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From: bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: question about nullptr
Date: Sun, 7 Jul 2024 18:40:02 +0100
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On 07/07/2024 05:55, Lawrence D'Oliveiro wrote:
> On Sat, 6 Jul 2024 14:51:19 +0100, bart wrote:
> 
>> Using actual zero for a pointer value is crass. This wouldn't work for
>> example:
>>
>>      char *p = 3;
> 
> But of course this does:
> 
>      char *p = 0;
> 
>  From the C23 spec, I found this footnote in §6.6:
> 
>      A named constant or compound literal constant of integer type and
>      value zero is a null pointer constant. A named constant or
>      compound literal constant with a pointer type and a value null is
>      a null pointer but not a null pointer constant; it may only be
>      used to initialize a pointer object if its type implicitly
>      converts to the target type.
> 
> That first sentence is so important, you’d think it would be in the main
> text somewhere.

It doesn't tell the full story, as you can have any arbitrary expression 
of such terms that results in a suitable value:

      char* p = width*height;

This is valid if width/height are enums and that area calculation yields 
zero. But update values slightly and it no longer compiles.