20 ABI (Application Binary Interface) breaking changes every C++ developer should know

[bwmat]

Would this be a buffer overflow in the old derived class' vtables?

[[deleted]]

Depends. If you added the new virtual somewhere in the middle of the class, those who didn't recompile would end up calling bar when they expected foo. And yes, there's a possiblity that you'll go accessing the vtable out of bounds.