友元函数

#include<iostream>
#include<cmath>
using namespace std;
class Dis
{
private:
    double x;
    double y;
public:
    Dis(double xx=0,double yy=0):x(xx),y(yy){}
    friend double distance1(Dis &,Dis &);
};
double distance1( Dis &t1, Dis &t2)
{
    double a=(t1.x-t2.x),b=(t1.y-t2.y);
    return sqrt(a*a+b*b);
}
int main()
{
    Dis t1(5,6),t2(7,8);
    cout<<distance1(t1,t2);
    return 0;
}