I don't see any problem with dispose() method (except that it doesn't nullifies the pointer, which can be a performance issue for some GC implementations). If you plan to go C# way, it's reasonable to adopt its techniques of destruction. Moreover C# and C++ approaches are compatible. If the programmer doesn't guarantee ownership of the object, it's just unreasonable to call delete, here adding the dispose() method to the Object and using it for destruction will help.
Your proposal is indeed better than the scheme above and it's not a pain to implement and use destruct+free function, but delete and dispose are already well-known idioms, as you were already told about.
