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Re: does a double cast to unsigned makes sense in any circumstance?

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"james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> writes:

> On Thursday, January 20, 2022 at 10:37:33 PM UTC-5, Tim Rentsch wrote:
>
>> James Kuyper <james...@alumni.caltech.edu> writes:
>
> ...
>
>>> Note that while members of the basic character set are
>>> required to be represented by non-negative values (6.2.5p3),
>>> members of the extended character set are not.  If char is
>>> signed, they can be as low as CHAR_MIN, in which case
>>> subtracting '0' necessarily results in undefined behavior.
>>> [...]
>
> As Keith pointed out (and I had already conceded four days
> before your message) that was a mistake on my part - the
> relevant clause says "Otherwise, the new type is signed and the
> value cannot be represented in it;  either the result is
> implementation-defined or an implementation-defined signal is
> raised."  (6.3.1.3p3).

I noticed that, and saw Keith's response, and so chose not to
post a followup myself.  But I wasn't saying anything about
converting back to a char, just about the subtraction.

>> No, it doesn't.  Subtracting one character value from another can
>> give undefined behavior, but it doesn't have to, and indeed
>> usually cannot.  The only circumstance where it can is when char
>> is signed and CHAR_MAX == INT_MAX.
>
> You're right - I was thinking about the possibility that
> CHAR_MAX == INT_MAX, but I forgot to specify that this was a
> pre-condition for my claim that the undefined behavior (which
> should instead have been a referrence to 6.3.1.3p3) was
> necessarily the result.  Thank you for pointing this out.

I was confident you would understand the point once I mentioned
it.  The followup was meant mainly for other readers who may
have been confused.

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does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-03 14:15 -0300
  Re: does a double cast to unsigned makes sense in any circumstance? Bart <bc@freeuk.com> - 2022-01-03 17:32 +0000
    Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-15 22:09 -0300
  Re: does a double cast to unsigned makes sense in any circumstance? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 10:06 -0800
    Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-15 22:12 -0300
      Re: does a double cast to unsigned makes sense in any circumstance? James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-15 23:12 -0500
        Re: does a double cast to unsigned makes sense in any circumstance? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-20 19:37 -0800
          Re: does a double cast to unsigned makes sense in any circumstance? "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-01-21 09:35 -0800
            Re: does a double cast to unsigned makes sense in any circumstance? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-21 17:19 -0800
  Re: does a double cast to unsigned makes sense in any circumstance? Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-03 19:31 +0100
    Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-15 22:14 -0300
      Re: does a double cast to unsigned makes sense in any circumstance? James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-15 23:24 -0500
        Re: does a double cast to unsigned makes sense in any circumstance? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-16 13:30 -0800
          Re: does a double cast to unsigned makes sense in any circumstance? James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-16 23:25 -0500
          Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-17 09:43 -0300
            Re: does a double cast to unsigned makes sense in any circumstance? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-17 13:59 -0800
              Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-28 22:13 -0300
                Re: does a double cast to unsigned makes sense in any circumstance? James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-28 21:29 -0500
                Re: does a double cast to unsigned makes sense in any circumstance? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-03 22:28 -0800
                Re: does a double cast to unsigned makes sense in any circumstance? "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-02-04 10:43 -0800
                Re: does a double cast to unsigned makes sense in any circumstance? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-28 19:37 -0800
                Re: does a double cast to unsigned makes sense in any circumstance? Manfred <noname@add.invalid> - 2022-01-30 03:45 +0100
                Re: does a double cast to unsigned makes sense in any circumstance? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-30 00:01 -0800
                Re: does a double cast to unsigned makes sense in any circumstance? Richard Damon <Richard@Damon-Family.org> - 2022-01-30 12:59 -0500
        Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-17 09:41 -0300
  Re: does a double cast to unsigned makes sense in any circumstance? Richard Damon <Richard@Damon-Family.org> - 2022-01-03 14:37 -0500
    Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-15 22:15 -0300
      Re: does a double cast to unsigned makes sense in any circumstance? Richard Damon <Richard@Damon-Family.org> - 2022-01-15 20:55 -0500
        Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-17 09:44 -0300
    Re: does a double cast to unsigned makes sense in any circumstance? Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-16 05:13 +0100
  Re: does a double cast to unsigned makes sense in any circumstance? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-15 18:53 -0800
    Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-17 09:46 -0300
    Re: does a double cast to unsigned makes sense in any circumstance? Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-29 16:57 +0100
      Re: does a double cast to unsigned makes sense in any circumstance? Öö Tiib <ootiib@hot.ee> - 2022-01-29 09:54 -0800
        Re: does a double cast to unsigned makes sense in any circumstance? Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-30 13:17 +0100
          Re: does a double cast to unsigned makes sense in any circumstance? Öö Tiib <ootiib@hot.ee> - 2022-01-30 23:53 -0800

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