std::is_signed (3) - Linux Manuals

std::is_signed: std::is_signed

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 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.

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
         (public static member constant)
[static]

Member functions

              converts the object to bool, returns value
operator bool (public member function)

operator() returns value
              (public member function)
(C++14)

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';
      std::cout << std::is_signed<float>::value << '\n';
      std::cout << std::is_signed<signed int>::value << '\n';
      std::cout << std::is_signed<unsigned int>::value << '\n';
      std::cout << std::is_signed<B>::value << '\n';
      std::cout << std::is_signed<C>::value << '\n';

      // shorter:
      std::cout << std::is_signed<signed int>() << '\n';
      std::cout << std::is_signed<unsigned int>() << '\n';
  }

Output:

  false
  true
  true
  false
  false
  false
  true
  false