How to test if float is NaN?

[Walter Bright]

Don Clugston wrote:
>> Since by definition a NaN is not equal to anything, including itself, 
>> I suppose it's a compiler bug?
> 
> Definitely. I've seen that sort of behaviour before in VC++.
> <cynic> Does anyone at Microsoft understand floating point arithmetic? 
> </cynic>

At least up until VC6, it did not check the parity bit for floating 
point comparisons (the P bit is set if one of the operands is NaN). In 
fact, I know of no C++ compiler other than DMC++ that does this correctly.

VC dropping support for 80 bit long doubles is not an encouraging sign.

This is typical for C++ compiler vendors. Handling of NaN arguments to 
the math.h functions is routinely erratic and buggy. This can be traced 
back to the Standards committee, who failed to specify NaN behavior.