Re: [boost] [math] floating point classification - testing helpwanted

From: John Maddock (john_at_[hidden])
Date: 2005-12-01 11:49:55

> This will be helpful in serializiing non-number floats and doubles.
>
> But there is even a bigger problem.  That is, we haven't found
> a portable way to create all the various flavors of these things.
>
> For native binary archives this isn't problem as the orginal
> bits are recovered.  But I can't see how to handle these
> kinds of floats in portable binary or text based arechives.

Depends what you want to recover, I'm assuming that norms, denorms and 
zero's are handled OK already?

Infinities should be OK too, and on most platforms 
std::numeric_limits<T>::infinity() will get you want you want (you can 
always negate it as required).

NaN's are a whole other issue, you should be able to get a NaN from 
numeric_limits<T>::quiet_NaN(), if they're supported, but it won't be the 
same NaN you had originally.  However, an interesting property of NaN's is 
that:

if x is a NaN then:

x == x

is *false* under IEEE arithmetic rules, so you can't actually tell whether 
you've got the same NaN back or not anyway!

The only thing you can tell is the sign of the NaN: according to C99 you can 
get the sign of a NaN from signbit, and set the sign of a NaN with copysign, 
but there's no way to portably implement those two :-(

In any case I'm not sure why anyone would want to care about the sign of a 
NaN.

John.