# std::isnan

From cppreference.com
< cpp‎ | numeric‎ | math

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).

### 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

### 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)

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