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Re: Supporting tabs for indentation, spaces for alignment

From: Ergus
Subject: Re: Supporting tabs for indentation, spaces for alignment
Date: Thu, 11 Apr 2019 12:24:05 +0200
User-agent: NeoMutt/20180716

Hi Alan:

Very thanks for replying. I will try to implement your solution for now
in my config.

But I'm wondering if after 11 years, with much better machines, improved
emacs with many new functionalities (most of them nothing to do with a
text editor) and the constant growing of languages that uses cc-mode as
a base (rust || C++>=11 for example).

But also with new indentation styles (like the google's one which is
by far the most popular package in melpa).
And this package which seems to work; so some experience already:


And the increase in popularity of other editors/ides which use different
policies (from vim, kdevelop, eclipse, notepad++, visual-studio-code);
so a emacs user (like me) needs to attach and don't break the global
project schema. Maybe it makes sense to think how to add other
tab-indent-policies and styles in the c-indent-alist list. Even if they
are not the defaults.

So the users could find their needs without writing 40 lisp lines and
read 20 manual pages full of therms that makes sense only after reading
1000 previous pages. Or needing to install an external package that
potentially gets abandon.

On Thu, Apr 11, 2019 at 09:02:23AM +0000, Alan Mackenzie wrote:
Hello again, Ergus.

You were asking in your post Subject: Re: C style alist question? for a
way of indenting CC Mode modes with "tabs for indentation and spaces for

The topic has come up before, and I answered back in 2008 with the
following post to the OP and address@hidden  It's a bit wordy, I
admit, but does provide a way of getting what you're asking for.  I
admit not having tried it out recently.

Alan Mackenzie (Nuremberg, Germany).

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Date: Fri, 11 Apr 2008 20:31:42 +0000
To: Mike Sullivan <address@hidden>
Subject: Re: Supporting tabs for indentation, spaces for alignment
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Hi, Mike,

Sorry it's taken me so long to answer.

On Wed, Jan 09, 2008 at 02:04:40AM -0600, Mike Sullivan wrote:
Currently, cc-mode offers no simple way to use tabs for indentation,
but spaces for alignment (as described many places on the web,
including http://wiki.chad.org/wiki?IndentationUsingTabs).  Is there
any chance that this will be added as a feature in a future version?

OK: Here's what I think that page means.  Supposing your tab-width is 8,

       if (a really long conditional that won't fit
           onto one line) { ....

, you want (I think), tabs here:
, yet spaces here:

If your tab-width were 2, you'd want to see this:

 if (a really long conditional that won't fit
     onto one line) { ....

, with tabs here:
, and spaces here:


Currently, assuming indent-tabs-mode is set, with a sufficiently large
tab-width (say, 8), this is what you'll get, but with a smaller
tab-width (2), the four spaces will be replaced by two tabs, not what
you want.

No, there are no plans for putting such a feature into CC Mode at the
moment.  One could opine that the opinions on that web page are somewhat
opinionated, capable of triggering religious wars.  ;-)  My own view is
that that indentation strategy would be a right royal pain, unless it
somehow got enforced 100% by the project's management and _every_ editor
in use supported it.  To implement the idea "properly" in CC Mode would
entail a non-trivial amount of work.

However, this is the sort of thing that seems tailor-made for
c-special-indent-hook (see page "Other Indentation" in the CC Mode
manual), and shows just how flexible Emacs (in general) and CC Mode (in
particular) are, and just how superior they are to lesser editors.  ;-)

I've written a quick hack, to give you an idea how this sort of thing
could work.  Here's the source file I tried it on, _without_ tabs here
(it goes more easily into an email that way), with c-basic-offset = 3:

int foo (char a, char b, char c, char d, char e,
        char f, char g) {
  if (a = 'a'
      && b = 'b'
      && c = 'c')
     print "hi!\n" ;
     return 0 ;
  return 1 ;

As a quick tip, to _see_ tabs in a buffer, do M-x hi-lock-mode, then C-x
w h.  On being prompted for a regexp just do C-q <tab> <CR>, then select
some highlighting face.

OK, here's the hack:

(defun ms-space-for-alignment ()
 "Make the current line use tabs for indentation and spaces for alignment.

It is intended to be called from the hook
`c-special-indent-hook'.  It assumes that `indent-tabs-mode' is
non-nil and probably assumes that `c-basic-offset' is the same as
     (let* ((indent-pos (progn (back-to-indentation) (point)))
             (indent-col (current-column))
             (syn-elt (car c-syntactic-context))
             (syn-sym (c-langelem-sym syn-elt)))
        (when (memq syn-sym '(arglist-cont-nonempty)) ;; <==============
          (let* ((syn-anchor (c-langelem-pos syn-elt))
                 (anchor-col (progn (goto-char syn-anchor)
            (goto-char indent-pos)
            (insert-char ?\t (/ anchor-col tab-width))
            (insert-char ?\  (- indent-col (current-column))))))))

To enable it, do this:

   M-: (setq indent-tabs-mode t)
   M-: (setq tab-width 3)
   M-: (setq c-basic-offset tab-width)
   (add-hook 'c-special-indent-hook 'ms-space-for-alignment nil t)

If you were to develop this hack into fully working code, I would be
willing to put it into the next full version of CC Mode (subject to the
customary copyright assignment to the FSF, which we all have to do).

Thanks, Mike Sullivan

Happy hacking!

Alan Mackenzie (Nuremberg, Germany).

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