Auto comment matching braces in emacs

Question

I am trying to find an emacs command that will auto comment the code similar to this:

Before command

if(condition){
        do ...
}

After command

if(condition){
        do ...
} //Matches if(condition){

I know that when the block of code between the parentheses is too big to fit on the current page, it will show me the "Matches if(condition)" part when I insert the closing brace. I however, want to select a block of code, and then execute this command in emacs so that it will put that comment there for me. Please let me know if anything is unclear and I will be glad to specify. I also apologize if this question has been asked before, however, I looked and I couldn't find it. If it has been, please point me to that question.

Thanks in advance.

Answer 1

Add this to the bottom of your ~/.emacs file and restart Emacs. Change the keybinding to any key sequence of your choice:

(defun my-add-end-block-comment ()
  "Documentation goes here ..."
  (interactive)
  (let (start text)
    (save-excursion
      (backward-sexp 3)
      (setq start (point))
      (end-of-line)
      (setq text (buffer-substring start (point))))
    (insert (concat "  // " text))))

(global-set-key (kbd "C-z /") 'my-add-end-block-comment)

Then position your cursor after the close brace (}) and press C-z /. This should also work for while and for loops.

Answer 2

You could use C-s (forward-search) to search for the if block, select the region and M-; (comment-dwim) to comment the region.

Anytime in doubt, use M-x apropos < RET > < your-search-term-here > < RET >.

Answer 3

You can use some snippet tool, like YASnippet, which will have another field at the end for the comment. It won't be the same behavior as you want, but at the end of the day it will do more or less the same.