Std::isnan (3) Linux Manual Page

std::isnan – std::isnan

Synopsis

Defined in header <cmath>
bool isnan( float arg ); (1) (since C++11)
bool isnan( double arg ); (2) (since C++11)
bool isnan( long double arg ); (3) (since C++11)
bool isnan( IntegralType arg ); (4) (since C++11)
1-3) Determines if the given floating point number arg is a not-a-number (NaN) value.
4) A set of overloads or a function template accepting the arg argument of any integral_type. Equivalent to (2) (the argument is cast to double).

Parameters

arg – floating point value

Return value

true if arg is a NaN, false otherwise

Notes

There are many different NaN values with different sign bits and payloads, see std::nan and std::numeric_limits::quiet_NaN.
NaN values never compare equal to themselves or to other NaN values. Copying a NaN is not required, by IEEE-754, to preserve its bit representation (sign and payload), though most implementation do.
Another way to test if a floating-point value is NaN is to compare it with itself: bool is_nan(double x) { return x != x; }

Example

// Run this code

#include <iostream>

#include <cmath>

#include <cfloat>
int main()

{
    std::cout << std::boolalpha
              << "isnan(NaN) = " << std::isnan(NAN) << '\n'
              << "isnan(Inf) = " << std::isnan(INFINITY) << '\n'
              << "isnan(0.0) = " << std::isnan(0.0) << '\n'
              << "isnan(DBL_MIN/2.0) = " << std::isnan(DBL_MIN / 2.0) << '\n'
              << "isnan(0.0 / 0.0)   = " << std::isnan(0.0 / 0.0) << '\n'
              << "isnan(Inf - Inf)   = " << std::isnan(INFINITY - INFINITY) << '\n';
}

Output:

  isnan(NaN) = true
  isnan(Inf) = false
  isnan(0.0) = false
  isnan(DBL_MIN/2.0) = false
  isnan(0.0 / 0.0) = true
  isnan(Inf – Inf) = true

nan
nanf
nanl not-a-number (NaN)
            (function)
(C++11)
(C++11)
(C++11)
fpclassify categorizes the given floating point value
            (function)
(C++11)
isfinite checks if the given number has finite value
            (function)
(C++11)
isinf checks if the given number is infinite
            (function)
(C++11)
isnormal checks if the given number is normal
            (function)
(C++11)
isunordered checks if two floating-point values are unordered
            (function)
(C++11)

std::isnan (3) Linux Manual Page