Enable autocomplete for C++ on vim

 Assuming you have a recent version of vim (8.1 should do) this is a lot easier now.

1. If you don't already have Vundle:
   git clone https://github.com/VundleVim/Vundle.vim.git ~/.vim/bundle/Vundle.vim
2. Add the following section to ~/.vimrc. The vundle section may already be there in which case you just add the Plugin line for Valloric/YouCompleteMe.
   set rtp+=~/.vim/bundle/Vundle.vim
   call vundle#begin()
   Plugin 'VundleVim/Vundle.vim'
   Plugin 'Valloric/YouCompleteMe'
   call vundle#end()
3. Clone YouCompleteMe.
   git clone https://github.com/Valloric/YouCompleteMe.git ~/.vim/bundle/YouCompleteMe
4. Update YouCompleteMe.
   cd ~/.vim/bundle/YouCompleteMe && git submodule update --init --recursive
   
5. Install the clang completer. This works but a better way may be to configure clangd.
   ./install.py --clang-completer
   
6. Add a file .ycm_extra_conf.py to some ancestor directory of your C++ projects.
   def Settings( **kwargs ):
     return {
       'flags': [ '-x', 'c++', '-Wall', '-Wextra', '-Werror' ],
     }
   
   There are other ways of doing the above that can give you more flexibility in your own sandbox. You can check them here: https://github.com/ycm-core/YouCompleteMe#general-semantic-completion