c++虚函数例子解析

#include <iostream>
using namespace std;
class Shape
{
public:
    //普通函数前加virtual，普通函数变成了虚函数，=0表示抽象虚函数，子类必须实现该函数
    virtual void printarea()=0;
    void hello(){cout<<"Shape hello function"<<endl;};
};
class Circle:public Shape
{
public:
    Circle(int a):r(a){}
    void hello(){cout<<"Circle hello function"<<endl;};
    void printarea(){cout<<"圆形面积为："<<r*r*3.14<<endl;}
private:
    int r;
};
class Rectangle:public Shape
{
public:
    Rectangle(int a,int b):x(a),y(b){}
    void hello(){cout<<"Rectangle hello function"<<endl;};
    void printarea(){cout<<"矩形面积为："<<x*y<<endl;}
private:
    int x;
    int y;
};
class Square:public Shape
{
public:
    Square(int a):l(a){}
    void hello(){cout<<"Square hello function"<<endl;};

    void printarea(){cout<<"正方形面积为："<<l*l<<endl;}
private:
    int l;
};

void prints(Shape *s) {
　　//基类指针s指向子类对象
   //s调用的函数printarea是虚函数，所以会调用子类的printarea
    s->printarea();
   //s调用的函数hello不是虚函数，所以只会调用基类的hello
    s->hello();
}

int main()
{
    Circle a(1);
    prints(&a);
    Square b(2);
    prints(&b);
    Rectangle c(1,2);
    prints(&c);
    return 0;
}

输出：

圆形面积为：3.14
Shape hello function
正方形面积为：4
Shape hello function
矩形面积为：2
Shape hello function