I code in C so can't speak for C++, but I imagine it works well for that too: irony-mode as a backend and company-mode as frontend. It pretty much gives the Xcode auto-complete experience, and I think it works great!
My one complaint is that clang's MSVC mode is imperfect, and sometimes suggests some strange things (or produces some false negative/positive in syntax checking). But they've been getting much better lately and it's never caused me much pain.
e: also, second what a sibling said - combo this with flycheck-mode and you have yourself a full-blown C++ IDE in emacs ;)