友元类

#include <iostream>
#include <math.h>
using namespace std;
class Point;
class distM{
public:
        static int dist(Point & p1,Point &p2);
 }; 
 /*友元类的使用较少 ，是因为对封装的破坏性过大*/
class Point{
public:
   Point(int x, int y)
       :_x(x),_y(y){}
  // friend int distM::dist(Point & p1,Point &p2);
  friend class distM;

  /*

           类distM中的每一个成员函数都是Point类的友元函数 

  友元类不受权限影响 public private protected 
  */ 
private:
   int _x;
   int _y;
 };
int distM::dist(Point & p1,Point &p2){ 
    int x = p1._x - p2._x;
    int y = p1._y - p2._y;
    return sqrt(x*x+y*y);
   }


int main(){
    Point p1(4,5);
    Point p2(7,8);
   int d =distM::dist(p1,p2);
    cout<<d<<endl;
    return 0;
 }