多态性的一个典型的例子

#include <iostream>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */
using namespace std;
class Point
{
    public:
        Point(float x=0,float y=0);
        void setPoint(float,float);
        float getX()const{return x;}
        float getY()const{return y;}
        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<<"]"<<endl;
    return output;
}
class Circle:public Point
{
    public:
        Circle(float x=0,float y=0,float r=0);
        void setRadius(float);
        float getRadius()const;
        float area()const;
        friend ostream &operator<<(ostream&,const Circle&);
    private:
        float radius;
};

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<<"Center=["<<c.x<<","<<c.y<<"],r="<<c.radius<<",area="<<c.area()<<endl;
    return output;
}
int main(int argc, char** argv) {
    Circle c(3.5,6.4,5.2);
    cout<<"original circle:
x="<<c.getX()<<",y="<<c.getY()<<",r="<<c.getRadius()<<",area="<<c.area()<<endl;
    c.setRadius(7.5);
    c.setPoint(5,5);
    cout<<"new circle:
"<<c;
    Point &pRef=c;
    cout <<"pRef:"<<pRef;
    return 0;
}