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

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-08-02 09:29:41

|  -----Original Message-----
|  From: boost-bounces_at_[hidden] 
|  [mailto:boost-bounces_at_[hidden]] On Behalf Of Johan Råde
|  Sent: 02 August 2006 09:44
|  To: boost_at_[hidden]
|  Subject: Re: [boost] [serialization] facets for non-finite numbers
|  
|  
|  
|  > Johan Råde wrote:
|  > |  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.
|  
|  Paul A. Bristow wrote:
|  
|  > 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.
|  
|  You are right, but our goals are different. You are working on a 
|  proposal to the C++ standardization committee. I writing code that 
|  should work with existing C++ compilers and std lib 
|  implementations. So I can not assume that the C99 signbit macro is
present.

OK, so why not write - or get someone else to write ;-)
a C++ is_negative function,
and use the signbit macro if available,
or make up your own bit test if not?

There are a few variations with type, endianness and compiler, of course,
but not TOO many?

|  What do you thing of Robert Ramey's idea of making it possible to 
|  configure a stream to report an error when a non-finite number is 
|  inserted or extracted? This could be done with a new stream flag and 
|  corresponding manipulators.

Some check would seem extremely wise, or archives may later prove unreadable
:-((

But I am uncertain how best to do it.

Paul

---
Paul A Bristow
Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB
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