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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: two's complement idea
Date: Tue, 25 Feb 2020 07:56:46 -0800
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cross@spitfire.i.gajendra.net (Dan Cross) writes:

> In article <r090qk$dv2$1@dont-email.me>,
> Alf P. Steinbach <alf.p.steinbach+usenet@gmail.com> wrote:
>
>> 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.
>
> Hmm, can't one simply compute:
>
>     ~(size_t)0 ^ (~(size_t)0 >> 1)
>
> ?

Why not just this?  (size_t)-1 /2 +1