C++虚函数需要注意的地方在基类上想要继承数据，数据类型不可以定义为私有类（private）,要定义成保护类（protected）。这样就可以继承。虚函数关键词virtual。

  看代码：

#include <iostream>
#include <string>

using namespace std;

class student
{
public:
	student(string str,int n,float s):name(str),age(n),score(s){}
	~student(){}
	virtual void display()
	{
		cout << "name = " << name << endl;
		cout << "age  = " << age << endl;
		cout << "score  = " << score << endl;
	}

protected:
	string name;
	int age;
	float score;

};

class grade :public student
{
public:
	grade(string str, int n, float s, float m) :student(str, n, s), wage(m) {  }
	void display()
	{
		cout << "name = " << name << endl;
		cout << "age  = " << age << endl;
		cout << "score  = " << score << endl;
		cout << "wage = " << wage << endl;
		
	}

protected:
	float wage;

};
int main()
{
	student stu("Li",18,98);
	grade g("Lao Wang",20,100,80);
	
	student *pt = &g;
	pt->display();
	pt = &stu;
	pt->display();



	
	return 0;
}

其中使用student类申明指针，然后赋值grade类的指针就可以转到grade类中，必须使用虚函数才可以。

其中重载是横向重载，虚函数是纵向重载。

虚析构函数，

一般把基类的析构函数设置为虚函数，这样有利于撤销动态分配空间得到正确的处理。

纯虚函数

纯虚函数的写法：virtual float area（） = 0；

virtual 函数类型 函数名称 （参数列表） = 0；

纯虚函数在派生类中应用的时候必须带有virtual 例如；

在派生类中： virtual float area （） {cout << "Point:";}

一个比较完成的例子：

#include <iostream>

using namespace std;

class Shape
{
public:
	Shape() {}
	~Shape() { cout << "Shape  " << endl; }
	virtual float area() const { return 0.0; }
	virtual float volume() const { return 0.0; }
	virtual void shapeName() const = 0;

};

class Point :public Shape
{
public:
	Point(float = 0,float = 0);
	~Point() { cout << "Point" << endl; }
	void setPoint(float,float);
	float getX()const { return x; }
	float getY()const { return y; }
	virtual void shapeName()const { cout << "Point:" << endl; };
	friend ostream & operator << (ostream &,const Point &);

protected:
	float x, y;
};

Point::Point(float a, float b)
{
	x = a;
	y = b;
}

void Point::setPoint(float a, float b)
{
	x = a; y = b;
}
ostream & operator << (ostream & output, const Point &p)
{
	output << "[" << p.x << "," << p.y << "]";
	return output;
}

class Circle :public Point
{
public:
	Circle(float x = 0,float y = 0, float r =0);
	void setRadius(float);
	float getRadius()const;
	virtual float area()const;
	virtual void shapeName()const { cout << "Circle:" << endl; };
	friend ostream & operator << (ostream &, const Circle &);
protected:
	float radius;

};

Circle::Circle(float a, float b, float r):Point(a, b), radius(r) {}
void Circle::setRadius(float r) { radius = r; }
float Circle::getRadius()const { return radius; }
float Circle::area()const { return 3.1415926*radius*radius; }
ostream & operator << (ostream & output, const Circle &c)
{
	output << "[" << c.x << "," << c.y << "]" << "r = " << c.radius;
	return output;
}

class Cylinder :public Circle
{
public:
	Cylinder(float x = 0, float y = 0,float r = 0,float h = 0);
	void setHeight(float);
	float getHeight()const { return height; }
	virtual float area() const;
	virtual float volume()const;
	virtual void shapeName()const { cout << "Cylinder:" << endl; };
	friend ostream & operator << (ostream &, const Cylinder &);
protected:
	float height;
};

Cylinder::Cylinder(float a, float b, float r, float h) :Circle(a, b, r), height(h) {}
void  Cylinder::setHeight(float h) { height = h; }
float Cylinder::area() const
{
	return 2 * Circle::area() + 2 * 3.1415926 * radius * radius;
}
float Cylinder::volume()const
{
	return Circle::area() * height;
}

ostream & operator << (ostream & output, const Cylinder &y)
{
	output << "[" << y.x<< "," << y.y << "]" << "r = " << y.radius  << ", h = " << y.height;
	return output;
}

int main()
{
	Point p(3.2,4.5);
	Circle c(2.4,1.2,5.6);
	Cylinder y(3.5,6.4,5.2,10.5);
	p.shapeName();
	cout << p << endl;
	c.shapeName();
	cout << c << endl;
	y.shapeName();
	cout << y << endl;
	Shape *pt;
	pt = &p;
	pt->shapeName();
	cout << " x = " << p.getX() << " y = " << p.getY() << "\narea" << pt->area() << "\nvolume = " << pt->volume() << "\n\n";
	pt = &c;
	pt->shapeName();
	cout << " x = " << p.getX() << " y = " << p.getY() << "\narea" << pt->area() << "\nvolume = " << pt->volume() << "\n\n";
	pt = &y;
	pt->shapeName();
	cout << " x = " << p.getX() << " y = " << p.getY() << "\narea" << pt->area() << "\nvolume = " << pt->volume() << "\n\n";
	return 0;
}