1.友元函数

class A{
  friend void MyFriend(class &A);
private:
  int m_a;
}

void MyFriend(class &A)
{
   std::cout << A.m_a << std::endl;
}

2.友元类

class A{
public:
  A();
  friend class B;
private:
  int m_a;
}

class B{
public:
  B();
  void UseA(A &a);
}

void B::UseA(A &a)
{
  std::cout<< a.m_a << std::endl;
}

3.友元成员函数

//注意声明顺序，保证可见
class A;//A做前置声明，因为B中要使用
class B{
public:
  B b();
  void UseA(A &a);
}

class A{
public:
  A();
  friend void B::UsaA(A &a);
private:
  int m_a;
}

void B::UseA(A &a){
  std::cout << a.m_a << std::endl;
}

4.共同友元函数 (一个函数是两个类的友元)

class B;
class A{
public:
  A();
  friend void UseAB(A &a,B&b);
private:
  int m_a;
}
class B{
public:
  B();
  friend void UsaAB(A &a,B &b);
private:
  int m_a;
}

void UseAB(A &a,B &b)
{
  std::cout << a.m_a + b.m_a << std::endl;
}

友元是对类接口的扩展，因为只有在类中才能指定谁为友元，so，不违反OOP思想!