Question

I’m trying to properly encapsulate a class A, which should only be operated on by class B.

However, I want to inherit from class B.

Having A friend B doesn’t work — friendship isn’t inherited.

What’s the generally accepted way of accomplish what I want, or am I making a mistake?

To give you a bit more color, class A represents a complex system’s state. It should only be modified by B, which are actions that can be applied to change class A’s state.

Answer 1

I assume you want to allow descendants of B to access A directly? If A and B are tightly coupled, you can make A a protected class definition within B itself, instead of being an independent definition. E.G.

class B
{
protected:
    class A
    {
    };
};

Another idea is to create protected methods on B that delegate their actions to A. E.G.

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

class B
{
protected:
    void DoSomething(A& a) { a.DoSomething(); }
};