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).


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

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