（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）；

#include <iostream>
#include <cmath>
using namespace std;
class Point
{
public:Point():x(0),y(0){}Point(double a,double b):x(a),y(b){}double distance(const Point &p)const;friend ostream & operator<<(ostream &,const Point &);
protected:double x;double y;
};
double Point::distance(const Point &p) const
{double dx = x-p.x;double dy = y-p.y;return sqrt(dx*dx+dy*dy);
}
ostream & operator<<(ostream &output,const Point &p)
{output<<"["<<p.x<<","<<p.y<<"]"<<endl;return output;
}class Circle:public Point
{
public:Circle():Point(),radius(1){};Circle(double a,double b,double r):Point(a,b),radius(r){}friend ostream &operator<<(ostream &,const Circle &);friend int locate(const Point &p, const Circle &c); //判断点p在圆上、圆内或圆外，返回值：<0圆内，==0圆上，>0 圆外
protected:double radius;
};
ostream &operator<<(ostream &output,const Circle &c)
{output<<"Center=["<<c.x<<", "<<c.y<<"], r="<<c.radius<<endl;return output;
}
int locate(const Point &p, const Circle &c)
{const Point cp(c.x,c.y); //圆心double d = cp.distance(p);if (abs(d - c.radius) < 1e-7)return 0;  //相等else if (d < c.radius)return -1;  //圆内elsereturn 1;  //圆外
}
int main( )
{Circle c1(3,2,4),c2(4,5,5);      //c2应该大于c1Point p1(1,1),p2(3,-2),p3(7,3);  //分别位于c1内、上、外cout<<"圆c1: "<<c1;cout<<"点p1: "<<p1;cout<<"点p1在圆c1之"<<((locate(p1, c1)>0)?"外":((locate(p1, c1)<0)?"内":"上"))<<endl;cout<<"点p2: "<<p2;cout<<"点p2在圆c1之"<<((locate(p2, c1)>0)?"外":((locate(p2, c1)<0)?"内":"上"))<<endl;cout<<"点p3: "<<p3;cout<<"点p3在圆c1之"<<((locate(p3, c1)>0)?"外":((locate(p3, c1)<0)?"内":"上"))<<endl;return 0;
}

运行结果：