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;
}