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std::is_signed

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

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

Inherited from std::integral_constant

Member constants

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

Member functions

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

#include <iostream>
#include <type_traits>
 
class A {};
enum B : int {};
enum class C : int {};
 
int main()
{
    static_assert(std::is_signed<A>::value == false);
    static_assert(std::is_signed<float>::value == true);
    static_assert(std::is_signed<signed int>::value == true);
    static_assert(std::is_signed<unsigned int>::value == false);
    static_assert(std::is_signed<B>::value == false);
    static_assert(std::is_signed<C>::value == false);
 
    // shorter:
    static_assert(std::is_signed_v<bool> == false);
    static_assert(std::is_signed<signed int>() == true);
    static_assert(std::is_signed<unsigned int>{} == false);
    static_assert(std::is_signed_v<signed char> == true);
    static_assert(std::is_signed_v<unsigned char> == false);
 
    // signedness of char is implementation-defined:
    std::cout << std::boolalpha << std::is_signed_v<char> << '\n';
}

Possible output:

true

See also

(C++11)
 checks if a type is an unsigned arithmetic type
(class template)
[static]
 identifies signed types
(public static member constant of std::numeric_limits<T>)
(C++11)
 checks if a type is an arithmetic type
(class template)
(C++11)
 makes the given integral type signed
(class template)
(C++11)
 makes the given integral type unsigned
(class template)

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https://en.cppreference.com/w/cpp/types/is_signed