" .vimrc
" It's better to change it only for specific types of files
autocmd FileType c,cpp let b:comment_leader = '// '
" Comment a text block by selecting it in V mode and pressing \cc
noremap <silent> <leader>cc :<C-B>silent <C-E>s/^/<C-R>=escape(b:comment_leader,'\/')<CR>/<CR>:nohlsearch<CR>
" Also
autocmd FileType c,cpp setlocal comments-=:// comments+=b://
Run Code Online (Sandbox Code Playgroud)