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Re: #import, #pragma once yadda yadda mumble grumble

From: Geoffrey Keating <geoffk at apple dot com>
To: Neil Booth <neil at daikokuya dot co dot uk>
Cc: Devang Patel <dpatel at apple dot com>, Jonathan Lennox <lennox at cs dot columbia dot edu>, gcc-patches at gcc dot gnu dot org, Mike Stump <mrs at apple dot com>, Nicola Pero <nicola at brainstorm dot co dot uk>, Stan Shebs <shebs at apple dot com>, Zack Weinberg <zack at codesourcery dot com>, David Ayers <d dot ayers at inode dot at>
Date: Wed, 30 Jul 2003 13:44:20 -0700
Subject: Re: #import, #pragma once yadda yadda mumble grumble

On Wednesday, July 30, 2003, at 01:07 PM, Neil Booth wrote:

Devang Patel wrote:-

Your patch changes semantics for following example.

	#include "a.h"
	#import "a.h"
	#include "a.h"

And a.h does not have include guards.

FSF GCC 3.4 before and after your patch behaves differently.

*If* we consider Darwin compilers as reference in this case then
following
compiler versions include content of a.h only once in this example.
- Darwin GCC 2.95 + Apple preprocessor
- Darwin GCC 2.95 + GNU preprocessor
- Darwin GCC 3.1 + Apple preprocessor
- Darwin GCC 3.1 + GNU preprocessor
- Darwin GCC 3.3


Does this matter to Apple?  If so, I'm willing to change it, it
only makes #import more expensive, encouraging people to use
header guards 8-)

Yes, unfortunately, some Apple code relies on it. (Most Apple headers have guards anyway, so the performance issue isn't so much of a problem.)

--
Geoff Keating <geoffk@apple.com>

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