In C++, should I almost always use virtual inheritance?

30
votes

I see from this entry that virtual inheritance adds sizeof(pointer) to an object's memory footprint. Other than that, are there any drawbacks to me just using virtual inheritance by default, and conventional inheritance only when needed? It seems like it'd lead to more future-proof class design, but maybe I'm missing some pitfall.

2
c++multiple-inheritancevirtual-inheritance
The order of initialization of base classes becomes non obvious to most people. Thus maintenance cost increase. - Martin York
Virtual inheritance adds an internal pointer only on some implementations. Itanium ABI doesn't use internal pointers, only vptr. - curiousguy

2 Answers

26
votes

The drawbacks are that

  1. All classes will have to initialize all its virtual bases all the time (e.g. if A is virtual base of B, and C derives from B, it also have to initialize A itself).
  2. You have to use more expensive dynamic_cast everywhere you use a static_cast (may or may not be the issue, depending on your system and whether your design requires it).

Point 1 alone makes it not worth it, since you can't hide your virtual bases. There is almost always a better way.

15
votes

In my experience, virtual inheritance (as opposed to virtual methods) is almost never needed. In C++ it's used to address the "diamond inheritance problem", which if you avoid multiple inheritance cannot actually happen.

I'm pretty sure that I've never encountered virtual inheritance outside C++ books, which includes both code I write and million+ line systems I maintain.