There is also an auto-complete provider that uses clang for giving completion suggestions. I also think there might be a company-mode backend in the works, thou

There is also an auto-complete provider that uses clang for giving completion suggestions. I also think there might be a company-mode backend in the works, though I'm not sure about this one (company-mode is the "official" completion framework for emacs, while auto-complete is the unofficial but most widespread one).