With a couple of minor exceptions, C++ is a superset of C.
It's fine for people to like Cxx, but I do wish we could stop perpetuating the falsehood that C is just a subset of Cxx. Cxx, when it started was a pure superset of C, but that is no longer the case.
Also, sizeof doesn't behave differently, the arguments that get passed to sizeof are different in a few cases, e.g. sizeof('x'). See Annex C: