Re: question about nullptr

From	Ben Bacarisse <ben@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: question about nullptr
Date	2024-07-09 11:14 +0100
Organization	A noiseless patient Spider
Message-ID	<874j8yswha.fsf@bsb.me.uk> (permalink)
References	(2 earlier) <v6bt15$3svoi$1@dont-email.me> <v6iik7$1948o$1@dont-email.me> <v6iklk$19cv8$1@dont-email.me> <v6inai$19q6r$1@dont-email.me> <v6insm$19uag$1@dont-email.me>

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"Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes:

> p = nullptr;

(p having been declared void *)

> (0 == p == nullptr == NULL == 0) == true ?
>
> Am I missing something here? If so, here is a preemptive: Shit!

You are missing that 0 == p == nullptr == NULL == 0 does not mean what
you want!  It means

 (((0 == p) == nullptr) == NULL) == 0

and that is a constraint violation.

Why?  Well 0 == p has value 1 and is of type int and equality
comparisons between int and nullptr_t values (of which there is only
one) are not permitted.  Catching this sort of thing is one of the
benefits of nullptr and its associated type nullptr_t.  It means that
while

#define nullptr ((void *)0)

can help to get C23 code to compile with a pre-C23 compiler, it might
result in some C23 constraint violations going undetected.

Anyway, that aside, I know what you meant.  To clarify, all of the
following have the value 1:

  0 == p
  p == nullptr
  nullptr == NULL
  NULL == 0
  0 == nullptr
  !p

Note that 0 == nullptr /is/ allowed even though 0 has type int.  That is
because 0 is also a null pointer constant, and the rules for == and !=
specifically allow it.

-- 
Ben.

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question about nullptr Thiago Adams <thiago.adams@gmail.com> - 2024-07-06 08:49 -0300
  Re: question about nullptr Bonita Montero <Bonita.Montero@gmail.com> - 2024-07-06 14:13 +0200
  Re: question about nullptr John McCue <jmccue@neutron.jmcunx.com> - 2024-07-06 12:33 +0000
  Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-06 12:54 +0000
    Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-06 15:07 +0200
      Re: question about nullptr scott@slp53.sl.home (Scott Lurndal) - 2024-07-06 14:04 +0000
        Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-06 16:42 +0200
          Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-06 16:45 +0200
        Re: question about nullptr Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-07-06 10:50 -0700
          Re: question about nullptr scott@slp53.sl.home (Scott Lurndal) - 2024-07-06 23:04 +0000
            Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-06 20:06 -0700
        Re: question about nullptr bart <bc@freeuk.com> - 2024-07-06 19:20 +0100
          Re: question about nullptr scott@slp53.sl.home (Scott Lurndal) - 2024-07-06 23:09 +0000
          Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-06 16:31 -0700
        Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-06 13:23 -0700
          Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-06 13:28 -0700
            Re: question about nullptr Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-07-07 08:01 +0000
              Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-08 22:08 -0700
          Re: question about nullptr Richard Damon <richard@damon-family.org> - 2024-07-06 17:04 -0400
            Re: question about nullptr scott@slp53.sl.home (Scott Lurndal) - 2024-07-06 23:10 +0000
          Re: question about nullptr scott@slp53.sl.home (Scott Lurndal) - 2024-07-06 23:03 +0000
          Re: question about nullptr James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-06 21:44 -0400
            Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-06 19:29 -0700
              Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-07 12:53 -0700
          Re: question about nullptr Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-07-07 08:03 +0000
        Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-06 23:14 +0100
          Re: question about nullptr scott@slp53.sl.home (Scott Lurndal) - 2024-07-06 23:13 +0000
            Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-06 20:10 -0700
            Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-07 08:46 +0000
            Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-07 19:59 +0100
              Re: question about nullptr scott@slp53.sl.home (Scott Lurndal) - 2024-07-07 19:40 +0000
                Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-07 15:17 -0700
                Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-08 20:11 +0200
                Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-08 13:32 -0700
                Re: question about nullptr Michael S <already5chosen@yahoo.com> - 2024-07-08 22:28 +0300
                Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-08 15:23 -0700
                Re: question about nullptr Michael S <already5chosen@yahoo.com> - 2024-07-09 10:48 +0300
                Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-09 04:32 -0700
                Re: question about nullptr Thiago Adams <thiago.adams@gmail.com> - 2024-07-09 09:10 -0300
                Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-09 07:52 -0700
                Re: question about nullptr Thiago Adams <thiago.adams@gmail.com> - 2024-07-09 13:16 -0300
                Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-09 14:22 -0700
                Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-09 22:25 +0000
                Re: question about nullptr Thiago Adams <thiago.adams@gmail.com> - 2024-07-10 08:50 -0300
                Re: question about nullptr Michael S <already5chosen@yahoo.com> - 2024-07-09 15:50 +0300
                Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-09 07:50 -0700
                Re: question about nullptr David Brown <david.brown@hesbynett.no> - 2024-07-09 18:19 +0200
                Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-09 02:49 +0000
                Re: question about nullptr Michael S <already5chosen@yahoo.com> - 2024-07-09 11:00 +0300
                Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-09 08:21 +0000
                Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-09 03:57 -0700
                Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-08 20:17 -0700
                Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-08 21:01 -0700
                Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-08 22:06 -0700
                Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-10 07:51 -0700
                Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-10 14:23 -0700
                Re: question about nullptr James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-10 18:50 -0400
                Re: question about nullptr scott@slp53.sl.home (Scott Lurndal) - 2024-07-08 14:09 +0000
                Re: question about nullptr Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-07-08 21:45 -0700
                Re: question about nullptr Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-07-08 21:49 -0700
                Re: question about nullptr Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-07-11 02:42 +0000
                Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-08 22:58 -0700
                Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-10 07:36 -0700
                Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-10 14:15 -0700
                Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-08 20:07 +0200
                Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-08 23:55 +0100
                Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-10 17:28 +0200
                Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-11 00:25 +0100
                Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-12 14:08 +0200
                Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-13 00:59 +0100
                Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-13 04:01 +0200
                Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-15 00:00 +0100
                Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-13 04:18 +0200
                Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-17 09:26 -0700
                Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-08 12:29 +0200
                Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-08 17:23 +0100
                Re: question about nullptr James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-08 11:03 -0400
                Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-09 02:45 +0000
                Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-08 11:08 +0200
                Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-08 11:18 +0100
                Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-09 02:32 +0000
                Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-08 22:02 -0700
                Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-09 10:21 +0100
                Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-10 17:32 -0700
                Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-11 02:12 +0100
                Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-10 18:41 -0700
                Re: question about nullptr Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-07-11 02:38 +0000
                Re: question about nullptr Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-07-12 14:11 +0200
                Re: question about nullptr Richard Damon <richard@damon-family.org> - 2024-07-08 07:18 -0400
                Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-08 07:19 +0000
                Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-08 00:42 +0100
          Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-06 20:36 -0700
      Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-06 22:11 +0000
    Re: question about nullptr bart <bc@freeuk.com> - 2024-07-06 14:51 +0100
      Re: question about nullptr Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-07-07 04:55 +0000
        Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-07 00:04 -0700
        Re: question about nullptr bart <bc@freeuk.com> - 2024-07-07 18:40 +0100
    Re: question about nullptr Richard Harnden <richard.nospam@gmail.invalid> - 2024-07-12 14:19 +0100
      Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-12 21:52 +0000
        Re: question about nullptr scott@slp53.sl.home (Scott Lurndal) - 2024-07-12 22:27 +0000
          Re: question about nullptr Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-12 23:28 +0000
            Re: question about nullptr scott@slp53.sl.home (Scott Lurndal) - 2024-07-13 00:10 +0000
              Re: question about nullptr James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-12 20:28 -0400
              Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-12 17:30 -0700
      Re: question about nullptr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-07-13 17:21 -0700
      Re: question about nullptr Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-07-15 22:51 +0000
        Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-15 16:32 -0700
        Re: question about nullptr James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-15 19:49 -0400
          Re: question about nullptr Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-07-15 18:22 -0700
  Re: question about nullptr Richard Damon <richard@damon-family.org> - 2024-07-06 10:39 -0400
    Re: question about nullptr David Brown <david.brown@hesbynett.no> - 2024-07-06 18:57 +0200
      Re: question about nullptr Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-07-07 08:00 +0000
        Re: question about nullptr David Brown <david.brown@hesbynett.no> - 2024-07-07 12:07 +0200
      Re: question about nullptr Thiago Adams <thiago.adams@gmail.com> - 2024-07-07 08:40 -0300
      Re: question about nullptr Richard Harnden <richard.nospam@gmail.invalid> - 2024-07-09 06:42 +0100
        Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-08 23:17 -0700
          Re: question about nullptr Richard Harnden <richard.nospam@gmail.invalid> - 2024-07-09 08:02 +0100
            Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-09 00:12 -0700
              Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-09 11:14 +0100
                Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-09 13:24 -0700
                Re: question about nullptr "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-07-09 13:28 -0700
                Re: question about nullptr Ben Bacarisse <ben@bsb.me.uk> - 2024-07-10 14:27 +0100
                Re: question about nullptr James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-10 11:09 -0400

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