YCM 对于clang的补全需要依赖clang++ 3.70及以上的版本。另外补全不一定是tab, 你得看看.vimrc怎么设置的，我的是ctrl + j或者kMake your choice

If everything is installed, then open vim之后，使用Vundle看看是不是安装成功，如果成功的话，那可能是配置问题了，
下面是我本机的YCMthe configuration in vimrc.bundle: See if it helps?

" YouCompleteMe {{{
    "youcompleteme  默认tab  s-tab 和自动补全冲突
    "let g:ycm_key_list_select_completion=['<c-n>']
    let g:ycm_key_list_select_completion = ['<Down>']
    "let g:ycm_key_list_previous_completion=['<c-p>']
    let g:ycm_key_list_previous_completion = ['<Up>']
    let g:ycm_complete_in_comments = 1  "在注释输入中也能补全
    let g:ycm_complete_in_strings = 1   "在字符串输入中也能补全
    let g:ycm_use_ultisnips_completer = 1 "提示UltiSnips
    let g:ycm_collect_identifiers_from_comments_and_strings = 1   "注释和字符串中的文字也会被收入补全
    let g:ycm_collect_identifiers_from_tags_files = 1
    " 开启语法关键字补全
    let g:ycm_seed_identifiers_with_syntax=1

    "let g:ycm_seed_identifiers_with_syntax=1   "语言关键字补全, 不过python关键字都很短，所以，需要的自己打开

    " 跳转到定义处, 分屏打开
    let g:ycm_goto_buffer_command = 'horizontal-split'
    " nnoremap <leader>jd :YcmCompleter GoToDefinition<CR>
    nnoremap <leader>jd :YcmCompleter GoToDefinitionElseDeclaration<CR>
    nnoremap <leader>gd :YcmCompleter GoToDeclaration<CR>

    " 引入，可以补全系统，以及python的第三方包 针对新老版本YCM做了兼容
    " old version
    if !empty(glob("~/.vim/bundle/YouCompleteMe/cpp/ycm/.ycm_extra_conf.py"))
        let g:ycm_global_ycm_extra_conf = "~/.vim/bundle/YouCompleteMe/cpp/ycm/.ycm_extra_conf.py"
    endif
    " new version
    if !empty(glob("~/.vim/bundle/YouCompleteMe/third_party/ycmd/cpp/ycm/.ycm_extra_conf.py"))
        let g:ycm_global_ycm_extra_conf = "~/.vim/bundle/YouCompleteMe/third_party/ycmd/cpp/ycm/.ycm_extra_conf.py"
    endif

    " 直接触发自动补全 insert模式下
    " let g:ycm_key_invoke_completion = '<C-Space>'
    " 黑名单,不启用
    let g:ycm_filetype_blacklist = {
        \ 'tagbar' : 1,
        \ 'gitcommit' : 1,
        \}
" }}}