第六周项目4成员函数、友元函数和一般函数有区别

#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 s1(CPoint&);
friend double s2(CPoint&,CPoint&);
double getX()
{
return x;
}
double getY()
{
return y;
}
};
double CPoint::s1(CPoint&p)
{
return sqrt((p.x-x)*(p.x-x)+(p.y-y)*(p.y-y));
}
double s3(CPoint&p1,CPoint&p2)
{
double dx=p1.getX()-p2.getX(),dy=p1.getY()-p2.getY();
return sqrt(dx*dx+dy*dy);
}
double s2(CPoint&p1,CPoint&p2)
{
double dx=p1.x-p2.x,dy=p1.y-p2.y;
return sqrt(dx*dx+dy*dy);

}
int main()
{

CPoint p1(10,13), p2(-5,6);
cout<<"1. "<<p1.s1(p2)<<endl;
cout<<"2. "<<s2(p1,p2)<<endl;
cout<<"3. "<<s3(p1,p2)<<endl;
return 0;

}