Re: [boost] is_nan

From: Johan Råde (rade_at_[hidden])
Date: 2006-07-26 10:20:56

Joe Gottman wrote:
>    Several libraries have an is_nan() function buried deep in the code.  Are 
> there any plans to make a general is_nan function available?  This would be 
> quite useful, as well as similar functions like is_finite.
> 
> Joe Gottman 
> 
> 
> 
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An interesting observation:

The is_nan implementation x!=x works, while !((x<=0)||(x>=0)) does not 
work on VC++ 7.1, with optimizations turned on. It seems that 
!((x<=0)||(x>=0)) is evaluated to false at compile time while x!=x is 
evaluated at run time.

Could it be that the compiler has a special rule that says,
do not optimize the expression x!=x, precisely because it is a common 
implementation of isnan?

Does anyone know if x!=x actually fails on any real world platform?

Maybe we should just keep using x!=x ?

--Johan Råde