Thread: Multiple inheritance: casting yields different address

Originally Posted by dwks

I wasn't using C-style casts, I was using dynamic_cast.


That was my first thought, but having data or having no data makes no difference. My understanding of this is pretty shaky, but here's what I think is happening. I think with multiple inheritance that instances of the class need two vtables, so that if an object is treated as one parent type, the appropriate methods can be looked up and called, and likewise for the other parent type.

So it would seem to me that there's no way to use multiple inheritance yet have the address of the object, no matter what base class it is casted to, be the same. Anyway, I'm not too concerned about this; I can always use the intrusive idea outlined above by brewbuck and bithub.

Or I could simply inherit every single class from a given (virtual) base class, then make ReferenceCounter always cast to this class when determining the address. I think that if this base class were virtual, it would always be located in the first vtable (at least that's what my experiments indicate), which would solve the problem.

It looks like you are indeed correct at least on Visual Studio. I just tested it. In addition what you said makes sense to me so I would imagine it works that way with a lot of compilers if not all of them. Well that's one more reason to avoid multiple inheritance

Edit: Sorry I didn't mean to imply you were using C style casts. I was just commenting about them in general.

Edit: Sorry I didn't mean to imply you were using C style casts. I was just commenting about them in general.

I didn't mean to imply your implication.

It should be mentioned that you can mess up just as badly with the C++-style cast reinterpret_cast<>(), which is pretty much just a C-style cast in disguise.

It looks like you are indeed correct at least on Visual Studio. I just tested it. In addition what you said makes sense to me so I would imagine it works that way with a lot of compilers if not all of them.

Since my tests were done with GCC, there are two compilers at least that seem to follow my reasoning.