std:: isnan

Parameters

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 ; } .

GCC and Clang support a -ffinite-math option (additionally implied by -ffast-math ), which allows the respective compiler to assume the nonexistence of special IEEE-754 floating point values such as NaN, infinity, or negative zero. In other words, std::isnan is assumed to always return false under this option.

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

#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';
}

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