Question

I would like to isolate the behavior when some double value is a NaN or an #INF. To detect a NaN, I test

doubleVal != doubleVal

What about an #INF? Will this test be true when the doubleVal is an #INF?

Answer 1

How about multiplication by 1. once you’re sure it’s not NaN?

http://ideone.com/97FNu

You could also use isinf and isnan, but those might incur some overhead depending on their implementation.

A third alternative is using the C max value macros (or equivalent std::numeric_limits):

 bool is_inf_or_nan(double x) 
{
    return !(x <= DBL_MAX && x >= -DBL_MAX); 
}