Path: utzoo!utgpu!watmath!watdragon!rose!ccplumb
From: ccplumb@rose.waterloo.edu (Colin Plumb)
Newsgroups: gnu.gcc
Subject: Re: #pragma once
Message-ID: <18028@watdragon.waterloo.edu>
Date: 9 Nov 89 19:44:43 GMT
References: <8910300524.AA11104@sugar-bombs.ai.mit.edu> <1989Nov1.094352.24529@paris.ics.uci.edu> <522@loft386.UUCP> <305@jhereg.Minnetech.MN.ORG>
Sender: daemon@watdragon.waterloo.edu
Reply-To: ccplumb@rose.waterloo.edu (Colin Plumb)
Distribution: gnu
Organization: U. of Waterloo, Ontario
Lines: 26

In article <305@jhereg.Minnetech.MN.ORG> mark@jhereg.UUCP (Mark H. Colburn) writes:
> Ok, I'll finally admit it, I am a little confused as to why there is such a
> construct as the #pragma once.
>
> If you want to avoid including the file twice, why not use the "standard"
> method of enclosing the entire thing in "#ifdefs" as shown above:
>
> This works on any machine with a preprocessor, is portable across any
> compiler, does not use a #pragma, and is in widespread use today.  Also,
> from the discussion which has ensued here about #pragma once, I assume that
> it does not always work correctly.

No, it works fine, it's just inefficient.  The preprocessor has to open the
file, parse it, including such pathological situations as

printf("\
#endif /* \
*/");

and all to include a few carriage returns in the source file.  #pragma once
lets the prorpocessor just skip over future #includes of the same file, not
even opening the file for examination.  With ANSI type-checking, header files
are bigger than ever, and reducing time spent parsing them becomes more
important.
-- 
	-Colin