----Original Message---- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Maarten Kronenburg Sent: 31 May 2006 21:25 To: boost@lists.boost.org Subject: Re: [boost] Infinite precision integer draft
This problem with unsigned_integer throwing an exception when negative is already apparent for the two expressions a-b and -a+b [presumably -b+a is intended] [snip]
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@Pitechnology.com Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ, ENGLAND Tel: +44 (0)1223 203894