Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!uw-june!pardo
From: pardo@cs.washington.edu (David Keppel)
Newsgroups: gnu.gcc
Subject: Re: #pragma once
Message-ID: <9754@june.cs.washington.edu>
Date: 9 Nov 89 17:37:30 GMT
References: <8910300524.AA11104@sugar-bombs.ai.mit.edu> <1989Nov1.094352.24529@paris.ics.uci.edu> <522@loft386.UUCP> <305@jhereg.Minnetech.MN.ORG>
Reply-To: pardo@june.cs.washington.edu (David Keppel)
Distribution: gnu
Organization: University of Washington, Computer Science, Seattle
Lines: 50

mark@jhereg.UUCP (Mark H. Colburn) writes:
>[Why `#pragma once'?  What's wrong with
>	#ifndef TOKEN
>	#define TOKEN
>	    ...
>	#endif TOKEN
>]

It's slow, particularly if you have nested includes.  The problem is
that with `#ifndef TOKEN ... #endif' the entire file must be scanned
each place that the #include directive appears, even if the #include
is redundent.  With `#pragma once', the compiler can avoid redundent
rescans.

An portable alternative convention avoids the overhead of rescanning
but requires more programmer work and allows more typos:

	#ifndef TOKEN
	#   include TOKEN
	#endif

I ususually combine the latter with lines in the .h that look like

	#ifdef TOKEN
	{ TOKEN multiply defined! }
	#else
	#define TOKEN
	    ...
	#endif

Alternatively, you can just do `#ifndef TOKEN ... #endif' and let
the compile go slow if you (somebody) does some repeated includes.

To summarize: `#pragma once' provides a facility that is available by
other mechanisms.  The advantage of `#pragma once' is that information
about the use of the .h is not spread through both the .h and the .c.
Using `#pragma once' does not compromise portability if the `#prgama
once' is ifdeffed.

#ifdef __STDC__
    #pragma once
#endif

Note that the octothorpe ('#') should be indented to keep older
compilers from gagging.

	;-D on  ( #pragma once upon a time )  Pardo
-- 
		    pardo@cs.washington.edu
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