How to do the equivalent of memset(this, …) without clobbering the vtbl?

For each class where you find a memset call, add a memset member function which ignores the pointer and size arguments and does assignments to all the data members.

edit:
Actually, it shouldn’t ignore the pointer, it should compare it to this. On a match, do the right thing for the object, on a mismatch, reroute to the global function.