Path: utzoo!attcan!uunet!husc6!purdue!gatech!ulysses!gsf
From: gsf@ulysses.homer.nj.att.com (Glenn Fowler[eww])
Newsgroups: comp.lang.c++
Subject: Re: Managing C/C++ Libraries: Dependencies
Summary: #include_once != #ifndef ...
Message-ID: <10853@ulysses.homer.nj.att.com>
Date: 14 Nov 88 15:54:30 GMT
References: <8387@nlm-mcs.arpa> <3414@geaclib.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 49

In article <3414@geaclib.UUCP>, dave@lethe.UUCP (David Collier-Brown) writes:
> > In article <124300001@inmet> stt@inmet writes:
> >>A simple language extension would be:
> >>
> >>#include_once "classname.h"
> [definition of include_once as equivalent to
>  #ifdef classname_h
>  #  include "classname.h"
>  #  define classname_h
>  #endif
> ]
#include_once is fundamentally different from #ifndef...#endif:

	#include_once	idempotency controlled by includer
	#ifndef...	idempotency controlled by includee

an include file intended to be included more than once is usually
written as such, so #include_once seems to be placing the control
in the wrong place

from an efficiency viewpoint, though, #include_once may be a win since
the include file contents need not be scanned

taking this into account, an experimental preprocessor, used by some
internal development organizations, has been doing the following
for the past year with no problems (other than stretching the dpANSI C
#include ``implementation defined'' meaning):

(1)	by default each #include file is included only once
(2)	a run time option forces the old behavior
(3)	a file to be included more than once must contain the statement
	``#pragma pp:multiple'' that is evaluated during the first include

comments:

(1)	how are files identified? does "x" == <x>, does "./x" == "x",
	with symlinks and ``ln -s a b'', does "a" == "b" (note that
	<dev,ino> is not a valid identity check on some distributed
	file system implementations)
(3)	pick your favorite #pragma syntax

this mechanism allows one to write headers in a dpANSI C comforming
manner while still exploiting efficient pp implementations:

(a)	idempotent include files are wrapped with #ifndef...#endif
(b)	multiply included headers contain ``#pragma pp:multiple''
-- 
Glenn Fowler    (201)-582-2195    AT&T Bell Laboratories, Murray Hill, NJ
uucp: {att,decvax,ucbvax}!ulysses!gsf       internet: gsf@ulysses.att.com