深入解析C++编程中的纯虚函数和抽象类

  virtual 函数类型 函数名 (参数表列) = 0;

#include <iostream>
using namespace std;
//声明抽象基类Shape
class Shape
{
public:
  virtual float area( )const {return 0.0;} //虚函数
  virtual float volume()const {return 0.0;} //虚函数
  virtual void shapeName()const =0; //纯虚函数
};

//声明Point类
class Point:public Shape//Point是Shape的公用派生类
{
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:";}//对虚函数进行再定义
  friend ostream & operator <<(ostream &,const Point &);
protected:
  float x,y;
};
//定义Point类成员函数
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;
}

//声明Circle类
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:";}//对虚函数进行再定义
  friend ostream &operator <<(ostream &,const Circle &);
protected:
  float radius;
};
//声明Circle类成员函数
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;
}

//声明Cylinder类
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:";
  }//对虚函数进行再定义
  friend ostream& operator <<(ostream&,const Cylinder&);
protected:
  float height;
};
//定义Cylinder类成员函数
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;
}

//main函数
int main( )
{
  Point point(3.2,4.5); //建立Point类对象point
  Circle circle(2.4,1.2,5.6);
  //建立Circle类对象circle
  Cylinder cylinder(3.5,6.4,5.2,10.5);
  //建立Cylinder类对象cylinder
  point.shapeName();
  //静态关联
  cout<<point<<endl;
  circle.shapeName(); //静态关联
  cout<<circle<<endl;
  cylinder.shapeName(); //静态关联
  cout<<cylinder<<endl<<endl;
  Shape *pt; //定义基类指针
  pt=&point; //指针指向Point类对象
  pt->shapeName( ); //动态关联
  cout<<"x="<<point.getX( )<<",y="<<point.getY( )<<"\narea="<<pt->area( )
   <<"\nvolume="<<pt->volume()<<"\n\n";
  pt=&circle; //指针指向Circle类对象
  pt->shapeName( ); //动态关联
  cout<<"x="<<circle.getX( )<<",y="<<circle.getY( )<<"\narea="<<pt->area( )
   <<"\nvolume="<<pt->volume( )<<"\n\n";
  pt=&cylinder; //指针指向Cylinder类对象
  pt->shapeName( ); //动态关联
  cout<<"x="<<cylinder.getX( )<<",y="<<cylinder.getY( )<<"\narea="<<pt->area( )
   <<"\nvolume="<<pt->volume( )<<"\n\n";
  return 0;
}

Point:[3.2,4.5](Point类对象point的数据:点的坐标)
Circle:[2.4,1.2], r=5.6 (Circle类对象circle的数据:圆心和半径)
Cylinder:[3.5,6.4], r=5.5, h=10.5 (Cylinder类对象cylinder的数据： 圆心、半径和高)

Point:x=3.2,y=4.5 (输出Point类对象point的数据:点的坐标)
area=0 (点的面积)
volume=0 (点的体积)

Circle:x=2.4,y=1.2 (输出Circle类对象circle的数据:圆心坐标)
area=98.5203 (圆的面积)
volume=0 (圆的体积)
Cylinder:x=3.5,y=6.4 (输出Cylinder类对象cylinder的数据:圆心坐标)
area=512.595 (圆的面积)
volume=891.96 (圆柱的体积)