how do I make a portable isnan/isinf function

Question

I\'ve been using isinf, isnan functions on Linux platforms which worked perfectly. But this didn\'t work on OS-X, so I decided to use std::is

Answer 1

This works under Visual Studio 2008:

#include <math.h>
#define isnan(x) _isnan(x)
#define isinf(x) (!_finite(x))
#define fpu_error(x) (isinf(x) || isnan(x))

For safety, I recommend using fpu_error(). I believe some numbers are picked up with isnan(), and some with isinf(), and you need both to be safe.

Here is some test code:

double zero=0;
double infinite=1/zero;
double proper_number=4;
printf("isinf(infinite)=%d.\n",isinf(infinite));
printf("isinf(proper_number)=%d.\n",isinf(proper_number));
printf("isnan(infinite)=%d.\n",isnan(infinite));
printf("isnan(proper_number)=%d.\n",isnan(proper_number));

double num=-4;
double neg_square_root=sqrt(num);
printf("isinf(neg_square_root)=%d.\n",isinf(neg_square_root));
printf("isinf(proper_number)=%d.\n",isinf(proper_number));
printf("isnan(neg_square_root)=%d.\n",isnan(neg_square_root));
printf("isnan(proper_number)=%d.\n",isnan(proper_number));

Here is the output:

isinf(infinite)=1.
isinf(proper_number)=0.
isnan(infinite)=0.
isnan(proper_number)=0.
isinf(neg_square_root)=1.
isinf(proper_number)=0.
isnan(neg_square_root)=1.
isnan(proper_number)=0.

Answer 2

The following article has some interesting tricks for isnan and isinf: http://jacksondunstan.com/articles/983

Answer 3

You could also use boost for this task:

#include <boost/math/special_functions/fpclassify.hpp> // isnan

if( boost::math::isnan( ... ) .... )

Answer 4

As brubelsabs said Boost offers this feature but, as reported here, instead of using

if (boost::math::isnan(number))

This should be used:

if ((boost::math::isnan)(number))

Answer 5

No-one seems to have mentioned the C99 function fpclassify which returns:

One of FP_INFINITE, FP_NAN, FP_NORMAL, FP_SUBNORMAL, FP_ZERO or implementation-defined type, specifying the category of arg.

This works with visual studio, but I don't know about OS-X.