This is deprecated.
Use neocomplcache and clang_complete.

" :help neocomplcache-faq
if !exists('g:neocomplcache_force_omni_patterns')
  let g:neocomplcache_force_omni_patterns = {}
endif
let g:neocomplcache_force_overwrite_completefunc = 1
let g:neocomplcache_force_omni_patterns.c =
      \ '[^.[:digit:] *\t]\%(\.\|->\)'
let g:neocomplcache_force_omni_patterns.cpp =
      \ '[^.[:digit:] *\t]\%(\.\|->\)\|\h\w*::'
let g:neocomplcache_force_omni_patterns.objc =
      \ '[^.[:digit:] *\t]\%(\.\|->\)'
let g:neocomplcache_force_omni_patterns.objcpp =
      \ '[^.[:digit:] *\t]\%(\.\|->\)\|\h\w*::'
let g:clang_complete_auto = 0
let g:clang_auto_select = 0
"let g:clang_use_library = 1