包含一個及一個以上的純虛函數的類就是抽象基類，抽象基類沒有也不能必要定義對象。

**#include <iostream> using namespace std; 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; //聲明一個純虛構函數//說明存在純虛構函數的基類不能夠定義對象，多以只能為抽象類 }; /////////////////////////////////////////////////////////////////////////////////////////////////// class Point :public Shape { public:Point(float=0,float=0); //定義構造函數void setPoint(float,float);float getX() const {return x;}float getY() const {return y;}virtual void shapeName() const{cout<<"Point";} //對純虛函數進行重新定義，以便實現函數的多態性friend ostream & operator<<(ostream &,const Point &); protected:float x,y;}; 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; } class Circle:public Point { public:Circle(float x=0,float y=0,float r=0); //聲明構造函數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 &); //重定義，第二個參數是對Circle常數據成員的引用 protected:float radius;}; Circle::Circle(float x, float y, float r):Point(x,y),radius(r) {} //函數初始化列表對函數進行初始化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; } //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<<"Cylinde:";} //對虛函數進行重定義friend ostream& operator<<(ostream&,const Cylinder &);protected:float height;}; 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;} int main() {Point point(3.2,4.5);Circle circle(2.4,1.2,5.6);Cylinder cylinder(3.5,6.4,5.2,10.5);point.shapeName();cout <<point<<endl;circle.shapeName();cout<<cylinder<<endl<<endl;cylinder.shapeName();cout<<cylinder<<endl<<endl;Shape *pt;pt=&point;pt->shapeName();cout<<"x="<<point.getX()<<"y="<<point.getY()<<"\narea"<<pt->area()<<"\nvolume="<<pt->volume()<<"\n\n";pt=&circle;pt->shapeName();cout<<"x="<<circle.getX()<<",y="<<circle.getY()<<"\narea="<<pt->area()<<"\nvolume="<<pt->volume()<<"\n\n";pt=&cylinder;cout<<"x="<<cylinder.getX()<<",y="<<cylinder.getY()<<"\narea="<<pt->area()<<"\nvolume="<<pt->volume()<<"\n\n";return 0; } **

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