I'm trying to add functionality found in some other code editors to my Emacs configuration, whereby C/C++ code within #if 0...#endif blocks is automatically set to the comment face/font. Based on my testing, cpp-highlight-mode does something like what I want, but requires user action. It seems like tying into the font-lock functionality is the correct option to make the behavior automatic.
I have successfully followed examples in the GNU documentation to change the face of single-line regular expressions. For example:
(add-hook 'c-mode-common-hook
(lambda ()
(font-lock-add-keywords nil
'(("\\<\\(FIXME\\|TODO\\|HACK\\|fixme\\|todo\\|hack\\)" 1
font-lock-warning-face t)))))
works fine to highlight debug related keywords anywhere in a file. However, I am having problems matching #if 0...#endif as a multiline regular expression. I found some useful information in this post (How to compose region like "<?php foo; bar; ?>"), that suggested that Emacs must be told specifically to allow for multiline matches. But this code:
(add-hook 'c-mode-common-hook
(lambda ()
'(progn
(setq font-lock-multiline t)
(font-lock-add-keywords nil
'(("#if 0\\(.\\|\n\\)*?#endif" 1
font-lock-comment-face t))))))
still does not work for me. Perhaps my regular expression is wrong (though it appears to work using M-x re-builder), I've messed up my syntax, or I'm following the wrong approach entirely. I'm using Aquamacs 2.1 (which is based on GNU Emacs 23.2.50.1) on OS X 10.6.5, if that makes a difference.
Any assistance would be appreciated!
Even if you got the multiline regexp to work, you'd still have problems with nested #ifdef/#endif's since it would stop font-locking at the first #endif. This code works, although I'm not sure if there will be a noticeable slow down for large files:
(defun my-c-mode-font-lock-if0 (limit)
(save-restriction
(widen)
(save-excursion
(goto-char (point-min))
(let ((depth 0) str start start-depth)
(while (re-search-forward "^\\s-*#\\s-*\\(if\\|else\\|endif\\)" limit 'move)
(setq str (match-string 1))
(if (string= str "if")
(progn
(setq depth (1+ depth))
(when (and (null start) (looking-at "\\s-+0"))
(setq start (match-end 0)
start-depth depth)))
(when (and start (= depth start-depth))
(c-put-font-lock-face start (match-beginning 0) 'font-lock-comment-face)
(setq start nil))
(when (string= str "endif")
(setq depth (1- depth)))))
(when (and start (> depth 0))
(c-put-font-lock-face start (point) 'font-lock-comment-face)))))
nil)
(defun my-c-mode-common-hook ()
(font-lock-add-keywords
nil
'((my-c-mode-font-lock-if0 (0 font-lock-comment-face prepend))) 'add-to-end))
(add-hook 'c-mode-common-hook 'my-c-mode-common-hook)
EDIT:
Take into account #else
EDIT #2: Niftier code to handle arbitrary nesting of if/else/endif's