成员函数、友元函数和一般函数有区别-两点间的距离

/*      
 *Copyright (c) 2016
 *All rights reserved.
 *文件名称：test.cpp
 *作    者：史红浩      
 *完成日期：2016年 5 月 9 日      
 *版 本 号：v1.0          
*/  
#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()
{
	int n1,n2,m1,m2;  
    cout<<"请输入点p1:";  
    cin>>n1>>n2;  
    cout<<"请输入点p2:";  
    cin>>m1>>m2;  

    CPoint p1(n1,n2), p2(m1,m2);
    cout<<"1. "<<p1.distance1(p2)<<endl;
    cout<<"2. "<<distance2(p1,p2)<<endl;
    cout<<"3. "<<distance3(p1,p2)<<endl;
    return 0;
}

运行结果：