Issue decoupling nested C++ classes

Question

I am having a simple C++ problem but I am not sure how to solve this.

I have two classes, A and B which are not inherited in anyway. A is Client class, and B is a class that stores a list of Client (A class).

I want only B to instantiate and destroy the class B, but I want other parts of my code to be able to call methods within class A.

Initially I had the following:

class B {
public:
    class A {
        public:
            void setX(int val);
            int getX();
        private:
            int x;
            A();
            ~A();
    };

    B()
    ~B()
    ....
};

This allows me to call A's constructor/destructor from B, and makes A's constructor/destructor private from the rest, but calling setX() and getX() on A became troublesome as I have to call B::A.

I thought putting A in it's own file (it's own class, not nested), as other classes can call methods on A. But now B cannot access A's constructor/destructor as it is private. If I make it public, then anyone can create an object of class A.

I am not sure what's the best way to do this. If A is it's own class, how can I keep the constructor/destructor available only to B please, while other classes can call setX() and getX() on an object from A please?

Egg · Accepted Answer · 2013-05-29T21:03:46.013

2

Declaring A in its own, non-nested class but with a friend class of B should do the trick.

Friend classes

Leaving you with:

class B{
public:
    B()
    ~B()
    ....
};

and

class A {
    friend class B;
    public:
        void setX(int val);
        int getX();
    private:
        int x;
        A();
        ~A();
};

edited May 29 '13 at 21:03

answered May 29 '13 at 20:50

Egg

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Thank you, this worked like a charm! Now I know about using the "friend" keyword, thanks a lot! – user1777907 May 29 '13 at 21:04

score 0 · Answer 2 · edited May 23 '17 at 11:50

I think you might messed up.(Or I did when reading your example! :P)

Declaring a nested class makes the nested class able to access all member of the nesting class and not the inverse as I understood from your explanation.
I mean: B within A means that A access al member of B, not the inverse!
Nested Classes Question(Check second response, not first!)

If when saying in your question "I want only B to instantiate and destroy the class B" you mean "I want only B to instantiate and destroy the class A" then you have two options.

You can either make B nested class of A, letting B access al private stuff of A or use friend classes.(Check @Egg response)

Issue decoupling nested C++ classes

2 Answers2