Apply settings in .dir-locals.el to both c and c++ major modes

Question

Is there a way to specify the same settings for c-mode and c++-mode without repeating the alist? I have:

((c-mode . ((comment-start . "/* ")
            (comment-end . " */")
            (comment-padding . 0))))

and would like to avoid:

((c-mode . ((comment-start . "/* ")
            (comment-end . " */")
            (comment-padding . 0)))

 (c++-mode . ((comment-start . "/* ")
              (comment-end . " */")
              (comment-padding . 0))))

Answer 1

Not as such (AFAIK).

However, both those modes derive from prog-mode which might be fine to target for this directory.

((prog-mode . ((comment-start . "/* ")
               (comment-end . " */")
               (comment-padding . 0))))

Or you could even use nil, which applies to every mode (again, maybe that's ok in practice).

((nil . ((comment-start . "/* ")
         (comment-end . " */")
         (comment-padding . 0))))

Otherwise, I think you just have to live with the duplication.

Unless it's just for you, in which case you might be happy with

((c-mode . ((eval . (my-comment-settings))))
 (c++-mode . ((eval . (my-comment-settings)))))

calling some my-comment-settings function defined in your init file.

(defun my-comment-settings ()
  "My comment settings."
  (setq-local comment-start "/* ")
  (setq-local comment-end " */")
  (setq-local comment-padding 0))