How to create a folder in the home directory?

I want to create a directory path = "$HOME/somedir".

I've tried using boost::filesystem::create_directory(path), but it fails - apparently the function doesn't expand system variables.

How can I do it the simplest way?

(note: in my case the string path is constant and I don't know for sure if it contains a variable)

edit: I'm working on Linux (although I'm planning to port my app to Windows in the near future).