项目4-成员函数友元函数有区别-成员函数

#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 distance(CPoint p);
};
double CPoint::distance(CPoint p)
{
    double l;
    l=sqrt((p.x-x)*(p.x-x)+(p.y-y)*(p.y-y));
      return l;
}
int main()
{
    CPoint t1(2,3),t2(3,4);
    cout<<t1.distance(t2);
}