std::copysign, std::copysignf, std::copysignl

Defined in header <cmath>
float       copysign ( float x, float y );
float       copysignf( float x, float y );
(1) (since C++11)
double      copysign ( double x, double y );
(2) (since C++11)
long double copysign ( long double x, long double y );
long double copysignl( long double x, long double y );
(3) (since C++11)
Promoted    copysign ( Arithmetic1 x, Arithmetic2 y );
(4) (since C++11)
1-3) Composes a floating point value with the magnitude of x and the sign of y.
4) A set of overloads or a function template for all combinations of arguments of arithmetic type not covered by (1-3). If any argument has integral type, it is cast to double. If any argument is long double, then the return type Promoted is also long double, otherwise the return type is always double.


x, y - floating point values

Return value

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

If x is NaN, then NaN with the sign of y is returned.

If y 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),


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


#include <iostream>
#include <cmath>
int main(void)
    std::cout << "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';


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

See also

absolute value of a floating point value (|x|)
checks if the given number is negative

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