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

According to this, infinity is easy to check:

• sign = either 0 or 1 bit indicating positive/negative infinity.
• exponent = all 1 bits.
• mantissa = all 0 bits.

NaN is a bit more complicated because it doesn't have a unique representation:

• sign = either 0 or 1.
• exponent = all 1 bits.
• mantissa = anything except all 0 bits (since all 0 bits represents infinity).

Below is the code for double-precision floating-point case. Single-precision can be similarly written (recall that the exponent is 11-bits for doubles and 8-bits for singles):

int isinf(double x)
{
    union { uint64 u; double f; } ieee754;
    ieee754.f = x;
    return ( (unsigned)(ieee754.u >> 32) & 0x7fffffff ) == 0x7ff00000 &&
           ( (unsigned)ieee754.u == 0 );
}

int isnan(double x)
{
    union { uint64 u; double f; } ieee754;
    ieee754.f = x;
    return ( (unsigned)(ieee754.u >> 32) & 0x7fffffff ) +
           ( (unsigned)ieee754.u != 0 ) > 0x7ff00000;
}

The implementation is pretty straightforward (I took those from the OpenCV header files). It uses a union over an equal-sized unsigned 64-bit integer which you might need to correctly declare:

#if defined _MSC_VER
  typedef unsigned __int64 uint64;
#else
  typedef uint64_t uint64;
#endif