Elisp-Function to always insert "*/" in C-Mode when "/*" was typed

Question

I want to implement the functionality, that i wrote about in the title. Since searching the web for existing solutions didn't give me anything useful, i figured i would just try and implement it myself.

I would want to just add a lambda or function to my init.el-File, that executes everytime, it looks at an opening multi-line-comment and inserts that matching closing chars. This is what i have so far:

(add-hook 'c-mode-hook
      (lambda ()
        (if (looking-at "/*")
            (insert "*/"))))

Probably not right at all. I just started with elisp a while ago, so help and explanation will be thankfully appreciated! If there is an existing solution, i also wouldn't hesitate to try that out. Is there an option to enable this in C-Mode, as well as in C++-Mode? Thanks in advance!

Answer 1

If you want to use an existing solution then you can look into smartparens which seems to be easily extensible, see the pair management section on their wiki.

As for your code, it's definitely on the right track, but looking-at looks at text after the point, so the insertion function should rather be something along the lines of

(defun close-c-comment ()
  (when (looking-back "/\\*" (- (point) 2))
    (insert "*/")))

(the first argument of looking-back is a regex, so the asterisk needs to be escaped). In addition, you probably want to add this function to the post-self-insert-hook in c-mode (to be automatically activated after insertions), so this part should be something like

(add-hook 'c-mode-hook
  (lambda () 
    (add-hook 'post-self-insert-hook #'close-c-comment)))

Note: The first version of this answer omitted the (optional) (- (point) 2) limit argument from the looking-back function call in the definition of close-c-comment, but -- as npostavs pointed out in his comments -- not specifying a limit for the backward search can seriously slow down the execution of the function, especially in the case of large buffers.