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std::is_signed(3)	      C++ Standard Libary	     std::is_signed(3)

NAME
       std::is_signed -	std::is_signed

Synopsis
	  Defined in header <type_traits>
	  template< class T >		   (since C++11)
	  struct is_signed;

	  If T is an arithmetic	type, provides the member constant value equal
       to true if
	  T(-1)	 < T(0): this results in true for the floating-point types and
       the signed
	  integer types, and in	false for the unsigned integer types  and  the
       type bool.

	  For any other	type, value is false.

	  The behavior of a program that adds specializations for is_signed
	  or is_signed_v
	  (since C++17)	is undefined.

Template parameters
	  T - a	type to	check

	 Helper	variable template

	  template<  class  T  >					(since
       C++17)
	  inline constexpr bool	is_signed_v = is_signed<T>::value;

       Inherited from std::integral_constant

Member constants
	  value	   true	if T is	a signed arithmetic type , false otherwise
	  [static] (public static member constant)

Member functions
	  operator bool	converts the object to bool, returns value
			(public	member function)
	  operator()	returns	value
	  (C++14)	(public	member function)

Member types
	  Type	     Definition
	  value_type bool
	  type	     std::integral_constant<bool, value>

Possible implementation
	  namespace detail {
	  template<typename T,bool = std::is_arithmetic<T>::value>
	  struct is_signed : std::integral_constant<bool, T(-1)	< T(0)>	{};

	  template<typename T>
	  struct is_signed<T,false> : std::false_type {};
	  } // namespace detail

	  template<typename T>
	  struct is_signed : detail::is_signed<T>::type	{};

Example
       // Run this code

	#include <iostream>
	#include <type_traits>

	class A	{};
	enum B : int {};
	enum class C : int {};

	int main()
	{
	    std::cout << std::boolalpha;
	    std::cout << std::is_signed<A>::value << '\n'; // false
	    std::cout << std::is_signed<float>::value << '\n'; // true
	    std::cout << std::is_signed<signed int>::value << '\n'; // true
	    std::cout << std::is_signed<unsigned int>::value <<	'\n'; // false
	    std::cout << std::is_signed<B>::value << '\n'; // false
	    std::cout << std::is_signed<C>::value << '\n'; // false

	    // shorter:
	    std::cout << std::is_signed_v<bool>	<< '\n'; // false
	    std::cout << std::is_signed<signed int>() << '\n'; // true
	    std::cout << std::is_signed<unsigned int>{}	<< '\n'; // false
	}

Output:
	false
	true
	true
	false
	false
	false
	false
	true
	false

See also
	  is_unsigned	checks if a type is an unsigned	arithmetic type
	  (C++11)	(class template)
	  is_signed	identifies signed types
	  [static]	(public	static member  constant	 of  std::numeric_lim-
       its<T>)
	  is_arithmetic	checks if a type is an arithmetic type
	  (C++11)	(class template)
	  make_signed	makes the given	integral type signed
	  (C++11)	(class template)
	  make_unsigned	makes the given	integral type unsigned
	  (C++11)	(class template)

http://cppreference.com		  2022.07.31		     std::is_signed(3)

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