Restrict method access to a specific class in C++

Question

I have two closely related classes which I'll call Widget and Sprocket. Sprocket has a set of methods which I want to be callable from Widget but not from any other class. I also don't want to just declare Widget a friend of Spocket because that would give Widget access to ALL protected and private members. I want to restrict Widget's access to only a specific set of methods.

One solution I came up with is to create a nested class inside Sprocket that contains wrappers for these methods and make Widget a friend of this nested class. For example:

class Sprocket
{
public:
    class WidgetInterface
    {
        friend class Widget;
        WidgetInterface(Sprocket* parent) : mParent(parent) {}

    private:
        void A() { mParent->A(); }
        void B() { mParent->B(); }

        Sprocket* mParent;
    };

private:
    void A() { ... }
    void B() { ... }
};

class Widget
{
public:
    Widget(Sprocket* sprock) : mSprocketIface(sprock) {}

    void doStuff() { mSprocketIface.A(); }  // Widget can call Sprocket::A()

private:
    Sprocket::WidgetInterface mSprocketIface;
};

This results in some code duplication because the method signatures are now declared in two places, but it works. But now suppose I want to add a subclass of Widget called SpecialWidget and I want that class to also have access to the Sprocket methods. I can simply add this new class to the Sprocket friends list or I can add yet another set of protected wrappers in Widget that SpecialWidget (and any other subclass) can access but you can see that this is now becoming a maintenance issue. I don't want to have to update the friends list or the wrappers if I add new classes or change the method signature. If I use the "add another set of wrappers" approach, the method signatures will be duplicated in three places!

Does anyone know of a simpler, cleaner way to do this?

Answer 1

If you have two tightly coupled classes, then it's really not worth trying to make friend access any more granular than it is. You control the implementation of both, and you should trust yourself enough to not abuse the ability to call some methods that you don't, strictly speaking, need to call.

If you want to make it clear for future code maintainers, add a comment to the friend declaration explaining why it is there (a good idea in general), and what private methods are allowed to be called by the friend class.

Answer 2

Sprocket has a set of methods which I want to be callable from Widget but not from any other class.

Why not save yourself some trouble & implement this set of methods in Widget, perhaps adding a Sprocket parameter to these methods?