2. 继承

#include  <iostream>
using  namespace  std;

class  Shape
{
public:
        Shape()  {}
        ~Shape()  {}
        virtual  float  GetArea()  {  return  -1;  }
};

class  Circle  :  public  Shape
{
public : 
    Circle(float r):r1(r) { }
    ~Circle() {}
    virtual float GetArea() { return 3.14 * r1 * r1; }
private : 
    float r1;

};

class  Rectangle  :  public  Shape
{
public : 
    Rectangle (float len ,float width):len (len) , width (width) {}
    ~Rectangle() { } 
    virtual float GetArea() { return len * width;  }
private : 
    float len, width; 

};

class  Square  :  public  Rectangle
{
public:
        Square(float  len);
        ~Square()  {}
};

Square::Square(float  len)  :Rectangle(len,  len)
{
}

int  main()
{
        Shape*  sp;

        sp  =  new  Circle(5);
        cout  <<  "The  area  of  the  Circle  is  "  <<  sp->GetArea()  <<  endl;
        delete  sp;
        sp  =  new  Rectangle(4,  6);
        cout  <<  "The  area  of  the  Rectangle  is  "  <<  sp->GetArea()  <<  endl;
        delete  sp;
        sp  =  new  Square(5);
        cout  <<  "The  area  of  the  Square  is  "  <<  sp->GetArea()  <<  endl;
        delete  sp;
}