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;
}
#include <iostream>
#include <cmath>
using name