Namespaces
Variants

std:: fpclassify

From cppreference.net
C++
Numerics library
Common mathematical functions
Nearest integer floating point operations
(C++11)
(C++11)
(C++11) (C++11) (C++11)
Floating point manipulation functions
(C++11) (C++11)
(C++11)
(C++11)
Classification and comparison
fpclassify
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Types
(C++11)
(C++11)
(C++11)
Macro constants
Defined in header <cmath>
(1)
int fpclassify ( float num ) ;

int fpclassify ( double num ) ;

int fpclassify ( long double num ) ;
(since C++11)
(until C++23)
constexpr int fpclassify ( /* floating-point-type */ num ) ;
(since C++23)
Defined in header <cmath>
template < class Integer >
int fpclassify ( Integer num ) ;
(A) (since C++11)
(constexpr since C++23)
1) Categorizes floating point value num into the following categories: zero, subnormal, normal, infinite, NAN, or implementation-defined category. The library provides overloads of std::fpclassify for all cv-unqualified floating-point types as the type of the parameter num . (since C++23)
A) Additional overloads are provided for all integer types, which are treated as double .

Contents

Parameters

num - floating-point or integer value

Return value

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

Notes

The additional overloads are not required to be provided exactly as (A) . They only need to be sufficient to ensure that for their argument num of integer type, std :: fpclassify ( num ) has the same effect as std :: fpclassify ( static_cast < double > ( num ) ) .

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)
checks if the given number has finite value
(function)
(C++11)
checks if the given number is infinite
(function)
(C++11)
checks if the given number is NaN
(function)
(C++11)
checks if the given number is normal
(function)
provides an interface to query properties of all fundamental numeric types
(class template)
C documentation for fpclassify