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Re: offsetOf

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Ben Bacarisse wrote:

> Jack Boot <jackboot@mailinator.com> writes:
> 
>> This came up in a code review recently, I've never heard of it before,
>> opinions welcome.
>>
>> There is a struct that looks a bit like this
>>
>> struct foo {
>>   int a[10];
>>   long b;
>>   ...
>> };
>>
>> Now I have a function that starts thusly:
>>
>> void
>> bar ( struct foo * f )
>> {
>>   int * ip = (int *) foo;
>>   ...
>> }
>>
>> The reviewer recommended, that for maximal portability I should replace
>> this by:
>>
>> int * ip = ( (int *) ( ( (unsigned char *) foo ) + offsetOf(struct foo,
>> a) ) );
>>
>> I've never seen this construction before, also I think type punning is
>> inherently non-portable so I'm not sure how much this buys you.
> 
> Nothing but obfuscation.   offsetOf(struct foo, a) is 0 (it can't be
> otherwise) so this just converts to a char * first.  This won't hurt but
> it won't help either.  This:
> 
>   int *ip = f->a;
> 
> is, to my mind, the clearest way to write it.  I can't see why it would
> be done with any casts at all.

Thanks for all the replies.

I wrote the code with the cast because I used struct foo as an Opaque Type 
in that file (only using pointers, the header with the struct definition 
not #include'd) to emulate C++-style encapsulation. Therefore I could not 
dereference f to get f->a.

The reviewer was concerned that there might be padding before a. I'm not 
sure any compiler would need to insert padding around int types, though I 
guess it's possible on a 64-bit arch with 32-bit ints. Anyways, to use his 
offsetOf solution you need to include the struct def, so in that case I 
agree with all you guys that f->a would be preferable.

Cheers,
JB

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offsetOf Jack Boot <jackboot@mailinator.com> - 2011-07-23 20:54 +0000
  Re: offsetOf Ian Collins <ian-news@hotmail.com> - 2011-07-24 09:05 +1200
  Re: offsetOf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-23 23:03 +0100
    Re: offsetOf Jack Boot <jackboot@mailinator.com> - 2011-07-24 10:30 +0000
      Re: offsetOf Ian Collins <ian-news@hotmail.com> - 2011-07-24 22:41 +1200
      Re: offsetOf James Kuyper <jameskuyper@verizon.net> - 2011-07-24 08:36 -0400
      Re: offsetOf jt@toerring.de (Jens Thoms Toerring) - 2011-07-24 14:50 +0000
      Re: offsetOf Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-24 10:28 -0600
      Re: offsetOf Nobody <nobody@nowhere.com> - 2011-07-24 17:30 +0100
      Re: offsetOf gordonb.gkpc8@burditt.org (Gordon Burditt) - 2011-07-27 08:43 -0500
    Re: offsetOf Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-07-24 14:18 +0300
      Re: offsetOf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-24 12:50 +0100
        Re: offsetOf Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-07-24 18:25 +0300
  Re: offsetOf Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-07-23 20:12 -0400
  Re: offsetOf "J. J. Farrell" <jjf@bcs.org.uk> - 2011-07-24 01:57 +0100

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