Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!cwjcc!hal!nic.MR.NET!shamash!nis!ems!amdahl!nsc!rfg
From: rfg@nsc.nsc.com (Ron Guilmette)
Newsgroups: comp.lang.c++
Subject: Re: Including header files minimally.
Message-ID: <8080@nsc.nsc.com>
Date: 27 Nov 88 08:17:56 GMT
References: <3561@pt.cs.cmu.edu> <7860@nsc.nsc.com> <3614@pt.cs.cmu.edu> <10873@ulysses.homer.nj.att.com> <1073@actnyc.UUCP> <738@quintus.UUCP> <1988Nov25.180309.9323@utzoo.uucp>
Reply-To: rfg@nsc.nsc.com.UUCP (Ron Guilmette)
Organization: National Semiconductor, Sunnyvale
Lines: 35

In article <1988Nov25.180309.9323@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>In most cases, the fact
>that #including a file n times is the same as including it once is a
>property of the file being included, not the file doing the including.

Absolutely right Henry.  I realized this very fact just before I suggested
using the sid-uid bit on header files which are not to be included more
than once.

In hindsight though, I now realize that even that was a silly suggestion.
I now think that putting something like:

	#once

or

	#pragma once

into such header files would be more appropriate, more clear, and a more
portable solution.

>The probability of trouble and mistakes will be much lower if it's the
>file being included that determines whether future inclusions have any
>effect.

Exactly right.

>The Waterloo "#pragma idempotent" strikes me as the right method;
>among other things, it means that your code is portable.

Is this (semantically) the same thing as I am suggesting?  I have never
heard of this before!  If it has the effect of forcing the file it is
found within to only be included once then I guess that there is at least
one clear (although not yet standard) precedent for an approach to this
problem.