Home Original page

std::signbit - cppreference.com

From cppreference.com

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)

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)

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 ith 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

#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