#include<iostream>

using namespace std;

//抽象类
class Shape
{
public:
    virtual double area() = 0;    //纯虚函数
};

//矩形
class Rect : public Shape
{
private:
    int ma;
    int mb;
public:
    Rect(int a, int b)
    {
        ma = a;
        mb = b;
    }
    double area()
    {
        return ma*mb;
    }
};

//圆
class Circle : public Shape
{
private:
    int mr;
public:
    Circle(int r)
    {
        mr = r;
    }
    double area()
    {
        return 3.14*mr*mr;
    }
};

void area(Shape* p)//但是可以定义指针，指向的是子类对象
{
    double r = p->area();//子类中重写后成为虚函数，展现多态
    
    cout << "r = " << r << endl;
}

int main()
{
    Rect rect(4, 5);
    Circle circle(10);
  
    //Shape s;//报错，抽象类不能产生对象
    
    area(&rect);//20
    area(&circle);//314
    return 0;
}

#include <iostream>
#include <string>

using namespace std;

//类中无成员变量
//特殊的抽象类
class Channel
{
public://成员函数均为public
    virtual bool open() = 0;//成员函数均为虚函数
    virtual void close() = 0;
    virtual bool send(char* buf, int len) = 0;
    virtual int receive(char* buf, int len) = 0;
};

int main()
{
    return 0;
}