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Re: offsetOf

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On 07/24/11 10:30 PM, Jack Boot wrote:
> 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.

There won't be.  As Eric said: A pointer to a struct can be converted to 
a pointer to its first element, and vice versa.

-- 
Ian Collins

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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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