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;}