Trivial to a C developer then. This has been typical of C code for literally decades. The only thing that sucks there is writing wrappers, but it's easy to generate them.

From a user point of view you end up doing things like:

  string * str = string_new();
  int len = string_length(str);

So it looks just like most other typical C code using prefixes per interface and passing pointers to structs around.

In practice, like in C++, you'd tend to only actually use the vtable approach for those things which you actually need to be able to override based on type, which tends to be rare.

Another approach is to use messages instead, and a call a generic message handler. E.g. "ob->call(MSG_TYPE, ... arguments)" and just have a pointer to a message handler function as the first pointer in your objects. It's more flexible, but slower.