friend class with limited access

Question

I want to make a class A friend class of class B. I want to do this as these interact very much and A needs to change internals of class B (which I dont want to expose using public). But I want to make sure it has access to only a few selected functions not all the functions.

Example:

class A
{
};

class B
{
private:
 void setState();
void setFlags();
friend class A
};

I want A to be able to access setState but not setFlags... Is there a design pattern or a nice way of doing this or am I left with giving full access or no access at all in this case.

Thanks

Answer 1

It depends on what you mean by "a nice way" :) At comp.lang.c++.moderated we had the same question a while ago. You may see the discussion it generated there.

IIRC, we ended up using the "friend of a nested key" approach. Applied to your example, this would yield:

class A
{
};

class B
{
public:
     class Key{
         friend class A;
         Key();
     };

    void setFlags(Key){setFlags();}         

private:
  void setState();
  void setFlags();
};

The idea is that the public setFlags() must be called with a "Key", and only friends of Key can create one, as its ctor is private.