Question

I’m creating little GUI for my OpenGL game. I’ve got two base classes Widget and Container. Widget class has methods like setPosition(x,y), setSize(w,h),… These methods should be visible only for classes derived from Container.

Edit: I can do it with friend and wrapper methods, but I’d like to do it without wrappers.

Answer 1

In case the suggestion was not clear from the comment, you can consider creating an interface with that subset of the methods and use private inheritance in Widget. Then use a protected (a single operation) in Container that would obtain that private interface for classes derived from it:

struct Drawable {    // choose a better name
  virtual void setPosition( int x, int y );
  virtual void setSize( int w, int h );
  virtual ~Drawable() {}
};
class Widget : private Drawable {
   friend class Container;
};
class Container {
protected:
    Drawable& getDrawable( Widget& w ) { return w; };
};
class MyContainer : public Container {
   void changeSize( Widget & w ) {
      getDrawable(w).setSize(10,20);
   }
};

The main difference with the forwarding options is that this scales better with the number of functions and is more maintainable (no need to update N-forwarders, just the accessor to the proper interface).

From a high level design point of view, a Widget is not Drawable in general, it is only Drawable from the context of Widget or Container.