C++: Reason why using ".hh" as extension for C++ header files

You can name your headers as you wish, they can even have no extension at all.

This FAQ mentions header naming conventions.

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. <vector> without any extension.

It's not for any specific reason, it's just a (developer- or project-specific) convention. As "file extension" is just part of name, it doesn't matter much, as source files are just plain text files. Furthermore, header files are just copy-pasted into file that included them (via #include, so file extension mean even less for them.

Borland's libraries' headers had .hpp extension. C++'s standard library headers have no extension at all. It's just a matter of convention and personal taste.

Some say, we are using ".cc" as extension for C++ files to differentiate from C files (which has an extension ".c"), likewise we are using using ".hh" as extension for C++ header files to differentiate from C header files (which has an extension ".h").

That is exactly the reason. It is just to differentiate CPP headers from C headers.

Some programmers and libraries, such as Boost, use .hpp for CPP headers. My personal choice is this:

• example.c
• example.cpp
• example.h
• example.h++

Even if they all belong to a huge project, you can still figure out which one is which. No description is needed.