is STD inside <iostream>?

Question

Is the std namespace inside the <iostream> header file? I wonder what other types of namespaces are inside there

Answer 1

Here are the sacred texts, at least in the case of gcc:
https://github.com/gcc-mirror/gcc/blob/master/libstdc++-v3/include/std/iostream

Answer 2

Namespaces don't have to be contiguous, i.e you can have two header files which both add functions/classes/namespaces etc without a redefinition error.

A lot of built in header files in C++ put everything inside the std namespace. So no, everything that's in std isn't defined only inside the iostream header. Only the iostream part of the std namespace.