It solves the problem by decoupling the public interface and necessary implementation details (object size), the register/unregister functions can be moved to an internal only header file, maybe even some file scoped functions. AFAIK this is not possible with a class declaration.

I think you could also solve this specific problem with an internal header file in c++ too (by moving register/deregister to it's own class) but this solution solves other problems like binary compatibility.