Re: two's complement idea

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c++
Subject	Re: two's complement idea
Date	2020-02-15 05:56 -0800
Organization	A noiseless patient Spider
Message-ID	<86a75kgdjh.fsf@linuxsc.com> (permalink)
References	<qppva0$cbj$1@news.albasani.net> <r08tpr$e3b$1@news.albasani.net> <r090qk$dv2$1@dont-email.me> <868slti7gm.fsf@linuxsc.com> <r0mhtd$j13$1@dont-email.me>

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"Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> writes:

> On 27.01.2020 10:17, Tim Rentsch wrote:
>
>> "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> writes:
>>
>>> On 22.01.2020 08:32, Bonita Montero wrote:
>>>
>>>> I recently had the problem that I wanted to have a constant which is
>>>> size_t wide and where the highest bit is set.  I simply solved it that
>>>> way (I assume ptrdiff_t as wide as size_t, but this should be common
>>>> today on all platforms):
>>>> std::size_t const SIZE_T_MSB = std::numeric_limits<ptrdiff_t>::min();
>>>> A guarantee from the standard that signed values are 2's-complement
>>>> woud be nice for that.
>>>
>>> C++20 will give you that guarantee.
>>>
>>> But e.g. `size_t(-1) - size_t(-1)/2` does the job with just C++98
>>> guarantees.
>>
>> Or just size_t(-1)/2+1.
>
> Or ~(size_t(-1)/2).

In practice that is highly likely to work, but strictly speaking
it is not guaranteed (at least not in C;  I believe that holds
also in C++ but I am less well-versed in the C++ standard than
the C standard).  The reason is size_t is not required to be at
least as wide as int, so the ~ could produce a negative value.

Thread

Re: two's complement idea Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-22 08:32 +0100
  Re: two's complement idea "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2020-01-22 09:23 +0100
    Re: two's complement idea Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-22 14:20 +0100
      Re: two's complement idea Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2020-01-22 13:37 +0000
    Re: two's complement idea Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-27 01:17 -0800
      Re: two's complement idea "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2020-01-27 12:35 +0100
        Re: two's complement idea Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-02-15 05:56 -0800
          Re: two's complement idea "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2020-02-16 09:17 +0100
            Re: two's complement idea Richard Damon <Richard@Damon-Family.org> - 2020-02-21 19:35 -0500
              Re: two's complement idea "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2020-02-22 07:01 +0100
            Re: two's complement idea Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-02-24 08:22 -0800
    Re: two's complement idea cross@spitfire.i.gajendra.net (Dan Cross) - 2020-02-24 03:12 +0000
      Re: two's complement idea cdalten@gmail.com - 2020-02-24 05:53 -0800
      Re: two's complement idea Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-02-25 07:56 -0800
        Re: two's complement idea cross@spitfire.i.gajendra.net (Dan Cross) - 2020-02-25 22:00 +0000
          Re: two's complement idea Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-02-25 16:14 -0800
            Re: two's complement idea cdalten@gmail.com - 2020-02-25 18:18 -0800
            Re: two's complement idea cross@spitfire.i.gajendra.net (Dan Cross) - 2020-02-26 02:22 +0000

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