成员函数、友元函数和一般函数有区别

#include <iostream>#include <cmath>using namespace std;class Cpoint{private:double x;//横坐标double y;//纵坐标public:Cpoint(double xx=0,double yy=0):x(xx),y(yy){}void distance1(Cpoint &c);friend void distance2(Cpoint &a,Cpoint &b);double getX(){return x;}double getY(){return y;}};void Cpoint::distance1(Cpoint &c)//成员函数{double d;d=sqrt((x-c.x)*(x-c.x)+(y-c.y)+(y-c.y));cout<<d<<endl;}void distance2(Cpoint &a,Cpoint &b)//友元函数{double d;d=sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)+(a.y-b.y));cout<<d<<endl;}void distance3(Cpoint &a,Cpoint &b)//一般函数{double d;d=sqrt((a.getX()-b.getX())*(a.getX()-b.getX())+(a.getY()-b.getY())+(a.getY()-b.getY()));cout<<d<<endl;}int main(){Cpoint a(2,3),b(-4,5);a.distance1(b);distance2(a,b);distance3(a,b);return 0;}