C++: Reason why using “.hh” as extension for C++ header files [closed]

Answer 1

I think it is mostly a social convention.

However, recent GCC compilers are able to compile a (single) header foo.hh or foo.h into something like foo.hh.gch or foo.h.gch which essentially contains a persistent memory heap image of the compiler (the cc1plus program started by g++) after it had parsed that header.

Notice that the C++11 standard library defines header files like e.g. without any extension.