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From: dld@F.GP.CS.CMU.EDU (David Detlefs)
Newsgroups: gnu.gcc
Subject: Re: Is "#pragma once" necessary?
Message-ID: <DLD.89Jul26124108@F.GP.CS.CMU.EDU>
Date: 26 Jul 89 16:41:08 GMT
References: <DLD.89Jul25130101@F.GP.CS.CMU.EDU> <GRUNWALD.89Jul25124852@flute.cs.uiuc.edu>
Organization: CMU CS Department
Lines: 41
In-reply-to: grunwald@flute.cs.uiuc.edu's message of 25 Jul 89 17:51:01 GMT

In response to my suggestion about modify cpp to get the efficiency of
"#pragma once" without the change in semantics, Dirk Grunwald writes:

>hacking cpp to recognize #ifndef sounds like even more of a problem;
>people have different conventions on where they place the #ifndef,
>the #define and so on.

I don't see the problem.  cpp can certainly distinguish between
comments and actual program text.  If you write a foo.h file of the form

--------------------------------------------------
<comments and whitespace>*
#ifndef FOO_H
<anything at all -- note that it doesn't matter where, or even *if*,
the #define FOO_H occurs>
#endif
<comments and whitespace>*
--------------------------------------------------
then cpp is certainly safe in disregarding subsequent includes of
foo.h that occur while FOO_H is defined.  If you have anything other
than a comment or whitespace before the #ifndef, or after the
corresponding #endif, then cpp might not be safe in ignoring
subsequent includes.  There is certainly no difficulty in recognizing
file of this form, and I believe that pretty much all files using the
#ifndef convention fit it.

As for #requires and #provides, I think we have to recognize that
we're going to exist for a long time in a sea of previously existing
code, and we shouldn't change a perfectly good mechanism without good
reason.  I fail to see adequate advantages with #requires and
#provides (though this does not of course imply thay such advantages
do not exist.)




--
Dave Detlefs			Any correlation between my employer's opinion
Carnegie-Mellon CS		and my own is statistical rather than causal,
dld@cs.cmu.edu			except in those cases where I have helped to
				form my employer's opinion.  (Null disclaimer.)