假设A类是B类的朋友,B是C类的朋友,A是C的朋友,A是A的朋友,还是两者,或者没有？

解决方法:

不,C中的友谊不是传递性的.

从C 03标准,11.4-8：

Friendship is neither inherited nor transitive. Example:

class A {
  friend class B;
  int a;
};

class B {
  friend class C;
};

class C {
  void f(A* p)
  {
    p->a++; 
    // error: C is not a friend of A
    // despite being a friend of a friend
  }
};

class D : public B {
  void f(A* p)
  {
    p->a++; 
    // error: D is not a friend of A
    // despite being derived from a friend
  }
};