Is there any penalty/cost of virtual inheritance in C++, when calling non-virtual base method?

Question

Does using virtual inheritance in C++ have a runtime penalty in compiled code, when we call a regular function member from its base class? Sample code:

Answer 1

At least in a typical implementation, virtual inheritance carries a (small!) penalty for (at least some) access to data members. In particular, you normally end up with an extra level of indirection to access the data members of the object from which you've derived virtually. This comes about because (at least in the normal case) two or more separate derived classes have not just the same base class, but the same base class object. To accomplish this, both of the derived classes have pointers to the same offset into the most derived object, and access those data members via that pointer.

Although it's technically not due to virtual inheritance, it's probably worth noting that there's a separate (again, small) penalty for multiple inheritance in general. In a typical implementation of single inheritance, you have a vtable pointer at some fixed offset in the object (quite often the very beginning). In the case of multiple inheritance, you obviously can't have two vtable pointers at the same offset, so you end up with a number of vtable pointers, each at a separate offset in the object.

IOW, the vtable pointer with single inheritance is normally just static_cast(object_address), but with multiple inheritance you get static_cast(object_address+offset).

Technically, the two are entirely separate -- but of course nearly the only use for virtual inheritance is in conjunction with multiple inheritance, so it's semi-relevant anyway.