Subject: Re: bug#37862: [PATCH] Use alternative C++ source files for determining C/C++ header files.

From: Alan Mackenzie <[email-masked]>
Date: Sun, 27 Oct 2019 13:30:38 +0000

Hello, Hong.

On Mon, Oct 21, 2019 at 20:12:51 -0700, Hong Xu wrote:

> * lisp/progmodes/cc-mode.el (c-or-c++-mode): Add check for the
> existence of alternative C++ source files.
> ---
>  lisp/progmodes/cc-mode.el | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)

> diff --git a/lisp/progmodes/cc-mode.el b/lisp/progmodes/cc-mode.el
> index 6bdfb170f1ba..940bd4be3f92 100644
> --- a/lisp/progmodes/cc-mode.el
> +++ b/lisp/progmodes/cc-mode.el
> @@ -2492,16 +2492,23 @@ c-or-c++-mode
>  matching on file name insufficient for detecting major mode that
>  should be used.
 
> -This function attempts to use file contents to determine whether
> -the code is C or C++ and based on that chooses whether to enable
> -`c-mode' or `c++-mode'."
> -  (if (save-excursion
> +This function attempts to use file contents and the existence of
> +alternative C++ source files to determine whether the code is C
> +or C++ and based on that chooses whether to enable `c-mode' or
> +`c++-mode'."
> +  (if (or
> +       (save-excursion
>          (save-restriction
>            (save-match-data
>              (widen)
>              (goto-char (point-min))
>              (re-search-forward c-or-c++-mode--regexp
>                                 (+ (point) c-guess-region-max) t))))
> +       ;; We could have gone through auto-mode-alist here, but it
> +       ;; might be too slow.
> +       (let ((base-name (file-name-sans-extension (buffer-file-name))))
> +	 (seq-find (lambda (ext) (file-exists-p (concat base-name ext)))
> +		   '(".cc" ".cpp" ".cxx" ".c++"))))
>        (c++-mode)
>      (c-mode)))
 
> -- 
> 2.20.1

Thanks for writing this proposal.

I think it would be better, given a foo.h, to find all files foo.*, and
test each one in turn for a match in auto-mode-alist.  Searching this
alist is really not that slow, particularly as it is only being done a
few times, and not in a tight loop.

For this, we really want a function in the Emacs core which would find
the match in auto-mode-alist.  Currently, this functionality is open
coded in set-auto-mode (in .../lisp/files.el).

However, I'm a bit sceptical about the use of this proposed facility.
Many projects (possibly most) have .c/.c++ files in a directory called
"src", and .h files in a separate directory called "include".  This
proposed facility wouldn't work in such a set up.  To extend it to cope
with separate "src"/"include" directories would make it messy.

-- 
Alan Mackenzie (Nuremberg, Germany).