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Re: Description of "." operator

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On 1/17/2013 23:19, Shao Miller wrote:
> On 1/7/2013 01:12, Keith Thompson wrote:
>> Shao Miller <sha0.miller@gmail.com> writes:
>>> On 1/6/2013 02:51, Keith Thompson wrote:
>> [...]
>>>> Shao Miller <sha0.miller@gmail.com> writes:
>>>>> On 12/7/2012 21:34, Keith Thompson wrote:
>> [...]
>>>>> 3.19:
>>>>>
>>>>>      "1 value
>>>>>         precise meaning of the contents of an object when
>>>>> interpreted as
>>>>> having a specific type"
>>>>
>>>> Like too many of the standard's "definitions", that's incomplete.
>>>> There
>>>> can be values that have nothing to do with the contents of an object.
>>>>
>>>
>>> Actually, I took it to have emphasis on "meaning."  So I agree with you,
>>> values exist independently of objects.
>>
>> Right -- and that should apply to struct types as well as to scalar
>> types.  At least, I can't think of any good reason why it shouldn't.
>>
>
> And unions too:
>
>    #include <stddef.h>
>    #include <stdio.h>
>
>    struct A {
>        int i;
>        int j;
>      };
>
>    struct B {
>        char ca[sizeof (int)];

Oops.  I meant some scalar type with the same size as 'int', not an 
array type.  Oh well.

>        int k;
>      };
>
>    union C {
>        struct A a;
>        struct B b;
>      };
>
>    int main(void) {
>        union C c1, c2;
>
>        if (offsetof(struct A, j) != offsetof(struct B, k))
>          return 0;
>
>        c1.a.j = 42;
>
>        /*
>         * Common initial sequence doesn't apply (no object)
>         * Effective type doesn't apply (no object)
>         * Type-punning applies
>         */
>        printf("%d\n", (c2 = c1).b.k);
>
>        return 0;
>      }
>
> Any problems, there?
>

Speaking of "temporary lifetime," I noticed that the following program 
yielded the same pointer value, but I would've expected them to be 
different:

#include <stdio.h>

int main(void) {
     struct { char ca[6]; } foo;

     printf("%p, ", (void *) foo.ca);
     printf("%p\n", (void *) (foo = foo).ca);
     return 0;
   }

It seems to me that the array member of the rvalue should be different 
than the array member of 'foo', since they're different objects; one is 
'foo' and one is an object with temporary lifetime.  Am I missing something?

gcc version 4.7.2 20120921 (Red Hat 4.7.2-2):

   gcc -std=c11 -pedantic -Wall -Wextra -Werror -o rvalobj rvalobj.c

clang seems to give the expected results.

clang version 3.0 (tags/RELEASE_30/final):

   clang -std=c1x -pedantic -Wall -Wextra -Werror -Wno-self-assign -o 
rvalobj rvalobj.c

Maybe it's just a GCC bug, but I'm wondering if another interpretation 
is possible.

-- 
- Shao Miller
--
"Thank you for the kind words; those are the kind of words I like to hear.

Cheerily,"  -- Richard Harter

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Thread

Description of "." operator Keith Thompson <kst-u@mib.org> - 2012-12-07 18:34 -0800
  Re: Description of "." operator Keith Thompson <kst-u@mib.org> - 2012-12-08 10:43 -0800
    Re: Description of "." operator Jens Gustedt <jens.gustedt@loria.fr> - 2012-12-10 00:35 +0100
      Re: Description of "." operator Keith Thompson <kst-u@mib.org> - 2012-12-09 20:02 -0800
  Re: Description of "." operator jgk@panix.com (Joe keane) - 2012-12-09 23:34 +0000
    Re: Description of "." operator Keith Thompson <kst-u@mib.org> - 2012-12-09 20:00 -0800
  Re: Description of "." operator Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-20 23:32 -0800
    Re: Description of "." operator Keith Thompson <kst-u@mib.org> - 2012-12-21 02:08 -0800
      Re: Description of "." operator Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-21 09:18 -0500
        Re: Description of "." operator Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-21 08:45 -0800
      Re: Description of "." operator Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-21 08:37 -0800
  Re: Description of "." operator Shao Miller <sha0.miller@gmail.com> - 2013-01-06 01:23 -0500
    Re: Description of "." operator Keith Thompson <kst-u@mib.org> - 2013-01-05 23:51 -0800
      Re: Description of "." operator Shao Miller <sha0.miller@gmail.com> - 2013-01-06 13:49 -0500
        Re: Description of "." operator Keith Thompson <kst-u@mib.org> - 2013-01-06 22:12 -0800
          Re: Description of "." operator Shao Miller <sha0.miller@gmail.com> - 2013-01-17 23:19 -0500
            Re: Description of "." operator Shao Miller <sha0.miller@gmail.com> - 2013-01-18 20:47 -0500
              Re: Description of "." operator Keith Thompson <kst-u@mib.org> - 2013-01-18 20:14 -0800
              Re: Description of "." operator jgk@panix.com (Joe keane) - 2013-01-20 20:07 +0000
                Re: Description of "." operator Shao Miller <sha0.miller@gmail.com> - 2013-01-20 15:36 -0500

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