C++第三次作业

#include<iostream>
#include<cmath>
double distance3(double a,double b,double c,double d)
{
    double ds;
    ds=sqrt((a-c)*(a-c)+(b-d)*(b-d));
    return ds;
}
using namespace std;
class Cpoint
{
public:
    Cpoint(double xx=0,double yy=0);
    double distance1(Cpoint x1);
    friend double distance2(Cpoint &,Cpoint &);
private:
    double x;
    double y;
};
Cpoint::Cpoint(double xx,double yy)
{
    x=xx;
    y=yy;
}
double Cpoint::distance1(Cpoint x1)
{
    double d;
    d=sqrt((x-x1.x)*(x-x1.x)+(y-x1.y)*(y-x1.y));
    cout<<d<<endl;
}
double distance2(Cpoint &t1,Cpoint &t2)
{
    double d;
    d=sqrt((t1.x-t2.x)*(t1.x-t2.x)+(t1.y-t2.y)*(t1.y-t2.y));
    cout<<d<<endl;
}
int main()
{
    Cpoint c1(2,2);
    Cpoint c2(1,1);
    c1.distance1(c2);
    distance2(c1,c2);
    cout<<distance3(1,1,2,2)<<endl;
}