Re: [boost] The meaning of -nan (was [serialization] facets for non-finite numbers)

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-08-18 04:36:58

|  -----Original Message-----
|  From: boost-bounces_at_[hidden] 
|  [mailto:boost-bounces_at_[hidden]] On Behalf Of Deane Yang
|  Sent: 17 August 2006 17:12
|  To: boost_at_[hidden]
|  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
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pbristow_at_[hidden]