Both YouCompleteMe and rtags are powerful tools that provide useful IDE features to both VIM and Emacs editors. Both use llvm front-end in order to provide code completion hints. Suggestion based on YCM are fine, however I believe that rtags is better tool because it has a concept of project and provides for instance GoToDefinition command, one that's not supported by YCM.

Goal of the project is to provide better code completion hints for rtags, where one has to filter out and sort results given by the FE. Apart from that, calling code completion for 'myvar.myf^', LLVM prefer to be given 'my_var.^', as it understands one expects member function and variables.

Many modern code completions (C++ ReSharper, MonoDevelop) are aware of concept of came case humps and provide nice hints when one provides starting letters of an identifier hunks. I'll try to implement also this behavior.