From: Martin Bonner (martin.bonner_at_[hidden])
Date: 2006-06-01 04:47:42
[mailto:boost-bounces_at_[hidden]] On Behalf Of Maarten Kronenburg
Sent: 31 May 2006 21:25 To: boost_at_[hidden]
Subject: Re: [boost] Infinite precision integer draft
> This problem with unsigned_integer
> throwing an exception when negative
> is already apparent for the two
> -a+b [presumably -b+a is intended]
> a true unsigned infinite precision integer
> which may or may not throw exceptions in
> equivalent expressions
But those expressions are not equivalent for non-negative integers.
a-b is defined as the value x such that b+x is equal to a.
For non-negative integers that means a-b is only defined if a >= b.
I would argue that -a is not really meaningful for non-negative
integers, but the most obvious definition is that -a is the value x such
that a+x = 0. That is only defined for a = 0 (and gives x=0).
Thus your first expression is defined for any b <= a, whereas your
second is only defined for b=0.
non-negative integers are not the same as Z (but are still useful).
-- Martin Bonner Martin.Bonner_at_[hidden] Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ, ENGLAND Tel: +44 (0)1223 203894
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk