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

Question

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 inher

Answer 1

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.

Answer 2

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.