FP_NORMAL, FP_SUBNORMAL, FP_ZERO, FP_INFINITE, FP_NAN

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Macro constants
Classification
FP_NORMALFP_SUBNORMALFP_ZEROFP_INFINITEFP_NAN
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Defined in header <cmath>
#define FP_NORMAL    /* implementation defined */
 (since C++11)
#define FP_SUBNORMAL /* implementation defined */
 (since C++11)
#define FP_ZERO      /* implementation defined */
 (since C++11)
#define FP_INFINITE  /* implementation defined */
 (since C++11)
#define FP_NAN       /* implementation defined */
 (since C++11)

The FP_NORMAL, FP_SUBNORMAL, FP_ZERO, FP_INFINITE, FP_NAN

Constant Explanation
FP_NORMAL indicates that the value is normal, i.e. not an infinity, subnormal, not-a-number or zero
FP_SUBNORMAL indicates that the value is subnormal
FP_ZERO indicates that the value is positive or negative zero
FP_INFINITE indicates that the value is not representable by the underlying type (positive or negative infinity)
FP_NAN indicates that the value is not-a-number (NaN)

Example

Run this code
#include <cfloat>
#include <cmath>
#include <iostream>
 
auto show_classification(double x)
{
    switch (std::fpclassify(x))
    {
        case FP_INFINITE:
            return "Inf";
        case FP_NAN:
            return "NaN";
        case FP_NORMAL:
            return "normal";
        case FP_SUBNORMAL:
            return "subnormal";
        case FP_ZERO:
            return "zero";
        default:
            return "unknown";
    }
}
 
int main()
{
    std::cout << "1.0/0.0 is " << show_classification(1 / 0.0) << '\n'
              << "0.0/0.0 is " << show_classification(0.0 / 0.0) << '\n'
              << "DBL_MIN/2 is " << show_classification(DBL_MIN / 2) << '\n'
              << "-0.0 is " << show_classification(-0.0) << '\n'
              << "1.0 is " << show_classification(1.0) << '\n';
}

Output: 

1.0/0.0 is Inf
0.0/0.0 is NaN
DBL_MIN/2 is subnormal
-0.0 is zero
1.0 is normal

See also

(C++11)
 categorizes the given floating-point value
(function)
C documentation for FP_categories
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