程序代码：

形状（一）
    编写C++程序完成以下功能：
声明一个基类Shape（形状），其中包含一个方法来计算面积；
从Shape派生两个类矩形和圆形；
从矩形派生正方形；
分别实现派生类构造函数、析构函数和其他方法；
创建派生类的对象，观察构造函数、析构函数调用次序；
不同对象计算面积。


#include<iostream>
using namespace std;
const double Pi=3.14;
class shape
{
   public:
      shape(){cout<<"shape constructor called"<<endl;}
      double Getarea(double x,double y){return area=x*y;}
      ~shape(){cout<<"shape destructor called"<<endl;}
   private:
      double area;
};
class rectangle:public shape
{
   public:
       rectangle();
       double rect(double X,double Y){return Getarea(X,Y);cout<<"rectangle constructor called"<<endl;}
        ~rectangle(){cout<<"rectangle destructor called"<<endl;}
   
};
class circle:public shape
{  public:
      circle();
      double cir_area(double r){cout<<"circle constructor called"<<endl;return  Pi*Getarea(r,r);}
      ~circle(){cout<<"circle destructor called"<<endl;}
};
class square:public rectangle
{
   public:
       square();
       double squa(double X){cout<<"square constructor called"<<endl;return Getarea(X,X);}
       ~square(){cout<<"aquare destructor called"<<endl;}
};
int main()
{   double l,w,r,x;
   
    cout<<"请输入长方形的长与宽"<<endl;
    cin>>l>>w;
    rectangle rec();
    cout<<"the area of rec is:"<<rec.rect(l,w)<<endl;
    cout<<"请输入圆的半径"<<endl;
    cin>>r;
    circle cir( );
    cout<<"the area of cir is:"<<cir.cir_area(r)<<endl;
    cout<<"请输入正方形的边长"<<endl;
    cin>>x;
    square squ( );
    cout<<"the area of squ is:"<<squ.squa(x)<<endl;
    return 0;
}