friend class with limited access

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

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.

One approach is through explicit interfaces, because the implementor of an interface can select who they give them to:

class NearlyPrivateInterface {
public:
   virtual void setState() = 0;
   virtual void setFlags() = 0;
};

class A {
public:
   void attach(NearlyPrivateInterface* instanceOfB);
};

class B: private NearlyPrivateInterface {
public:
   void attach(A& a) { a.attach(this); }
};

You can do following thing..

class A{
};

class B{
private: 
    void setFlags();
protected:
    void setState();

}; 

class RestrictedB :public B{  
    friend class A;
};