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Re: null pointer constant, address constant

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On 12/20/2012 10:33 AM, James Kuyper wrote:
> On 12/20/2012 08:34 AM, christian.bau wrote:
>> On Dec 18, 11:52�pm, Keith Thompson <ks...@mib.org> wrote:
>>
>>> The standard says that a constant arithmetic expression with the value
>>> 0, converted to a pointer type, yields a null pointer. �It does *not*
>>> say the same thing about a non-constant arithmetic expression with the
>>> value 0.
>>
>> It doesn't have to. The standard doesn't have to state something
>> explicitly if it follows from other rules that are stated.
>>
>> Converting _any_ integer to _any_ pointer gives an implementation-
>> defined result.
> 
> Not so. "An integer may be converted to any pointer type. Except as
> previously specified, the result is implementation-defined, ..."
> 6.3.2.3p5. Since null pointer constants are described in 6.3.2.3p4, they
> are covered by "Except as previously specified". Conversion of
> non-constant arithmetic expressions with a value of 0 is NOT covered by
> that exception. This means that the implementation-defined result of
> such a conversion need not match the standard-defined result for null
> pointer constants.

After thinking about it a little while, I realized I hadn't worded that
quite right, and that my citation was incorrect. Here's a corrected version:

Since null pointers are described in 6.3.2.3p3, they are covered by
"Except as previously specified". Conversions of integer expressions
with a value of 0 to pointer types which are not covered by 6.3.2.3p3
are therefore not covered by that exception, and need not be defined by
the implementation to produce the same result.

I think that you're assuming that there's some kind of consistency
requirement imposed by the standard on the implementation's definition
of the result. For instance, that it is determined solely by the
integer's value and the type of the pointer. The standard imposes no
such requirement. The result could depend upon the type of the integer.
It could depend upon the kind of expression used to produce that
integer. It could depend upon the phase of the moon, and whether or not
today is Friday the 13th. It could be completely random. I can't come up
with a good reason for any of those things to be true, but it would not
render the implementation non-conforming if they were.

If there were such a consistency requirement, it would depend upon the
precise wording of that requirement whether or not it requires
consistency between the result, when it's implementation-defined, and
the result when the implementation has no obligation to provide a
definition, because it's standard-defined (by 6.3.2.3p3). However, that
issue doesn't come up, because there is no such consistency requirement.

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null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2012-12-18 13:25 +0000
  Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-18 09:53 -0500
    Re: null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2012-12-18 20:06 +0000
      Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-18 15:39 -0500
        Re: null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2012-12-19 00:44 +0000
          Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-18 20:24 -0500
            Re: null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2012-12-19 10:27 +0000
              Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-19 07:15 -0500
              Re: null pointer constant, address constant Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-12-21 03:03 +0200
                Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-21 12:13 -0500
                Re: null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2013-01-09 00:44 +0000
                Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2013-01-08 20:21 -0500
                Re: null pointer constant, address constant Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-08 20:46 -0500
          Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 20:37 -0500
      Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 16:15 -0500
        Re: null pointer constant, address constant Keith Thompson <kst-u@mib.org> - 2012-12-18 15:52 -0800
          Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 19:12 -0500
            Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 19:51 -0500
          Re: null pointer constant, address constant Keith Thompson <kst-u@mib.org> - 2012-12-18 17:02 -0800
          Re: null pointer constant, address constant "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-12-20 05:34 -0800
            Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-20 10:33 -0500
              Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-20 12:28 -0500
                Re: null pointer constant, address constant Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-20 21:13 -0800
              Re: null pointer constant, address constant Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-20 21:05 -0800
            Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-20 12:17 -0500
            Re: null pointer constant, address constant Keith Thompson <kst-u@mib.org> - 2012-12-20 12:10 -0800
              Re: null pointer constant, address constant Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-20 21:25 -0800
  Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 14:02 -0500
    Re: null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2012-12-18 20:09 +0000
      Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-18 15:54 -0500
        Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 16:18 -0500

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