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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../numeric/math/isunordered.html below:

std::isunordered - cppreference.com

(1) bool isunordered( float x, float y ); bool isunordered( double x, double y ); bool isunordered( long double x, long double y ); (since C++11)
(until C++23)

constexpr bool isunordered( /* floating-point-type */ x, /* floating-point-type */ y );

(since C++23)

template< class Arithmetic1, class Arithmetic2 > bool isunordered( Arithmetic1 x, Arithmetic2 y );

(A) (since C++11)
(constexpr since C++23)

1) Determines if the floating point numbers x and y are unordered, that is, one or both are NaN and thus cannot be meaningfully compared with each other. The library provides overloads for all cv-unqualified floating-point types as the type of the parameters x and y.(since C++23)

A) Additional overloads are provided for all other combinations of arithmetic types.

[edit] Parameters x, y - floating-point or integer values [edit] Return value

true if either x or y is NaN, false otherwise.

[edit] Notes

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their first argument num1 and second argument num2:

If num1 or num2 has type long double, then std::isunordered(num1, num2) has the same effect as std::isunordered(static_cast<long double>(num1), static_cast<long double>(num2)).
Otherwise, if num1 and/or num2 has type double or an integer type, then std::isunordered(num1, num2) has the same effect as std::isunordered(static_cast<double>(num1), static_cast<double>(num2)).
Otherwise, if num1 or num2 has type float, then std::isunordered(num1, num2) has the same effect as std::isunordered(static_cast<float>(num1), static_cast<float>(num2)).

(until C++23)

If num1 and num2 have arithmetic types, then std::isunordered(num1, num2) has the same effect as std::isunordered(static_cast</*common-floating-point-type*/>(num1), static_cast</*common-floating-point-type*/>(num2)), where /*common-floating-point-type*/ is the floating-point type with the greatest floating-point conversion rank and greatest floating-point conversion subrank between the types of num1 and num2, arguments of integer type are considered to have the same floating-point conversion rank as double.

If no such floating-point type with the greatest rank and subrank exists, then overload resolution does not result in a usable candidate from the overloads provided.

(since C++23) [edit] Example

#include <cmath>
#include <iostream>
 
#define SHOW_UNORDERED(x, y) \
    std::cout << std::boolalpha << "isunordered(" \
              << #x << ", " << #y << "): " \
              << std::isunordered(x, y) << '\n'
 
int main()
{
    SHOW_UNORDERED(10, 01);
    SHOW_UNORDERED(INFINITY, NAN);
    SHOW_UNORDERED(INFINITY, INFINITY);
    SHOW_UNORDERED(NAN, NAN);
}

Output:

isunordered(10, 01): false
isunordered(INFINITY, NAN): true
isunordered(INFINITY, INFINITY): false
isunordered(NAN, NAN): true

[edit] See also

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