定义一个基类shape，在shape 的基础上派生出 rectangle 和circle ……

#include <bits/stdc++.h>
using namespace std;
const double pi=3.14159;
class Shape{
public:
    void initshape()
    {
        area=0;
    }
    double getArea()
    {
        return area;
    }
protected:
    double area;
};
class Rectangle:public Shape
{
public:
    void initRectangle(int x,int y)
    {
        initshape();
        a=x;
        b=y;
    }
    void setarea()
    {
        area=a*b;
    }
protected:
    int a;
    int b;
};
class Circle:public Shape
{
public:
    void initCircle(double ridus)
    {
        initshape();
        r=ridus;
    }
    void setarea()
    {
        area=r*r*pi;
    }
protected:
    double r;
};

class Square:public Rectangle
{
public:
    void initSquare(int a)
    {
        initshape();
        initRectangle(a,a);
    }
    void setarea()
    {
        area=a*b;
    }
};
int main()
{
    Square t;
    t.initSquare(3);
    t.setarea();
    cout<<t.getArea()<<endl;
    Circle m;
    m.initCircle(5);
    m.setarea();
    cout<<m.getArea()<<endl;
    return 0;
}