Friend declaration for single member instead of whole class?

Question

Normally in C++ when class A declares friendship with class B, class B has full access to class A private members. What I need to do is to allow class B to access only one private member of class A and nothing else. Is there any way for it, maybe something new in C++11?

Answer 1

Not that I'm aware of. If it's really important to only allow access to a single member, you could wrap the data in another class (C) that has entirely private members, make that class a friend of B and provide a public accessor for the C object in A.

Something like this:

template<class T> class MatesWithB
{
     friend class B;

  public:
     MatesWithB( T & val ) : data(val) {}

  private:
     T& data;
     operator T&()             { return data; }
     operator const T&() const { return data; }
};

class A
{
      // ...
      MatesWithB<int> GetPrivateThingy() { return MatesWithB(thingy); }

   private:
      int thingy;            // Shared with class B
};

Something like that. I haven't run this through a compiler to check.

But I just wonder... When you find yourself needing to do this, isn't something in your design fundamentally flawed?