C++语言基础 例程 抽象类

贺老师的教学链接  本课讲解

实例：顶层的Shape作为抽象类

#include <iostream>using namespace std;//声明抽象基类Shapeclass 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;}