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).


arg - floating point value

Return value

true if arg is a NaN, false otherwise.


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; }


#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';


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

See also

not-a-number (NaN)
categorizes the given floating-point value
checks if the given number has finite value
checks if the given number is infinite
checks if the given number is normal
checks if two floating-point values are unordered
C documentation for isnan

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