Right. C++ objects are values, not references to values like in almost all other languages. So C++ needs to know the sizes of everything to construct them. If you tried to write out the mathematical series describing the ultimate size you would need to allocate for A or B, you would end up with a value approaching infinity.

This is probably my favorite thing about C++. Values are so much easier to reason about than references.

It's more of a simplification. Although, I did try references and they didn't compile either. In fact, if I remember correctly the error was "forward declaration forbidden".

After spending about 10-12 years away from C++, I was pretty much on novice level. I remember some things, but most details and day to day specifics are gone. It did serve as a fresh reminder why I hate C++. Or more precisely, why I hate C++ compilers.