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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../ranges/../numeric/math/isnan.html below:

std::isnan - cppreference.com

(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) template< /*math-floating-point*/ V > constexpr typename /*deduced-simd-t*/<V>::mask_type isnan ( const V& v_num ); (S) (since C++26)

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.

[edit] Parameters num - floating-point or integer value v_num - a data-parallel object of std::basic_simd specialization where its element type is a floating-point type [edit] Return value

1) true if num is a NaN, false otherwise.

S) A data-parallel mask object where the i^th element equals true if v_num[i] is a NaN or false otherwise for all i in the range [â0â, v_num.size()).

[edit] 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)).

[edit] 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';
}

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

[edit] See also

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