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Re: null pointer constant, address constant

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On 12/18/2012 07:44 PM, Szabolcs Nagy wrote:
...
>> Can you give an example where it was reasonable, in a context where an
>> ICE was required, to use an expression that pushed (or exceeded) the
>> limits on what is permitted for an ICE?
> 
> it is required for an elaborate hack to workaround a
> silliness of the standard that is tgmath.h
> 
> (ie. i have an implementation of tgmath.h which is
> done with minimal compiler support using the typeof
> extension present in many compilers)
> (yes c11 _Generic could solve the issue but is not
> practical as it is not supported on the compilers i
> care about and i cannot modify the compilers, i'm
> only implementing the standard library)
> 
> while testing the implementation it turned out that the
> tested compilers disagreed about what is an ICE or a
> null pointer constant (so it's not that trivial matter
> after all)

That is, I hope you'll admit, a rather unusual situation.

> eg on some compilers
> 
>  #define __is_integral(x) !((1?1:x)/2)
>  __is_integral(1.0)
> 
> is an ICE and null pointer constant on others not

Because 1.0 is not the immediate operand of a cast, that doesn't qualify
as an ICE, and therefore does not qualify as an NPC. The compilers that
disagree are wrong.

How did you use this macro, such that it needed to be an ICE or an NPC?

>  0 ? (double*)0 : 0 ? (void*)0 : (void*)0
> 
> has type double* on some compilers on others void*

The inner ?: expression is a constant expression with a null pointer
value, but not a null pointer constant. Therefore, the outer ?:
expression should have the type void*.

> none of the tested compilers are completely standard
> conformant (gcc, clang) which means i can implement
> tgmath.h with typeof only, if they were conformant i
> could not

I believe that gcc can fully conform to C90, and probably to C99, with
the right compiler options. I thought the same was true of clang, but
I'm not as familiar with that compiler. If you need to use the compiler
in a non-conforming mode, I suppose it doesn't help to know that it also
has a conforming mode.
-- 
James Kuyper

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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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