Signbit

Defined in header <cmath>.

Description

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).

​Additional Overloads are provided for all integer types, which are treated as double.

Declarations

C++23

// 1)
constexpr bool signbit( /* floating-point-type */ num );

Additional Overloads

// 2)
template< class Integer >
constexpr bool signbit( Integer num );

C++11

// 1)
bool signbit( float num );
// 2)
bool signbit( double num );
// 3)
bool signbit( long double num );

Additional Overloads

// 4)
template< class Integer >
bool signbit( Integer num );

Parameters

​num - floating-point or integer value

Return value

true if num is negative, false otherwise.

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 Additional Overloads. 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)).

Examples

#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'
}

Result

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