NAME

std::isnan - std::isnan

Synopsis

Defined in header <cmath>
bool isnan( float arg ); (1) (since C++11)
(constexpr since C++23)
bool isnan( double arg ); (2) (since C++11)
(constexpr since C++23)
bool isnan( long double arg ); (3) (since C++11)
(constexpr since C++23)
bool isnan( IntegralType arg ); (4) (since C++11)
(constexpr since C++23)

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