Path: utzoo!attcan!uunet!actnyc!prh
From: prh@actnyc.UUCP (Paul R. Haas)
Newsgroups: comp.lang.c++
Subject: Re: Including header files minimally.
Message-ID: <1073@actnyc.UUCP>
Date: 21 Nov 88 20:05:55 GMT
References: <3561@pt.cs.cmu.edu> <7860@nsc.nsc.com> <3614@pt.cs.cmu.edu> <10873@ulysses.homer.nj.att.com>
Reply-To: prh@actnyc.UUCP (Paul R. Haas)
Organization: InterACT Corporation
Lines: 40

In article <10873@ulysses.homer.nj.att.com> gsf@ulysses.homer.nj.att.com (Glenn Fowler[eww]) writes:
>In article <3614@pt.cs.cmu.edu>, dld@f.gp.cs.cmu.edu (David Detlefs) writes:
>> ...
>> [David discusses his scheme for avoiding including files to often.]
>> ...
>
> [Glenn shows a flaw with David's method.]

I think I have a solution which is ANSI compatible and requires minimal
source code changes (some people write their header files this way already).

Make the C preprocessor recognize files of the form:
	comments and whitespace
	#ifndef SOME_SYMBOL
	any properly nested stuff, hopefully including "#define SOME_SYMBOL"
	#end
	comments and whitespace
And only include them a second time if the symbol is not defined.  The
preprocessor would have to keep a table of include file names and
symbols that they depend on.  Obviously, this should be turned off if
the preprocessor is sending comments through.

Advantages:
  o Code will behave the same way with preprocessors without this feature.
  o No #pragmas required
  o Easy to explain to people
Disadvantages:
  o In order for this to speed things up, most header files should be
    written in a particular form.
  o I am not sure how much effort this will take.  It looks like I will
    end up putting some ugly hacks into cccp.c.
  o Will slightly slow down the preprocessor for header files which lack
    the #ifndef ... #endif structure.

Feel free to point out something that I missed.  I am going to look into
adding this to the Gnu preprocessor, cccp.  If anyone else is doing something
similar, let me know.
-----
Paul Haas	uunet!actnyc!prh or prh@frith.egr.msu.edu
(212) 696-3653