中。

2.thread

2.1`<thread>` 头文件摘要

<thread> 头文件声明了 std::thread 线程类及 std::swap (交换两个线程对象)辅助函数。另外命名空间 std::this_thread 也声明在 <thread> 头文件中。下面是 C++11 标准所定义的 <thread> 头文件摘要：

namespace std {
    #define __STDCPP_THREADS__ __cplusplus
    class thread;
    void swap(thread& x, thread& y);
    namespace this_thread {
        thread::id get_id();
        void yield();

        template <class Clock, class Duration>
        void sleep_until(const chrono::time_point<Clock, Duration>& abs_time);

        template <class Rep, class Period>
        void sleep_for(const chrono::duration<Rep, Period>& rel_time);
    }        
}

2.2 `std::thread` 类摘要

std::thread 代表了一个线程对象，C++11 标准声明如下：

namespace std {
    class thread {
        public:
            // 类型声明:
            class id;
            typedef implementation-defined native_handle_type;

            // 构造函数、拷贝构造函数和析构函数声明:
            thread() noexcept;
            template <class F, class ...Args> explicit thread(F&& f, Args&&... args);
            ~thread();
            thread(const thread&) = delete;
            thread(thread&&) noexcept;
            thread& operator=(const thread&) = delete;
            thread& operator=(thread&&) noexcept;

            // 成员函数声明:
            void swap(thread&) noexcept;
            bool joinable() const noexcept;
            void join();
            void detach();
            id get_id() const noexcept;
            native_handle_type native_handle();
            
            // 静态成员函数声明:
            static unsigned hardware_concurrency() noexcept;
    };
}

std::thread 中主要声明三类函数：(1). 构造函数、拷贝构造函数及析构函数；(2). 成员函数；(3). 静态成员函数。另外， std::thread::id 表示线程 ID，同时 C++11 声明如下：

namespace std {
    class thread::id {
        public:
            id() noexcept;
    };

    bool operator==(thread::id x, thread::id y) noexcept;
    bool operator!=(thread::id x, thread::id y) noexcept;
    bool operator<(thread::id x, thread::id y) noexcept;
    bool operator<=(thread::id x, thread::id y) noexcept;
    bool operator>(thread::id x, thread::id y) noexcept;
    bool operator>=(thread::id x, thread::id y) noexcept;

    template<class charT, class traits>
    basic_ostream<charT, traits>&
        operator<< (basic_ostream<charT, traits>& out, thread::id id);


    // Hash 支持
    template <class T> struct hash;
    template <> struct hash<thread::id>;
}

2.3 `std::thread` 详解

2.3.1 `std::thread` 构造和赋值

2.3.1.1 std::thread 构造函数

默认构造函数 (1)	thread() noexcept;
初始化构造函数 (2)	template <class Fn, class... Args> explicit thread(Fn&& fn, Args&&... args);
拷贝构造函数 [deleted] (3)	thread(const thread&) = delete;
Move 构造函数 (4)	thread(thread&& x) noexcept;

默认构造函数(1)，创建一个空的 std::thread 执行对象。
初始化构造函数(2)，创建一个 std::thread 对象，该 std::thread 对象可被 joinable，新产生的线程会调用 fn 函数，该函数的参数由 args 给出。
拷贝构造函数(被禁用)(3)，意味着 std::thread 对象不可拷贝构造。
Move 构造函数(4)，move 构造函数(move 语义是 C++11 新出现的概念，详见附录)，调用成功之后 x 不代表任何 std::thread 执行对象。

注意：可被 joinable 的 std::thread 对象必须在他们销毁之前被主线程 join 或者将其设置为 detached.

In the destructor of std::thread, std::terminate is called if:

    #1 the thread was not joined (with t.join())
    #2 and was not detached either (with t.detach())

Thus, you should always either join or detach a thread before the flows of execution reaches the destructor.

std::thread 各种构造函数例子如下:

#include <iostream>
#include <utility>
#include <thread>
#include <chrono>
 
void f1(int n)
{
    for (int i = 0; i < 5; ++i) {
        std::cout << "Thread 1 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}
 
void f2(int& n)
{
    for (int i = 0; i < 5; ++i) {
        std::cout << "Thread 2 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}
 
class foo
{
public:
    void bar()
    {
        for (int i = 0; i < 5; ++i) {
            std::cout << "Thread 3 executing\n";
            ++n;
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
    }
    int n = 0;
};
 
class baz
{
public:
    void operator()()
    {
        for (int i = 0; i < 5; ++i) {
            std::cout << "Thread 4 executing\n";
            ++n;
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
    }
    int n = 0;
};
 
int main()
{
    int n = 0;
    foo f;
    baz b;
    std::thread t1; // t1 is not a thread
    std::thread t2(f1, n + 1); // pass by value
    std::thread t3(f2, std::ref(n)); // pass by reference
    std::thread t4(std::move(t3)); // t4 is now running f2(). t3 is no longer a thread
    std::thread t5(&foo::bar, &f); // t5 runs foo::bar() on object f
    std::thread t6(b); // t6 runs baz::operator() on object b
    t2.join();
    t4.join();
    t5.join();
    t6.join();
    std::cout << "Final value of n is " << n << '\n';
    std::cout << "Final value of foo::n is " << f.n << '\n';
}

2.3.1.2 std::thread 赋值操作

Move 赋值操作 (1)	thread& operator=(thread&& rhs) noexcept;
拷贝赋值操作 [deleted] (2)	thread& operator=(const thread&) = delete;

Move 赋值操作(1)，如果*this不可 joinable，需要传递一个右值引用(rhs)给 move 赋值操作；如果*this仍然有与之关联的running thread(i.e. joinable() == true)，则会调用 terminate() 报错。
拷贝赋值操作(2)，被禁用，因此 std::thread 对象不可拷贝赋值。

请看下面的例子：

#include <stdio.h>
#include <stdlib.h>

#include <chrono>    // std::chrono::seconds
#include <iostream>  // std::cout
#include <thread>    // std::thread, std::this_thread::sleep_for

void thread_task(int n) {
    std::this_thread::sleep_for(std::chrono::seconds(n));
    std::cout << "hello thread "
        << std::this_thread::get_id()
        << " paused " << n << " seconds" << std::endl;
}

int main(int argc, const char *argv[])
{
    std::thread threads[5];
    std::cout << "Spawning 5 threads...\n";
    for (int i = 0; i < 5; i++) {
        threads[i] = std::thread(thread_task, i + 1);
    }
    std::cout << "Done spawning threads! Now wait for them to join\n";
    for (auto& t: threads) {
        t.join();
    }
    std::cout << "All threads joined.\n";

    return EXIT_SUCCESS;
}

2.3.1.3 其他成员函数

get_id: 获取线程 ID，返回一个类型为 std::thread::id 的对象。请看下面例子:

  #include <iostream>
  #include <thread>
  #include <chrono>
   
  void foo()
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
   
  int main()
  {
      std::thread t1(foo);
      std::thread::id t1_id = t1.get_id();
   
      std::thread t2(foo);
      std::thread::id t2_id = t2.get_id();
   
      std::cout << "t1's id: " << t1_id << '\n';
      std::cout << "t2's id: " << t2_id << '\n';
   
      t1.join();
      t2.join();
  }

joinable: 检查线程是否可被 join。检查当前的线程对象是否表示了一个活动的执行线程，由默认构造函数创建的线程是不能被 join 的。另外，如果某个线程已经执行完任务，但是没有被 join 的话，该线程依然会被认为是一个活动的执行线程，因此也是可以被 join 的。

  #include <iostream>
  #include <thread>
  #include <chrono>
   
  void foo()
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
   
  int main()
  {
      std::thread t;
      std::cout << "before starting, joinable: " << t.joinable() << '\n';
   
      t = std::thread(foo);
      std::cout << "after starting, joinable: " << t.joinable() << '\n';
   
      t.join();
  }

join: join 线程，调用该函数会阻塞当前线程，直到由 *this 所标示的线程执行完毕 join 才返回。

 #include <iostream>
  #include <thread>
  #include <chrono>
   
  void foo()
  {
      // simulate expensive operation
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
   
  void bar()
  {
      // simulate expensive operation
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
   
  int main()
  {
      std::cout << "starting first helper...\n";
      std::thread helper1(foo);
   
      std::cout << "starting second helper...\n";
      std::thread helper2(bar);
   
      std::cout << "waiting for helpers to finish..." << std::endl;
      helper1.join();
      helper2.join();
   
      std::cout << "done!\n";
  }

detach: Detach 线程。将当前线程对象所代表的执行实例与该线程对象分离，使得线程的执行可以单独进行。一旦线程执行完毕，它所分配的资源将会被释放。

调用 detach 函数之后：

*this 不再代表任何的线程执行实例。
joinable() == false
get_id() == std::thread::id()

另外，如果出错或者 joinable() == false，则会抛出 std::system_error.

    #include <iostream>
    #include <chrono>
    #include <thread>
     
    void independentThread() 
    {
        std::cout << "Starting concurrent thread.\n";
        std::this_thread::sleep_for(std::chrono::seconds(2));
        std::cout << "Exiting concurrent thread.\n";
    }
     
    void threadCaller() 
    {
        std::cout << "Starting thread caller.\n";
        std::thread t(independentThread);
        t.detach();
        std::this_thread::sleep_for(std::chrono::seconds(1));
        std::cout << "Exiting thread caller.\n";
    }
     
    int main() 
    {
        threadCaller();
        std::this_thread::sleep_for(std::chrono::seconds(5));
    }

swap: Swap 线程，交换两个线程对象所代表的底层句柄(underlying handles)。

  #include <iostream>
  #include <thread>
  #include <chrono>
   
  void foo()
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
   
  void bar()
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
  }
   
  int main()
  {
      std::thread t1(foo);
      std::thread t2(bar);
   
      std::cout << "thread 1 id: " << t1.get_id() << std::endl;
      std::cout << "thread 2 id: " << t2.get_id() << std::endl;
   
      std::swap(t1, t2);
   
      std::cout << "after std::swap(t1, t2):" << std::endl;
      std::cout << "thread 1 id: " << t1.get_id() << std::endl;
      std::cout << "thread 2 id: " << t2.get_id() << std::endl;
   
      t1.swap(t2);
   
      std::cout << "after t1.swap(t2):" << std::endl;
      std::cout << "thread 1 id: " << t1.get_id() << std::endl;
      std::cout << "thread 2 id: " << t2.get_id() << std::endl;
   
      t1.join();
      t2.join();
  }

native_handle: 返回 native handle（由于 std::thread 的实现和操作系统相关，因此该函数返回与 std::thread 具体实现相关的线程句柄，例如在符合 Posix 标准的平台下(如 Unix/Linux)是 Pthread 库）。
hardware_concurrency [static]: 检测硬件并发特性，返回当前平台的线程实现所支持的线程并发数目，但返回值仅仅只作为系统提示(hint)。

  #include <iostream>
  #include <thread>
   
  int main() {
      unsigned int n = std::thread::hardware_concurrency();
      std::cout << n << " concurrent threads are supported.\n";
  }

2.3.1.4 std::this_thread 命名空间中相关辅助函数介绍

get_id: 获取线程 ID。
yield: 当前线程放弃执行，操作系统调度另一线程继续执行。

  #include <iostream>
  #include <chrono>
  #include <thread>
   
  // "busy sleep" while suggesting that other threads run 
  // for a small amount of time
  void little_sleep(std::chrono::microseconds us)
  {
      auto start = std::chrono::high_resolution_clock::now();
      auto end = start + us;
      do {
          std::this_thread::yield();
      } while (std::chrono::high_resolution_clock::now() < end);
  }
   
  int main()
  {
      auto start = std::chrono::high_resolution_clock::now();
   
      little_sleep(std::chrono::microseconds(100));
   
      auto elapsed = std::chrono::high_resolution_clock::now() - start;
      std::cout << "waited for "
                << std::chrono::duration_cast<std::chrono::microseconds>(elapsed).count()
                << " microseconds\n";
  }

sleep_until: 线程休眠至某个指定的时刻(time point)，该线程才被重新唤醒。

  template< class Clock, class Duration >
  void sleep_until( const std::chrono::time_point<Clock,Duration>& sleep_time );

sleep_for: 线程休眠某个指定的时间片(time span)，该线程才被重新唤醒，不过由于线程调度等原因，实际休眠时间可能比 sleep_duration 所表示的时间片更长。

  template< class Rep, class Period >
  void sleep_for( const std::chrono::duration<Rep,Period>& sleep_duration );

  #include <iostream>
  #include <chrono>
  #include <thread>
   
  int main()
  {
      std::cout << "Hello waiter" << std::endl;
      std::chrono::milliseconds dura( 2000 );
      std::this_thread::sleep_for( dura );
      std::cout << "Waited 2000 ms\n";
  }