animal a = new cat(); a.f = new food() auto c = cast(cat)a;as now f in cat will be food rather than catfood.
I think the problem is in your hierarchy. If Animal can have Food, that means that any animal should be able to accept any food, without knowing what kind of food is this. Cat requiring cat food is a leaky abstraction, the cat shouldn't know nor care what kind of food it gets, as it's an animal and it will eat any food.
