class Point 
{ 
  friend double Distance(const Point &p1, const Point &p2); //类体中声明 
 
public: 
  Point(int x, int y):x_(x), y_(y) 
  {}; 
 
private: 
  int x_; 
  int y_; 
}; 
 
//类体外定义友元函数 
double Distance(const Point &p1, const Point &p2) 
{ 
  double dx = p1.x_ - p2.x_; //直接访问类的私有成员 
  double dy = p1.y_ - p2.y_; 
  return sqrt(dx*dx+dy*dy); 
} 
 
int main() 
{ 
  Point p1(3,4); 
  Point P2(6,8); 
  cout << Distance(p1,p2) << endl; 
 
  return 0; 
} 

class Television //电视类 
{ 
  friend class TeleController;//友元类声明 
   
public: 
  Television(int volume, int chanel): volume_(volume), chanel_(chanel){} 
 
private: 
  int volume_; //音量 
  int chanel_; //频道 
}; 
 
class TeleController //遥控器类 
{ 
public: //类中的每个成员函数都是电视类的友元函数，都可以访问电视类的私有数据成员 
  void VolumeUp(Television &tv) 
  { 
    tv.volume_ += 1; //音量加1 
  } 
 
  void VolumeDown(Television &tv) 
  { 
    tv.volume_ -= 1; 
  } 
 
  void ChanelUp(Television &tv) 
  { 
    tv.chanel_ += 1; //频道加1 
  } 
 
  void ChanelDown(Television &tv) 
  { 
    tv.chanel_ -= 1; //频道减1 
  } 
}; 
 
 
int main() 
{ 
  Television tv(1,1); 
  TeleController tc; 
  tc.ChanelUp(tv); 
  return 0; 
}