11.4.2

#include <iostream>using namespace std;class Point{    public:    Point(double xx=0,double yy=0):x(xx),y(yy){}    friend ostream&operator<<(ostream &,const Point &);    protected:    double x,y;};ostream&operator<<(ostream &output,const Point &p){    output<<"("<<p.x<<","<<p.y<<")"<<endl;    return output;}class Circle:public Point{    public:    Circle(double xx=0,double yy=0,double r=0):Point(xx,yy),R(r){}    double area();    friend ostream&operator<<(ostream&,const Circle &);    protected:    double R;};ostream&operator<<(ostream&output,const Circle &c){    output<<"center:"<<endl;    cout<<"("<<c.x<<","<<c.y<<")"<<" "<<"r:"<<c.R<<endl;    return output;}double Circle::area(){    double s;    return s=3.14*R*R;}int main(){    Circle c(2,3,4);    cout<<c;    cout<<"圆的面积为:"<<c.area();    return 0;}