例12.4 虛函數和抽象基類的應用。
在本章例12.1介紹了以Point為基類的點—圓—圓柱體類的層次結構。現在要對它進行改寫，在程序中使用虛函數和抽象基類。類的層次結構的頂層是抽象基類Shape(形狀)。Point(點), Circle(圓), Cylinder(圓柱體)都是Shape類的直接派生類和間接派生類。
下面是一個完整的程序，為了便于閱讀，分段插入了一些文字說明。
程序如下: 
第(1)部分
#include <iostream>
using namespace std;
//聲明抽象基類Shape
class Shape
{public:
 virtual float area( ) const {return 0.0;}//虛函數
 virtual float volume() const {return 0.0;}      //虛函數
 virtual void shapeName() const =0;              //純虛函數
};
第(2)部分
//聲明Point類
class Point:public Shape//Point是Shape的公用派生類
{public:
  Point(float=0,float=0);
  void setPoint(float,float);
  float getX( ) const {return x;}
  float getY( ) const {return y;}
  virtual void shapeName( ) const {cout<<″Point:″;}      //對虛函數進行再定義
  friend ostream & operator<<(ostream &,const Point &);
protected:
  float x,y;
};
//定義Point類成員函數
Point::Point(float a,float b)
{x=a;y=b;}

void Point::setPoint(float a,float b)
{x=a;y=b;}

ostream & operator<<(ostream &output,const Point &p)
{output<<″[″<<p.x<<″,″<<p.y<<″]″;
return output;
}
第(3)部分
//聲明Circle類
class Circle:public Point
{public:
  Circle(float x=0,float y=0,float r=0);
  void setRadius(float);
  float getRadius( ) const;
  virtual float area( ) const;
  virtual void shapeName( ) const {cout<<″Circle:″;}//對虛函數進行再定義
  friend ostream &operator<<(ostream &,const Circle &);
protected:
  float radius;
};
//聲明Circle類成員函數
Circle::Circle(float a,float b,float r):Point(a,b),radius(r){ }

void Circle::setRadius(float r):radius(r){ }

float Circle::getRadius( ) const {return radius;}

float Circle::area( ) const {return 3.14159*radius*radius;}

ostream &operator<<(ostream &output,const Circle &c)
{output<<″[″<<c.x<<″,″<<c.y<<″], r=″<<c.radius;
 return output;
}
第(4)部分
//聲明Cylinder類
class Cylinder:public Circle
{public:
  Cylinder (float x=0,float y=0,float r=0,float h=0);
  void setHeight(float);
  virtual float area( ) const;
  virtual float volume( ) const;
  virtual void shapeName( ) const {cout<<″Cylinder:″;}//對虛函數進行再定義
  friend ostream& operator<<(ostream&,const Cylinder&);
 protected:
  float height;
};
//定義Cylinder類成員函數
Cylinder::Cylinder(float a,float b,float r,float h)
    :Circle(a,b,r),height(h){ }

void Cylinder::setHeight(float h){height=h;}

float Cylinder::area( ) const
{ return 2*Circle::area( )+2*3.14159*radius*height;}

float Cylinder::volume( ) const
{return Circle::area( )*height;}

ostream &operator<<(ostream &output,const Cylinder& cy)
{output<<″[″<<cy.x<<″,″<<cy.y<<″], r=″<<cy.radius<<″, h=″<<cy.height;
return output;
}
第(5)部分
//main函數
int main( )
{Point point(3.2,4.5);//建立Point類對象point
 Circle circle(2.4,1.2,5.6);                     //建立Circle類對象circle
 Cylinder cylinder(3.5,6.4,5.2,10.5);            //建立Cylinder類對象cylinder
 point.shapeName();                              //靜態關聯
 cout<<point<<endl;

 circle.shapeName();                             //靜態關聯
 cout<<circle<<endl;

 cylinder.shapeName();                           //靜態關聯
 cout<<cylinder<<endl<<endl;

 Shape *pt;                                      //定義基類指針

 pt=&point;                                      //指針指向Point類對象
 pt->shapeName( );                                //動態關聯
 cout<<″x=″<<point.getX( )<<″,y=″<<point.getY( )<<″\\narea=″<<pt->area( )
     <<″\\nvolume=″<<pt->volume()<<″\\n\\n″;

 pt=&circle;                                     //指針指向Circle類對象
 pt->shapeName( );                                //動態關聯
 cout<<″x=″<<circle.getX( )<<″,y=″<<circle.getY( )<<″\\narea=″<<pt->area( )
     <<″\\nvolume=″<<pt->volume( )<<″\\n\\n″;

 pt=&cylinder;                                   //指針指向Cylinder類對象
 pt->shapeName( );                                //動態關聯
 cout<<″x=″<<cylinder.getX( )<<″,y=″<<cylinder.getY( )<<″\\narea=″<<pt->area( )
     <<″\\nvolume=″<<pt->volume( )<<″\\n\\n″;
 return 0;
}