Copysign

Defined in header <cmath>.

Description

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 mag and sgn  (since C++23).

​Additional Overloads are provided for all other combinations of arithmetic types.

Declarations

C++23

// 1)
constexpr /* floating-point-type */
            copysign ( /* floating-point-type */ mag,
                       /* floating-point-type */ sgn );
// 2)
constexpr float copysignf( float mag, float sgn );
// 3)
constexpr long double copysignl( long double mag, long double sgn );

Additional Overloads

// 4)
template< class Arithmetic1, class Arithmetic2 >
constexpr /* common-floating-point-type */
            copysign( Arithmetic1 mag, Arithmetic2 sgn );

C++11

// 1)
float copysign ( float mag, float sgn );
// 2)
double copysign ( double mag, double sgn );
// 3)
long double copysign ( long double mag, long double sgn );
// 4)
float copysignf( float mag, float sgn );
// 5)
long double copysignl( long double mag, long double sgn );

Additional Overloads

// 6)
template< class Arithmetic1, class Arithmetic2 >
/* common-floating-point-type */
    copysign( Arithmetic1 mag, Arithmetic2 sgn );

Parameters

mag, sgn - floating-point or integer values

Return value

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.

Error handling

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.

Notes

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 Additional Overloads. 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)). (until C++23)

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.

Examples

#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';
}

Result

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