Re: [boost] [serialization] facets for non-finite numbers

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-08-01 16:18:18

|  -----Original Message-----
|  From: boost-bounces_at_[hidden] 
|  [mailto:boost-bounces_at_[hidden]] On Behalf Of Johan Råde
|  Sent: 01 August 2006 15:49
|  To: boost_at_[hidden]
|  Subject: Re: [boost] [serialization] facets for non-finite numbers
|  
|  Paul A Bristow wrote:
|  > For the record, as a punishment for raising this issue at 
|  the UK C++ BSI
|  > group meeting, I was tasked with raising the issue for the 
|  C++ Standards
|  > group.
|  > 
|  > Some rambling thoughts on this issue are at
|  > 
|  > http://www2.open-std.org/JTC1/SC22/WG21/docs/papers/2006/n2022.pdf 
|  > 
|  > Your implementation almost certainly provides a 
|  'proof_of_concept' for this
|  > - as if it was needed.
|  > 
|  > And I suspect more than a few will be well advised to use 
|  it - otherwise
|  > they may find to their cost (possibly considerable cost) 
|  that some archives
|  > turn out to be not readable because some floating-point 
|  calculation has
|  > produced an infinity or NaN.
|  > 
|  > If you don't do this, it would seem prudent to ensure that every
|  > floating-point serialized is checked for finiteness before 
|  writing to the
|  > archive, and worrying about what to do it any are not!
|  > 
|  > Thanks.
|  > 
|  > Paul
|  > 
|  
|  I read your document. It seems that we have reached more or less the 
|  same conclusions about the representation of inifinites and 
|  nan in text streams. :-)

Not rocket science!

|  Thanks for informing me that there is a C99 standard in this area.
|  As a VC++ 7.1 user, I have never used C99.

Not sure that many C users have either ;-)

http://www2.open-std.org/JTC1/SC22/WG14/www/C99RationaleV5.10.pdf

http://www.dinkumware.com/c99.aspx

May help.

|  I note that:
|  
|  The output from the extended_num_put facet is consistent 
|  with the C99 
|  standard, except that I do not put signs on NaNs. I will not change 
|  that, since there is no reasonable way for a C++ program to 
|  detect that sign.

Surely C99 signbit tests the sign bit? (And is best used for testing the
sign of infinity?)

 (And the sign has no meaning anyway, unlike infinity 
|  where a sign sometimes, but not always, does have meaning.)

Mathematically, you are absolutely correct, but I have argued, 
and continue to argue strongly, that there really IS a sign bit,
and that it is unnecessary, indeed wasteful,
to prevent users from making some entirely private use
of this potentially precious sign bit,
when it costs almost nothing to simply output a '-' if the NaN has the sign
bit set.

|  The extended_num_get facet should probably be modified to accept as 
|  input all the representations allowed under the C99 
|  standard. This means adding support for

|  1. infinity as an alternative to inf
|  2. +nan and -nan
|  3. nan with extra info, i.e. nan(...some characters...)

Would be good if you can spare the time.

Paul

---
Paul A Bristow
Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB
[phone] & SMS, Mobile [phone] & SMS
pbristow_at_[hidden]