Must the entire class hierarchy be recompiled if a virtual or non-virtual function is added to the base class in C++?

Question

I am trying to learn how sensitive the vtable of a class is in C++ and, for that, I need to know whether recompilation of the entire class hierarchy (total, 3 header files) is n

Answer 1

The C++ one-definition-rule makes it clear that definitions of entities in different translation units (ie: files) must all be identical if you're going to link them together. As such, if you change the definition of a class at all, public, private, virtual, non-virtual, whatever, all translation units that use that definition have to be looking at the new class definition. And that will require recompiling it.

Failure to do this is il-formed, but no diagnostic (linker-error) is required. So your project may appear to link just fine. Indeed, it may actually work in some cases. But there's nothing which guarantees in which cases they will work and in which cases they will not.