类模板的友元声明

Question

I'm facing problems with friend class templates. Here's how my code looks like

template<typename T>
class A{
private:

  template<typename U>
  friend class A;

  int m_num = 0;

public:
//...

};

while

template <typename T>
class B{
private:

  template<typename U>
  friend class B;

  template<typename U>
  friend class A; // Set A as friend class template of B

public:

  template<typename U>
  int GetNumOfA(const A<U>& a){
    return a.m_num; // Cannot Access A's private member!
  }

};

I expected A's private members to be accessible since I declared A as friend class of B. But it couldn't access it. Could you help me figure out why this happens? Or any ideas to make m_num accessible from B?

Answer 1

You have it the wrong way around. You want B to be friend of A . So you need to declare B a friend in A 's definition:

template<typename T>
class A{

//...

  template<typename U>
  friend class B;

//...

};

Do however think about whether you actually want all of these classes and their different specializations to be friend . The point of making a member private is that it represents a state internal to the class and should not be accessed directly by unrelated code. Usually, if you need direct access to a member, then you should either make it public or if there is a class invariant that doesn't allow that, give access to it through a public member function which preserves the invariant.