an empty constructor should probably be included since a typical use of an interface involves putting a pointer to some concrete object in a container, which will otherwise call the wrong destructer and will not clean the memory correctly.
so if anyone is going to delete derived objects through a pointer to Ianimal make a virtual destructor, else make your destructor nonvirtual and protected.
making your destructor pure virtual is probably not such a good idea, since it forces implementers of derived classes to override your destructor, eventhough they might want to do nothing
