At the very least, the proper abstraction is the one that guarantees that you pass the same pointer as the first argument of the method call as the one you used to read the vtable from.

And no, you don't have to put everything in the header with C++, and especially not if you're using it in "C with classes" mode. C++ only needs it for templates - and even then only if you want to use implicit instantiation, with `extern template` available for fine-grained control allowing you to have those specializations separately compiled and implementations kept out of the header file.