std::signbit

 

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Defined in header <cmath>
	(1)
bool signbit( float num ); bool signbit( double num ); bool signbit( long double num );			(since C++11) (until C++23)
constexpr bool signbit( /*floating-point-type*/ num );			(since C++23)
SIMD overload (since C++26)
Defined in header <simd>
template< /*math-floating-point*/ V > constexpr typename /*deduced-simd-t*/<V>::mask_type   signbit ( const V& v_num );		(S)	(since C++26)
Additional overloads
Defined in header <cmath>
template< class Integer > bool signbit( Integer num );		(A)	(since C++11) (constexpr since C++23)

1) Determines if the given floating point number num is negative. The library provides overloads for all cv-unqualified floating-point types as the type of the parameter num.(since C++23)

S) The SIMD overload performs an element-wise std::signbit on v_num. (See math-floating-point and deduced-simd-t for their definitions.)

(since C++26)

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

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

Return value

1) true if num is negative, false otherwise.

S) A data-parallel mask object where the i^th element equals true if v_num[i] is negative or false otherwise for all i in the range [​0​, v_num.size()).

Notes

This function detects the sign bit of zeroes, infinities, and NaNs. Along with std::copysign, std::signbit is one of the only two portable ways to examine the sign of a NaN.

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::signbit(num) has the same effect as std::signbit(static_cast<double>(num)).

Example

Run this code

#include <cmath>
#include <iostream>
 
int main()
{
    std::cout << std::boolalpha
              << "signbit(+0.0) = " << std::signbit(+0.0) << '\n'
              << "signbit(-0.0) = " << std::signbit(-0.0) << '\n'
              << "signbit(+nan) = " << std::signbit(+NAN) << '\n'
              << "signbit(-nan) = " << std::signbit(-NAN) << '\n'
              << "signbit(+inf) = " << std::signbit(+INFINITY) << '\n'
              << "signbit(-inf) = " << std::signbit(-INFINITY) << '\n';
}

Output:

signbit(+0.0) = false
signbit(-0.0) = true
signbit(+nan) = false
signbit(-nan) = true
signbit(+inf) = false
signbit(-inf) = true

See also

abs(float)fabsfabsffabsl (C++11)(C++11)	absolute value of a floating point value (\(\small{|x|}\)|x|) (function)
copysigncopysignfcopysignl (C++11)(C++11)(C++11)	copies the sign of a floating point value (function)
C documentation for signbit