vs2010编译C++ 友元函数

// CTest.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <iostream>
#include <math.h>
using namespace std;

class Cpoint{
private:
    double x,y;
public:
    Cpoint(double x1,double y1){
        x=x1;y=y1;
    }
    void print(){
        cout<<"("<<x<<","<<y<<")"<<endl;
    }
    friend double dist(Cpoint a,Cpoint b);

};
double dist(Cpoint a,Cpoint b){
    double dx = b.x-a.x;
    double dy = b.y-a.y;
    return sqrt(dx*dx+dy*dy);
}
int _tmain(int argc, _TCHAR* argv[])
{
    Cpoint p1(,),p2(,);//实例化
    p1.print();
    p2.print();
    double d = dist(p1,p2);//执行友元函数
    cout<<"d="<<d<<endl;
    system("pause");
    return ;
}