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Re: NULL versus 0

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Bo Persson <bo@bo-persson.se> writes:
> On 2021-09-26 at 09:47, Branimir Maksimovic wrote:
>> On 2021-09-26, Jorgen Grahn <grahn+nntp@snipabacken.se> wrote:
>>> On Sat, 2021-09-25, see.my....@gmail.com wrote:
>>>>> int *pInt = 0;
>>>>>
>>>>> Nothing inherently wrong with that, but in C, it would be more
>>>>> traditional to use NULL.
>>>>
>>>> It would be more traditional in C++, as well.
>>>
>>> Opinions varied.  Before nullptr, I always used NULL in C and 0 in
>>> C++.  IIRC using 0 was a common recommendation here, too, for whatever
>>> reason.
>> #define NULL (void*)0
>> simple, problem is that in C++, one have to cast from void
>> and that does not works, and this is BuG in C++.
>> 
>  It is not a bug when it is done very much on purpose.
>
> Stroustrup says:
>
> "A pointer to any type of object can be assigned to a variable of type
> void*, a void* can be assigned to another void*, void* can be compared 
> for equality and inequality, and a void* can be explicitly converted
> to another type. Other operations would be unsafe because the compiler 
> cannot know what kind of object is really pointed to. Consequently,
> other operations result in compile-time errors."

"The C++ Programming Language", 4th edition, section 7.2.1.

As Stroustrup says, converting from void* to another pointer type
requires an explicit conversion, but converting NULL to any pointer
type does not.  (void*)0 is a null pointer constant in C, but not in C++.

This:
    int *p;
    p = (void*)0;
is invalid in C++, but this:
    int *p;
    p = NULL;
is valid.  Because of that, a C++ implementation that defines NULL as
either (void*)0 or ((void*)0) is non-conforming.  (It's unlikely that
any actual C++ implementations make that mistake.)

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */

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NULL versus 0 "das...@gmail.com" <dashley@gmail.com> - 2021-09-25 11:03 -0700
  Re: NULL versus 0 Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-25 18:16 +0000
  Re: NULL versus 0 Paavo Helde <myfirstname@osa.pri.ee> - 2021-09-25 21:20 +0300
    Re: NULL versus 0 Paavo Helde <myfirstname@osa.pri.ee> - 2021-09-25 21:32 +0300
      Re: NULL versus 0 "das...@gmail.com" <dashley@gmail.com> - 2021-09-25 12:08 -0700
    Re: NULL versus 0 Juha Nieminen <nospam@thanks.invalid> - 2021-09-27 05:47 +0000
      Re: NULL versus 0 Bo Persson <bo@bo-persson.se> - 2021-09-27 08:32 +0200
        Re: NULL versus 0 "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-09-26 23:37 -0700
        Re: NULL versus 0 "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-09-27 10:32 +0200
          Re: NULL versus 0 Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-27 11:32 -0700
      Re: NULL versus 0 James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-09-27 15:03 -0400
  Re: NULL versus 0 "see.my....@gmail.com" <see.my.homepage@gmail.com> - 2021-09-25 13:05 -0700
    Re: NULL versus 0 Jorgen Grahn <grahn+nntp@snipabacken.se> - 2021-09-26 06:58 +0000
      Re: NULL versus 0 Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-26 07:47 +0000
        Re: NULL versus 0 Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-26 02:15 -0700
          Re: NULL versus 0 Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-26 09:38 +0000
            Re: NULL versus 0 Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-26 15:03 -0700
              Re: NULL versus 0 Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-29 02:29 +0000
                Re: NULL versus 0 Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-28 20:34 -0700
                Re: NULL versus 0 David Brown <david.brown@hesbynett.no> - 2021-09-29 08:52 +0200
        Re: NULL versus 0 Bo Persson <bo@bo-persson.se> - 2021-09-26 13:54 +0200
          Re: NULL versus 0 Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-26 15:17 -0700
          Re: NULL versus 0 Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-29 02:26 +0000
      Re: NULL versus 0 Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-26 02:17 -0700
      Re: NULL versus 0 HorseyWorsey@the_stables.com - 2021-09-26 14:20 +0000
  Re: NULL versus 0 James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-09-25 17:36 -0400
  Re: NULL versus 0 "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-09-26 23:05 -0700

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