C++ templates and inheritance

Question

My C++ framework has Buttons. A Button derives from Control. So a function accepting a Control can take a Button as its argument. So far so good.

I also have List<T>. However, List<Button> doesn't derive from List<Control>, which means a function accepting a list of Controls can't take a list of Buttons as its argument. This is unfortunate.

Maybe this is a stupid question, but I don't see how can I solve this :( List<Button> should derive from List<Control>, but I don't see a way to make this happen "automatically".

Answer 1

Stroustrup has an item on this in his FAQ:

Why can't I assign a vector<Apple*> to a vector<Fruit*>

You can solve it in two ways:

• Make the List contain pointers to Control . Then accept List<Control*>
• Make your function a template. You can still use List<Button> and List<Control> then, but it's more boilerplate code, and not necassary most of the time.

Here is code for the second alternative. The first alternative is already explained by other answers:

class MyWindow {
    template<typename T>
    void doSomething(List<T> & l) {
        // do something with the list...
        if(boost::is_same<Control, T>::value) {
            // special casing Control

        } else if(boost::is_same<Button, T>::value) {
            // special casing Button

        }

    }
};

To restrict doSomething only for List<derived from Control>, some more code is needed (look for enable_if if you want to know).

Note that this kind of code (looking what type you have) is rather to avoid. You should handle such things with virtual functions. Add a function doSomething to Control, and override it in Button.

Answer 2

I hate to tell you but if you're using a list of instances to Control instead of pointers to Control, your buttons will be garbage anyway (Google "object slicing"). If they're lists of pointers, then either make the list<button*> into list<control*> as others have suggested, or do a copy to a new list<control*> from the list<button*> and pass that into the function instead. Or rewrite the function as a template.

So if you previously had a function called doSomething that took a list of controls as an argument, you'd rewrite it as:

template <class TControl>
void doSomething( const std::list<TControl*>& myControls ) {
  ... whatever the function is currently doing ...
}

void doSomethingElse() {
   std::list<Button*> buttons;
   std::list<Control*> controls;
   doSomething( buttons );
   doSomething( controls );
}