std::isnan

 

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Defined in header <cmath>
	(1)
bool isnan( float num ); bool isnan( double num ); bool isnan( long double num );			(since C++11) (until C++23)
constexpr bool isnan( /* floating-point-type */ num );			(since C++23)
Additional overloads
Defined in header <cmath>
template< class Integer > bool isnan( Integer num );		(A)	(since C++11) (constexpr since C++23)

1) Determines if the given floating point number num is a not-a-number (NaN) value. The library provides overloads 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.

Parameters

num

-

floating-point or integer value

Return value

true if num 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; }.

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::isnan(num) has the same effect as std::isnan(static_cast<double>(num)).

Example

Run this code

#include <cfloat>
#include <cmath>
#include <iostream>
 
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

See also

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