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Gmane
From: Alan Mackenzie <acm <at> muc.de>
Subject: Re: Supporting tabs for indentation, spaces for alignment
Newsgroups: gmane.emacs.cc-mode.general
Date: Friday 11th April 2008 20:31:42 UTC (over 8 years ago)
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,
in

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

, you want (I think), tabs here:
tttttttt
, 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:
tt
, 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  , 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
`tab-width'."
  (save-excursion
      (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)
				    (back-to-indentation)
				    (current-column)))
		 num-tabs)
	;; 
	    (goto-char indent-pos)
	    (delete-horizontal-space)
	    (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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