It's an extension, but pretty much ubiquitous across all systems. It wouldn't be part of POSIX though, since this standard doesn't deal with memory allocation of any sort.
Yes, and I can make a stack of pizza boxes. It's still not a memory stack.
Anyways, more to the point: If your purpose all along was to name functional alternatives to non-existing language features, then you already lost. Because there is too nothing that C++ does that can't be done in C.