Namespaces
Variants

std:: signbit

From cppreference.net
C++
Numerics library
Common mathematical functions
Nearest integer floating point operations
(C++11)
(C++11)
(C++11) (C++11) (C++11)
Floating point manipulation functions
(C++11) (C++11)
(C++11)
(C++11)
Classification and comparison
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
signbit
(C++11)
Types
(C++11)
(C++11)
(C++11)
Macro constants
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)
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 .

Contents

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

(C++11) (C++11)
absolute value of a floating point value ( |x| )
(function)
(C++11) (C++11) (C++11)
copies the sign of a floating point value
(function)
C documentation for signbit