Use `.dir-locals.el' to append to `before-save-hook' as a buffer local variable

Question

I want to use .dir-locals.el to append to the before-save-hook in a project for c++ files.

Currently I'm using:

(add-hook 'c++-mode-hook
          (lambda ()
            (add-hook 'before-save-hook 'clang-format-buffer nil t)))

This will set before-save-hook as a buffer local variable ignoring everything that I set globally.

How can I create a buffer-local variable of 'before-save-hook' that takes the value from the global version and then adds clang-format-buffer?

Another thing that is a little bit annoying is that this will only be applied after the second file I open in the project. In the first cpp file that I open the hook is not applied.

Answer 1

(add-hook 'before-save-hook #'clang-format-buffer nil t)

This is the right way to add a function locally to a hook.

This will set before-save-hook as a buffer local variable ignoring everything that I set globally.

No, it won't. The global value will still exist, and run-hooks will run both values. before-save-hook is no exception here, try it out and you will see. Nothing will get ignored.

Another thing that is a little bit annoying is that this will only be applied after the second file I open in the project. In the first cpp file that I open the hook is not applied.

This is what your .dir-locals.el should look like:

;;; Directory Local Variables
;;; For more information see (info "(emacs) Directory Variables")

((c++-mode
  (eval add-hook 'before-save-hook #'clang-format-buffer nil t)))