Re: structs in Mutable containers
Re: structs in Mutable containers
- Subject: Re: structs in Mutable containers
- From: "Sean McBride" <email@hidden>
- Date: Tue, 16 Sep 2008 12:57:35 -0400
- Organization: Rogue Research
On 9/16/08 9:45 AM, Clark Cox said:
>> NSPoint and CGPoint may be the same now, but they may not always be.
>> (Consider that NSAffineTransformStruct and vImage_AffineTransform were
>> the same, but the former changed from float to CGFloat and the latter
>> stayed float, even in 64 bit.)
>>
>> Also, they are separate structs to the compiler, so it is free (though
>> unlikely) to align and pack them differently.
>
>No, it is not. The C standard guarantees that two structs with the
>same initial sequence of members have the same layout as far as the
>common members are concerned. The following is perfectly legal code:
>
>#include <stdio.h>
>
>typedef struct Foo { float x, y; } Foo;
>typedef struct Bar { float x, y; } Bar;
>
>int main() {
> Foo f = {123,456};
>
> //Treat f as if it is a Bar:
> printf("x = %f\n", ((Bar*)&f)->x);
> printf("y = %f\n", ((Bar*)&f)->y);
>
> return 0;
>}
Well, I am certainly no language lawyer. Perhaps I am confusing
things. I am thinking of gcc's "warning: dereferencing type-punned
pointer will break strict-aliasing rules". For example, compile your
snippit above like so:
$ gcc-4.2 -Wall -Wextra -fstrict-aliasing test.m
test.m: In function 'main':
test.m:10: warning: dereferencing type-punned pointer will break strict-
aliasing rules
test.m:11: warning: dereferencing type-punned pointer will break strict-
aliasing rules
This is what I was remembering:
<http://www.cellperformance.com/mike_acton/2006/06/
understanding_strict_aliasing.html#introduction>
--
____________________________________________________________
Sean McBride, B. Eng email@hidden
Rogue Research www.rogue-research.com
Mac Software Developer Montréal, Québec, Canada
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