std::signbit

Since C++23

Header: <cmath>

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)

# Declarations

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)

template< /*math-floating-point*/ V >
constexpr typename /*deduced-simd-t*/<V>::mask_type
signbit ( const V& v_num );

(since C++26)

Additional overloads

template< class Integer >
bool signbit( Integer num );

(since C++11) (constexpr since C++23)

# 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

# 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(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';
}