Impossible to inherit from this object?

Question

Following up on this question, people have suggested I go with "option 3" which might look like this:

class b2Body {
private:
    b2Body() {}
    friend class b2World;
};

class Body : public b2Body {
private:
    Body() {}
    friend class World;
};

class b2World {
public:
    b2Body *CreateBody() {
        return new b2Body;
    }
};

class World : public b2World {
public:
    Body *CreateBody() {
        return new Body;
    }
};

int main() {
    World w;
    Body *b = w.CreateBody();
    return 0;
}

Try it online

But there are two major problems with this:

1. Body can never be constructed, even with World::CreateBody (because b2Body is private)
2. Even if it could, then the b2Body part wouldn't be initialized correctly (b2World::CreateBody needs to be called)

Does this mean I can never inherit from b2Body/b2World and follow this same design pattern? (keep in mind that I can't edit the b2* classes)

If so, I suppose you guys would recommend I just keep b2World and b2Body as member variables instead?

---

I think it comes down to these two options now:

• option 3c
• option 2

Answer 1

Only b2World can create a b2Body so this just doesn't go anywhere. These classes clearly aren't designed to be inherited from so yes, aggregate them instead. What about something along the lines of

class Body {
public:
    Body( b2Body* b ) : b2b( b ) {}

private:
    b2Body*const b2b;
};

class World {
public:
    World() : b2w( /* create b2w here */ ) {}

    Body *CreateBody() {
            return new Body( b2w->CreateBody( /*...*/ ); }
    }
private:
    b2World*const b2w;
};