成员函数友元函数和一般函数有区别

////  main.cpp//  project////  Created by 徐伟 on 6/14/16.//  Copyright © 2016 fizz_i. All rights reserved.//#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) {}    double distance1(CPoint &); //参数用const CPoint &更佳    friend double distance2(CPoint &, CPoint &) ;    double getX()    {        return x;    }    double getY()    {        return y;    }};double distance3(CPoint &,CPoint &) ;double CPoint::distance1(CPoint &p){    double dx=(x-p.x), dy=y-p.y;    return sqrt(dx*dx+dy*dy);}double distance2(CPoint &p1,CPoint &p2){    double dx=(p1.x-p2.x), dy=p1.y-p2.y;    return sqrt(dx*dx+dy*dy);}double distance3(CPoint &p1,CPoint &p2){    double dx=(p1.getX()-p2.getX()), dy=p1.getY()-p2.getY();    return sqrt(dx*dx+dy*dy);}int main(){    CPoint p1(10,13), p2(-5,6);    cout<<"1. "<<p1.distance1(p2)<<endl;    cout<<"2. "<<distance2(p1,p2)<<endl;    cout<<"3. "<<distance3(p1,p2)<<endl;    return 0;}