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Re: C11 6.3.2.3p7 Referenced Type

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On 06/01/2013 11:30 PM, Shao Miller wrote:
> On 1/6/2013 22:13, Wojtek Lerch wrote:
>>
>>    int main( void ) {
>>      if ( _Alignof(int) <= _Alignof(float) ) {
>>        float f;
>>        int n, *p = (int*) &f;
>>        if ( p == &n )
>>          puts( "They're equal!" );
>>      }
>>    }
>>
>> If this program produces output on some implementation, which
>> requirement of the standard does the implementation violate?  (Chapter
>> and verse please.)
...
> If I've followed along correctly, the question is: Is a converted
> pointer defined to point to the same object as the original, or only the
> result of its conversion back to the original pointer type?

Yes.  And if it's the first one, then where does the standard say it?

> I don't see why this example should be prohibited.
>   'n' is never
> accessed, so an implementation could choose to put it in the same place
> where 'f' is.

The implementation is free to completely optimize them out and not put 
them anywhere at all, but that does not change the fact that in the 
abstract machine their addresses must be different.  If I changed the 
check to "if ( (char*)&f == (char*)&n )", it would be required to return 
false, regardless of whether f and n are accessed or not.  Of course, 
any decent implementation will generate the same code that it would if I 
wrote if(0), without any actual comparing of addresses at runtime.

How about this then:

   float f;
   int n, *p = (int*) &f;
   char *x = (char*)&f, *y = (char*) p;
   if ( x != y ) ...

In other words, does the standard guarantee that (char*)(int*)&x, if 
defined, must be equal to (char*)&x?


>   As an implementation, I'd enjoy that kind of freedom, for
> conservation of storage.

Sure, as long you follow the observable behaviour of the abstract 
machine.  In the abstract machine, n and f are distinct objects and they 
don't overlap.

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C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-04 02:08 -0500
  Re: C11 6.3.2.3p7 Referenced Type Keith Thompson <kst-u@mib.org> - 2013-01-04 08:09 -0800
    Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-04 12:25 -0500
    Re: C11 6.3.2.3p7 Referenced Type lawrence.jones@siemens.com - 2013-01-04 13:47 -0500
      Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-04 14:56 -0500
        Re: C11 6.3.2.3p7 Referenced Type lawrence.jones@siemens.com - 2013-01-04 17:10 -0500
          Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-04 20:34 -0500
  Re: C11 6.3.2.3p7 Referenced Type Wojtek Lerch <wojtek_l@yahoo.ca> - 2013-01-04 23:45 -0500
    Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-05 02:07 -0500
      Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-05 02:16 -0500
    Re: C11 6.3.2.3p7 Referenced Type Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 17:25 -0800
      Re: C11 6.3.2.3p7 Referenced Type Wojtek Lerch <wojtek_l@yahoo.ca> - 2013-01-06 22:13 -0500
        Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-06 23:30 -0500
          Re: C11 6.3.2.3p7 Referenced Type Wojtek Lerch <wojtek_l@yahoo.ca> - 2013-01-07 00:07 -0500
            Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-07 01:29 -0500
              Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-07 02:18 -0500
              Re: C11 6.3.2.3p7 Referenced Type Wojtek Lerch <wojtek_l@yahoo.ca> - 2013-01-10 17:45 -0500
          Re: C11 6.3.2.3p7 Referenced Type Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 22:30 -0800
            Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-07 01:58 -0500
              Re: C11 6.3.2.3p7 Referenced Type Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 23:16 -0800
        Re: C11 6.3.2.3p7 Referenced Type Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 22:02 -0800

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