From cppreference.com
Defined in header
boolsignbit(floatnum);boolsignbit(doublenum);boolsignbit(longdoublenum); (1)(since C++11)
(until C++23)constexprboolsignbit(/*floating-point-type*/num);(since C++23)
(since C++26)
Defined in header
template</*math-floating-point*/V>constexprtypename/*deduced-simd-t*/<V>::mask_typesignbit(constV&v_num); (S) (since C++26)
Defined in header
template<classInteger>boolsignbit(Integernum); (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::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>intmain(){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