#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; 
 
} 

//A.h  
#pragma once  
#include <iostream>  
using namespace std;  
class A  
{  
  //friend class B; //如果不写这句话将会出现编译错误 
public:  
  ~A(void);  
  A(); 
private:   
  int m_nItem; 
};  
//A.cpp 
#include "A.h" 
 
A::A() 
{ 
  m_nItem =3; 
} 
 
A::~A(void) 
{ 
} 
//B.h 
#pragma once  
 
class B  
{  
public:  
  B(void);  
  ~B(void);  
  int func();  
};  
//B.cpp 
#include "StdAfx.h" 
 
#include "B.h"  
#include "A.h" //must include A.h  
#include <iostream> 
 
 
B::B(void) 
{ 
} 
 
B::~B(void) 
{ 
} 
 
int B::func()  
{  
  cout<<"This is in B"<<endl;  
  A a; 
  return a.m_nItem; 
}  

//A.h 
#pragma once 
#include "B.h" 
class A 
{ 
friend int B::func(A xx); 
public: 
  A(void):mx(20),my(30){} 
  ~A(void){} 
private: 
  int mx; 
  int my; 
}; 
//B.h 
#pragma once 
class A; 
class B 
{ 
public: 
  B(void); 
  ~B(void); 
  int func(A xx); 
}; 
//B.cpp 
#include "B.h" 
#include "A.h" 
 
B::B(void) 
{ 
} 
B::~B(void) 
{ 
} 
int B::func(A xx) 
{ 
  return xx.mx * xx.my; 
} 
//main.cpp 
#include "A.h" 
#include "B.h" 
#include <iostream> 
using namespace std; 
void main() 
{ 
  A a; 
  B b; 
  cout<<b.func(a)<<endl; 
  system("pause"); 
} 

class CMySingleton 
{ 
public: 
  friend CMySingleton& InstanceMEC(); 
 
private: 
  CMySingleton() {}; 
  CMySingleton(const CMySingleton &lxSington) {}; 
  ~CMySingleton(){}; 
}; 
 
CMySingleton& InstanceMEC() 
{ 
  //因为函数InstanceMEC()是类ClxSingletonMEC的友元函数，所以可以访问类所有的成员函数.所以不会有编译错误 
  static CMySingleton Instance;  
  return Instance; 
}