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From: gsf@ulysses.homer.nj.att.com (Glenn Fowler[drew])
Newsgroups: comp.lang.c++
Subject: Re: #once cpp directive implemented!
Summary: #once -> #pragma once
Message-ID: <11338@ulysses.homer.nj.att.com>
Date: 20 Mar 89 14:17:43 GMT
References: <124@riunite.ACA.MCC.COM>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 50

In article <124@riunite.ACA.MCC.COM>, rfg@riunite.ACA.MCC.COM (Ron Guilmette) writes:
> As someone pointed out recently, the subject of multiple
> inclusion of individual include files was beaten to death
> here not long ago.
> At the risk of starting it all up again, I would like to
> announce the availability of patches for the the GNU C
> preprocessor (version 1.34) which implement the new #once
> directive.  These patches may be aquired by simply sending
> me mail and requesting the patches.  I hope that these patches
> will soon become a standard part of the GNU releases, but till
> then, I will distribute them.

implementing
	#once
as
	#pragma once 
would be more in line with the current standards efforts
since #pragma's may/must be ignored by certain implementations

the paradigm would be
	#ifndef <magic_symbol>
	#define <magic_symbol>
	#pragma once
	...
	#endif
this would allow for:
(1)	portability -- works for all implementations that either
	ignore #pragma's or that implement once
(2)	efficiency for those implementations that implement once
(3)	compatibility with those implementations that implement
	the equivalent of once by detecting #ifndef...#endif
	wrapped include files
(4)	correctness for any errors/choices in the file identity
	algorithm (excuse the os-dependent examples):
		ln a.h b.h
		# "a.h" == "b.h" ?
		ln -s a.h b.h
		# "a.h" == "b.h" ?
		cd /tmp
		echo '' > stdio.h
		echo '#include "stdio.h"' > a.c
		echo '#include <stdio.h>' >> a.c
		# "stdio.h" == <stdio.h> ?
		# "./a.h" == "a.h" ?
		# "$PWD/a.h" == "./a.h" ? (where $PWD really expanded)
	note that with NFS and RFS the same physical file accessed
	by different paths may have different <dev,ino>
-- 
Glenn Fowler    (201)-582-2195    AT&T Bell Laboratories, Murray Hill, NJ
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