before-save-hook for cc-mode

Question

I want to customize cc-mode to execute a function before saving. One solution would be to modify cc-mode keymap to bind C-x C-s to a function which performs the work I need and then saves it.

My question is whether there is a hook I could customize for this? The CC Hooks docs don't mention a before-save-hook specifically for cc-mode. How should this be done through a hook?

Alternatively, how do I do this through defadvice? From reading the manual, the only way I could see was to enable the advice before saving and then disable after the work is done. This sounds cumbersome. What would be the idiomatic way of using advices?

Answer 1

One fairly common way of restricting the action of a hook to a particular major mode is to check the value of major-mode.

For example, this hook prints a message before saving buffers using c++-mode:

(defun my-c++-mode-before-save-hook ()
  (when (eq major-mode 'c++-mode)
    (message "It's never too early to start saving (C++ code)!")))

(add-hook 'before-save-hook #'my-c++-mode-before-save-hook)

As a side note: sometimes it can be useful to check if a major mode is derived from CC Mode; we can do this by checking if c-buffer-is-cc-mode is not nil:

(defun my-cc-mode-before-save-hook ()
  (when c-buffer-is-cc-mode
    (message "Saving a buffer with a major-mode derived from CC Mode.")))

Answer 2

Alternatively, use a local hook:

(add-hook 'c++-mode-hook
          (lambda () (add-hook 'before-save-hook MY-HOOK-FUNC nil 'local)))

This adds MY-HOOK-FUNC to the buffer-local before-save-hook of each C++ Mode buffer, or any buffer in a mode derived of C++ Mode.

I find this more elegant that explicitly checking major-mode, and it has the advantage that you can disable the before-save-hook locally for each buffer with M-: (remove-hook 'before-save-hook MY-HOOK-FUNC 'local) without affecting any other buffers.

Answer 3

I once needed something like that for autocompiling my elisp files when saved, I used a approach like this (converted the approach to cc-mode):

(add-hook 'c-common-hook #'(lambda ()
                              (add-hook 'before-save-hook #'(lambda ()
                                                               (funcall #'YOUR-FUNC-HOOK)))))