C++中的friend友元函数详细解析

#include "cmath"
#include "iostream"
using namespace std;
class Point
{
public:
      Point(double xx,double yy)
      {
          x=xx;
          y=yy;
      }
      void GetXY();
      friend double Distance(Point &a,Point &b);
protected:
private:
      double x,y;
};
void Point::GetXY()
{
     //cout<<"("<<this->x<<","<<this->y<<")"<<endl;
     cout<<"("<<x<<","<<y<<")"<<endl;
}
double Distance(Point &a,Point &b)
{
     double length;
     length=sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));     //它可以引用类中的私有成员
     return length;
}
int main(void)
{
     Point p1(3.0,4.0),p2(6.0,8.0);
     p1.GetXY();    //成员函数的调用方法，通过使用对象来调用
     p2.GetXY();
     double d = Distance(p1,p2);     //友元函数的调用方法，同普通函数的调用一样，不要像成员函数那样调用
     cout<<d<<endl;
     system("pause");
     return 0;
}

#include <cmath>
#include <iostream>
using namespace std;
class Point
{
public:
      Point(double xx,double yy)
      {
           x=xx;
           y=yy;
      }
      void GetXY();
      friend double Distance(Point &a,Point &b);
     friend ostream &operator <<(ostream &a,Point &b);
protected:
private:
      double x,y;
};
// friend ostream& operator<<(ostream& o,A& another);
ostream &operator <<(ostream &out,Point &b)   //在类中声明的时候，可以是ostream &a，函数定义的时候也可以是ostream &out
{
     out<<"("<<b.x<<","<<b.y<<")"<<endl;
     return out;
}
void Point::GetXY()
{
     //cout<<"("<<this->x<<","<<this->y<<")"<<endl;
     //cout<<"("<<x<<","<<y<<")"<<endl;
     cout<<*this;
}
double Distance(Point &a,Point &b)
{
     double length;
     length=sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
     return length;
}
int main(void)
{
     Point p1(3.0,4.0),p2(6.0,8.0);
     p1.GetXY();
     p2.GetXY();
     double d = Distance(p1,p2);
     cout<<d<<endl;
     system("pause");
     return 0;

}