Many people have asked for C++ auto completion in Emacs. Until recently no really good alternative has been available. Parsing C++ is very hard and using tools like ctags works well for C but not so much for C++. CEDET/Semantics works for small C++ projects but is slow and quite complicated to install and configure.
LLVM/Clang is a compiler architecture supported by Apple that seeks to become a serious alternative to GCC. The project started just a few years ago but it already provides a high quality C and Objective C compiler that performs better than GCC in several benchmarks. The C++ support is still not complete but good enough to compile many parts of boost. Clang is not just a compiler, it also has includes great tools for code analysis. Clang is BSD licensed which enables close interaction with third part applications without licensing issues.
https://llvm.org/svn/llvm-project/cfe/trunk/utils/clang-completion-mode.el is a wonderful hack that lets Emacs interface with Clang. The result is great and fast auto completion inside Emacs. I’m sure we will see more uses of Clang in other projects.