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From: red floyd on 16 Mar 2010 10:41
On Mar 15, 9:27 pm, "Johannes Schaub (litb)" <schaub-johan...(a)web.de>
wrote:
> red floyd wrote:
> > On Mar 6, 6:58 am, Olaf Klein <olaf.klei...(a)googlemail.com> wrote:
> >> Dear group,
>
> >> is it possible to initialize an unsigned variable with its maximum
> >> value with the following lines?
>
> >> unsigned u = -1;
> >> std::size_t s = -1;
>
> > If you're trying to get all 1's, may I suggest
>
> > unsigned u = ~0;
>
> That's a bad idea. It only works on two's complement (on one's complement,
> this can have the opposite effect of initializing "u" to all-zero bits). The
> following one is more reliable:
>

ISO/IEC 14882:2003 3.9.1/4:
"Unsigned integers, declared unsigned, shall obey the laws of
arithmetic modulo 2^n
where n is the number of bits in the value representation of that
particular size of integer."



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From: Bart van Ingen Schenau on 17 Mar 2010 01:49
On Mar 17, 2:41 am, red floyd <redfl...(a)gmail.com> wrote:
> On Mar 15, 9:27 pm, "Johannes Schaub (litb)" <schaub-johan...(a)web.de>
> wrote:
>
>
>
> > red floyd wrote:
> > > On Mar 6, 6:58 am, Olaf Klein <olaf.klei...(a)googlemail.com> wrote:
> > >> Dear group,
>
> > >> is it possible to initialize an unsigned variable with its maximum
> > >> value with the following lines?
>
> > >> unsigned u = -1;
> > >> std::size_t s = -1;
>
> > > If you're trying to get all 1's, may I suggest
>
> > > unsigned u = ~0;
>
> > That's a bad idea. It only works on two's complement (on one's complement,
> > this can have the opposite effect of initializing "u" to all-zero bits). The
> > following one is more reliable:
>
> ISO/IEC 14882:2003 3.9.1/4:
> "Unsigned integers, declared unsigned, shall obey the laws of
> arithmetic modulo 2^n
> where n is the number of bits in the value representation of that
> particular size of integer."

And how does that quote pertain to the value of the expression `~0`?
It is an operation on signed integers.

Bart v Ingen Schenau


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