I'm wondering if there's any standard about it. In case there isn't, which is the most common between .h and .hpp extensions? I'll use g++ as compiler.

I use .h for both as I don't see a particularly good reason to make a difference between C and C++ headers (I code nearly 100% in C++). Good file managers can make difference without looking the extension

http://www.new-brunswick.net/workshop/c++/faq/coding-standards.html#faq-27.4

And for naming your own headers

But basically some editors can figure out the type of the file based on the extension so .h, .hpp, or .hh are ok. I tend to use .h.

Um, just 'question' I suppose.

As for the original question: .h and .hpp are the most popular ones.

Technically it doesn't matter what you use, because an #include is just a brute-force text replacement regardless of file type or language: the preprocessor doesn't know any actual C or C++, and consequently doesn't need to know what the language is. Theoretically, you could just as well use the preprocessor for any other language. The .c vs. .cpp/.cc difference DOES matter though, since the compiler uses the extension to determine the language.

In practice, it is convenient to use something that your editor 'knows'; since highlighting rules are probably the same for C and C++, you can just use .h for both, unless you have a project that's partially C and partially C++ (like a library that works with C++ behind the scenes, but has a plain C API); in that case, using different extensions may avoid confusion.

Whatever you do, using capital letters (.H and .C for C++ vs. .h and .c for plain C) is obviously not a good idea if you want your code to be remotely cross-platform.

I'm going with .hpp for consistency with .cpp
Thank you