Re: [boost] serialization - NaN, +/Inf and others

From: Kim Barrett (kab_at_[hidden])
Date: 2005-11-17 15:01:51

At 6:57 PM +0000 11/17/05, Paul A Bristow wrote:
>fpclassify
>
>#define fpclassify(x) <int rvalue> [added with C99, int functions in C++]
>
>[...]
>
>The positive and negative infinties and zeros really are different in
>representation in MS world at least, but I fear you may just have to ignore
>that.
>
>http://www2.open-std.org/JTC1/SC22/WG14/www/C99RationaleV5.10.pdf
>
>does not provide any rationale for the lack of sign on infinity and zero.
>
>Getting -zero is unlikely to be a problem, but getting +infinity instead of
>-infinity could be most confusing - as different as you can get ;-)
>
>I suspect C99 put it in the 'too difficult' box?

To test for negative infinity and negative zero, use fpclassify(x) and
signbit(x). To build a negative infinity, I think copysign of a
positive infinity and a negative sign supplier ought to work, and
likely will work in practice; C99 specifies that it works for NaNs,
but is actually silent about what it does for infinities (sigh!).
copysign can also be used to obtain a negative zero, for those platforms
which actually support negative zero.