Function overriding with argument inheritance

Question

I have a generic items list class to create a more specific listing using it as base class, something like this..

ref class ItemBase { }
ref class ItemA : ItemBase { }
ref class ItemList abstract {
 public:
  virtual void add(ItemBase ^p);
}
ref class ItemListA : ItemList {
 public:
  virtual void add(ItemA ^p) override; // it doesn't works :(
}

I want to restric adding specific type of items in each class.

Answer 1

The accepted pattern for doing this is making the base class method protected:

ref class ItemBase { }
ref class ItemA : ItemBase { }
ref class ItemList abstract {
 protected:
  virtual void addInternal(ItemBase ^p);
}
ref class ItemListA : ItemList {
 public:
  virtual void add(ItemA ^p){addInternal(p);} 
}

Here is a better solution using generics. Note how we constrain the generic parameter T to ItemBase, to enforce that this collection must only ne used with ItemBase or its subclasses.

ref class ItemBase { };
ref class ItemA : public ItemBase { };

generic <class T>
where T: ItemBase
ref class ItemList abstract {
 public:
     virtual void Add(T p){}
};

ref class ItemListA : ItemList<ItemA^> {
   //no need to override Add here
};

//usage
int main(array<System::String ^> ^args)
{
    ItemListA^ list = gcnew ItemListA();
    list->Add(gcnew ItemA());
}