std::isnan

 

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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( Integral 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 from 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 may change its bit pattern.

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

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