I used clang https://github.com/Rip-Rip/clang_complete in my vim, and try to make it work on omnicompletion. The menu can be called out when I type . or ::, but then, I don't know how to choose from the items in the menu. Tab or space or enter key simply don't work. So how to choose from the menu? How to make it complete its guess?
AFAIK, this pluging simply uses the same menu as Vim's default insert mode completion. If I'm right, you are supposed to use the arrows to go up and down the list and <enter> to actually do the completion.
See :h ins-completion-menu.