(unsigned)-1

[Karl Heuer]

In article <690001 at hplsla.HP.COM> bam at hplsla.HP.COM writes:
>Using (unsigned) -1 is asking for trouble.  A better approach is
>    int allones = ~0;

This is probably okay.

>    This works for any size integer.

Well, not quite.  If the type of "~0" is "int" (which it is on this compiler;
no official reference at hand), then "unsigned u = ~0" is equivalent to
"unsigned u = (unsigned)~0", and if "~0" is negative zero, then casting it
from int to unsigned should yield "the value congruent to it mod 2**n", namely
zero! (I presume +0 and -0 are arithmetically equal; never did understand the
logic of ones-complement.)  And on the pdp-11, "long x = ~0" works only
because the 16-bit result of "~0" gets sign-extended.

I'd say the safest notation is "~(target-type)0".

Karl W. Z. Heuer (ima!haddock!karl or karl at haddock.isc.com), The Walking Lint