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Date: Mon, 14 Jul 2003 06:38:23 +1000 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Marcel Moolenaar <marcel@xcllnt.net>
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cc: Craig Rodrigues <rodrigc@crodrigues.org>
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Subject: Re: GCC 3.3.1, new warnings with <limits>
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On Sun, 13 Jul 2003, Marcel Moolenaar wrote:

> On Sun, Jul 13, 2003 at 08:23:54AM -0500, David Leimbach wrote:
> >
> > This is a good policy in general, however, one could easily argue that
> > what
> > is trying to be determined with signedness  and such being
> > less-than-compared
> > to 0 isn't really a big deal and possibly the only way to implement this
> > numeric_limits<T>::digits thing without any type introspection which
> > C++ currently
> > lacks.
>
> What about?
>
> 	#define issigned(T)	(((T)(0)>(T)(~0)) ? 1 : 0)

This is worse than any version that uses -1 instead of ~0, since ~0 gives
implementation-defined behaviour.  I think it can set trap bits.  Anyway,
the value of ~0 is unclear.  I think it is -0 == 0 on 1's complement
machines.  Since its value is unclear, the result of converting this value
to even an unsigned type T is also unclear.  I think the above actually does
work for the 3 representations in C99 iff ~0 has no trap bits, but this is
unclear.  These problems are avoided by using plain -1.

Bruce