Executing a function on buffer enter and on buffer leave events?

Question

I would like to execute a certain function every time I leave a buffer associated with C++, and another function every time I enter a C++ buffer. I came up with this so far:

" Buffer Leave
augroup myBufferLeave
    autocmd BufLeave *.cpp,*.c,*.h call s:ExecuteLeave()
augroup END

" Buffer Enter
augroup myBufferEnter
    autocmd BufEnter *.cpp,*.c,*.h call s:ExecuteEnter()
augroup END

Another alternative is to hook into the global BufEnter event and set a buffer variable or check the current &ft and execute the appropriate function:

autocmd BufEnter * if &ft = 'cpp' |
    \ call s:ExecuteEnter() |
    \ endif

autocmd BufLeave * if &ft = 'cpp' |
    \ call s:ExecuteLeave() |
    \ endif

Is there a cleaner way to do this? Ideally, without tapping into the * global buffer events. Ideally, I would like to use something like this:

 autocmd BufLeave cpp call s:ExecuteLeave()
 autocmd BufEnter cpp call s:ExecuteEnter()

Is there a way to pack the *.cpp,*.c,*.h condition into one single definition such as cpp? The cpp definition is a FileType, but file-types are not triggered on BufEnter and BufLeave. If one needs to add another file extension, such as *.CXX for example, the list will become bloated eventually *.cpp,*.c,*.h, *.CXX, *.C, and a bit hard to maintain.

Answer 1

As the completeopt is a global option, you are correct in that you need a way to toggle the preview setting. I suggest you do this via the BufEnter autocmd with a * as the pattern.

augroup TogglePreview
  autocmd!
  autocmd Filetype,BufEnter * execute 'set completeopt'.(&ft == 'cpp' ? '+' : '-').'=preview'
augroup END

The idea is that the autocmd uses 'filetype' upon entering a new buffer or settings the buffer's 'filetype' to decide to turn on/off the preview option.

You may want to consider using :pclose to close the preview window as well.

For more help see:

:h :au
:h BufEnter
:h 'completeopt'
:h :exe
:h :pclose