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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: question about nullptr
Date: Sun, 25 Aug 2024 18:18:23 -0700
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James Kuyper <jameskuyper@alumni.caltech.edu> writes:
> On 8/25/24 16:56, dave_thompson_2@comcast.net wrote:
>> On Wed, 10 Jul 2024 11:09:41 -0400, James Kuyper
>> <jameskuyper@alumni.caltech.edu> wrote:
>> 
>>> NULL is required to expand
>>> into a null pointer constant ... 0 and (void*)0 are the
>>> two most likely and common choices.
>>>
>> ((void*)0) 
>> 
>> Otherwise NULL["foo"] gives quite the wrong result
>
> Correct. Sorry.

A mostly meaningless price of trivia: In C17 and earlier, an excessively
literal reading of the standard implies that ((void*)0) is not a null
pointer constant.  It says that a null pointer constant is "An integer
constant expression with the value 0, or such an expression cast to type
void *".  It does not say that a parenthesized null pointer constant is
a null pointer constant.  (And (void*)0 is a null pointer constant but
not a valid definition for NULL.)

C23 fixes this by updating the wording for parenthesized expressions.

C17: "A *parenthesized expression* is a primary expression. Its type and
value are identical to those of the unparenthesized expression. It is an
lvalue, a function designator, or a void expression if the
unparenthesized expression is, respectively, an lvalue, a function
designator, or a void expression."

C23: "A *parenthesized expression* is a primary expression. Its type,
value, and semantics are identical to those of the unparenthesized
expression."

This was surely the intent all along.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */