dll interface class sharing or something

I''ve got a dll system much like half life/quake2, the engine passes function pointers to the dll to use as an interface. I want the engine and the game dll to share certain classes. This has been ok so far, but how much of a class does one of the systems need to support the other? Take an entity class for example: class CEntity { public: EventCollide(CEntity *other); }; obviously I''m missing out most of the stuff. But say EventCollide contains code specific to the game dll (where it was originally intended to be). Does this code need to be in the engine code as well for the engine to support the class, or will the function declaration above do? Thanks, Frank