成员函数 友元函数 和 一般函数的区别

#include <iostream>#include <cmath>using namespace std;class CPoint{private:    double x;    double y;public:     CPoint(double xx=0,double yy=0):x(xx),y(yy){}     double getx(){return x;}     double gety(){return y;}     double distance(CPoint p1);     friend double  distance1(CPoint p1,CPoint  p2);};double CPoint::distance(CPoint p1){    double d;    d=sqrt((p1.x-x)*(p1.x-x)+(p1.y-y)*(p1.y-y));     return d;}double  distance1(CPoint p1,CPoint p2){   double d;   d=sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y));   return d;}double  distance2(CPoint p1,CPoint p2){   double d;   d=sqrt((p1.getx()-p2.getx())*(p1.getx()-p2.getx())+(p1.gety()-p2.gety())*(p1.gety()-p2.gety()));   return d;}int main(){    CPoint p1(0,1),p2(0,2);    cout<<p1.distance(p2)<<endl;    cout<<distance1(p1,p2)<<endl;    cout<<distance2(p1,p2)<<endl;    return 0;}

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