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

std::copysign, std::copysignf, std::copysignl - cppreference.com

float       copysignf( float mag, float sgn );

long double copysignl( long double mag, long double sgn );

template< class Integer >
double      copysign ( Integer mag, Integer sgn );

1-3) Composes a floating point value with the magnitude of mag and the sign of sgn. The library provides overloads of std::copysign for all cv-unqualified floating-point types as the type of the parameters.(since C++23)

S) The SIMD overload performs an element-wise std::copysign on v_magand v_sgn.

(See math-common-simd-t for its definition.)

A) Additional overloads are provided for all integer types, which are treated as double.

If no errors occur, the floating point value with the magnitude of mag and the sign of sgn is returned.

If mag is NaN, then NaN with the sign of sgn is returned.

If sgn is -0, the result is only negative if the implementation supports the signed zero consistently in arithmetic operations.

This function is not subject to any errors specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

• The returned value is exact (FE_INEXACT is never raised) and independent of the current rounding mode.

std::copysign is the only portable way to manipulate the sign of a NaN value (to examine the sign of a NaN, std::signbit may also be used).

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::copysign(num1, num2) has the same effect as std::copysign(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::copysign(num1, num2) has the same effect as std::copysign(static_cast<double>(num1),
                static_cast<double>(num2)).
• Otherwise, if num1 or num2 has type float, then std::copysign(num1, num2) has the same effect as std::copysign(static_cast<float>(num1),
                static_cast<float>(num2)).

If num1 and num2 have arithmetic types, then std::copysign(num1, num2) has the same effect as std::copysign(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.

#include <cmath>
#include <iostream>
 
int main()
{
    std::cout << std::showpos
              << "copysign(1.0,+2.0) = " << std::copysign(1.0, +2.0) << '\n'
              << "copysign(1.0,-2.0) = " << std::copysign(1.0, -2.0) << '\n'
              << "copysign(inf,-2.0) = " << std::copysign(INFINITY, -2.0) << '\n'
              << "copysign(NaN,-2.0) = " << std::copysign(NAN, -2.0) << '\n';
}

Output:

copysign(1.0,+2.0) = +1
copysign(1.0,-2.0) = -1
copysign(inf,-2.0) = -inf
copysign(NaN,-2.0) = -nan

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