| -----Original Message----- | From: boost-bounces@lists.boost.org | [mailto:boost-bounces@lists.boost.org] On Behalf Of Deane Yang | Sent: 17 August 2006 17:12 | To: boost@lists.boost.org | Subject: Re: [boost] The meaning of -nan (was | [serialization] facets for non-finite numbers) | | Paul A Bristow wrote: | > | Paul Bristow wrote | > | > That is indeed the *intention* - but entirely USER-DEFINED | > | and with NO MATH | > | > MEANING. | > | > | > | > NaNs are never > < or == anyway? | > | | > | But it tells you where how you got there doesnt it? | > | > Maybe - but this is getting OT - the topic was Octonionic | Not-A-Numbers ;-) | > | | Is this really off-topic? I've seen everyone assert that -nan has no | mathematical meaning. Does this mean that there is no pfficial | specification (whether it is "mathematically meaningful" or | not) of how a -nan can arise as a result of an arithmetic operation? Well the 'official' position seems entirely unclear to me with IEEE754, IEEE754R - still unreleased, and a high degree of non-compliance/disagreement among manufacturers and vendors. But a NaN is a NaN, whatever the sign, only the exponent field determines NaN-ness AFAIK. Apart from sign there are also lots of significand bits whose meaning is officially undefined. I suspect that -NaN can only arise from the use of copysign on an existing NaN. But I am confident that this whole business is 'User-Defined'. But the ability to stream out a signed NaN and stream back in a signed NaN is potentially VERY useful. Paul --- Paul A Bristow Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB +44 1539561830 & SMS, Mobile +44 7714 330204 & SMS pbristow@hetp.u-net.com