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From: Ben Bacarisse <ben@bsb.me.uk>
Newsgroups: comp.lang.c
Subject: Re: question about nullptr
Date: Tue, 09 Jul 2024 11:14:41 +0100
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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.