13.2

/*（1）先建立一个Point(点)类，包含数据成员x,y(坐标点)；（2）以Point为基类，派生出一个Circle(圆)类，增加数据成员(半径)，基类的成员表示圆心；（3）编写上述两类中的构造、析构函数及必要运算符重载函数（本项目主要是输入输出）；（4）定义友元函数int locate，判断点p与圆的位置关系（返回值<0圆内，==0圆上，>0 圆外）；    在项目1的基础上，在圆类上重载关系运算符（6种），使之能够按圆的面积比较两个圆的大小。    自编main函数完成测试。*/#include <iostream>#include <cmath>using namespace std;class Point{    public:    Point(double xx,double yy):x(xx),y(yy){}    protected:    double x;    double y;};class Circle:public Point{    public:    Circle(double xx,double yy,double R):Point(xx,yy),r(R){}    friend ostream &operator<<(ostream &,Circle &);    double area();    bool operator>(const Circle &);    bool operator<(const Circle &);    bool operator>=(const Circle &);    bool operator<=(const Circle &);    bool operator==(const Circle &);    bool operator!=(const Circle &);    protected:    double r;};ostream &operator<<(ostream &output,Circle &c){    output<<"为（"<<c.x<<", "<<c.y<<"）, r="<<c.r<<" ";    return output;}double Circle::area(){    double s;    s=3*r*r;    return s;}bool Circle::operator>(const Circle &c){    return (r-c.r)>0;}bool Circle::operator<(const Circle &c){    return (r-c.r)<0;}bool Circle::operator<=(const Circle &c){    return !(r>c.r);}bool Circle::operator>=(const Circle &c){    return !(r<c.r);}bool Circle::operator==(const Circle &c){    return (c.r-r)==0;}bool Circle::operator!=(const Circle &c){    return (c.r-r)!=0;}int main(){    Circle c1(3,2,4),c2(4,5,5);    cout<<"圆c1"<<c1<<"面积为"<<c1.area()<<endl;    cout<<"圆c2"<<c2<<"面积是 "<<c2.area()<<endl;    cout<<"圆c1 ";    if(c1>c2) cout<<"大于, ";    if(c1<c2) cout<<"小于, ";    if(c1>=c2) cout<<"大于等于, ";    if(c1<=c2) cout<<"小于等于, ";    if(c1==c2) cout<<"等于, ";    if(c1!=c2) cout<<"不等于, ";    cout<<"圆c2"<<endl;    return 0;}