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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../../cpp/types/is_signed.html below:

std::is_signed - cppreference.com

template< class T > struct is_signed;

(since C++11)

std::is_signed is a UnaryTypeTrait.

Checks whether T is a signed arithmetic type.

If std::is_arithmetic<T>::value is true, provides the member constant value equal to T(-1) < T(0).
Otherwise, provides the member constant value equal to false.

If the program adds specializations for std::is_signed or std::is_signed_v, the behavior is undefined.

[edit] Template parameters [edit] Helper variable template

template< class T > constexpr bool is_signed_v = is_signed<T>::value;

(since C++17) Inherited from std::integral_constant Member constants true if T is a signed arithmetic type, false otherwise
(public static member constant) Member functions converts the object to bool, returns value
(public member function) returns value
(public member function) Member types [edit] 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 {};
}
 
template<typename T>
struct is_signed : detail::is_signed<T>::type {};

[edit] Example

#include <iostream>
#include <type_traits>
 
class A {};
static_assert(std::is_signed_v<A> == false);
 
class B { int i; };
static_assert(std::is_signed_v<B> == false);
 
enum C : int {};
static_assert(std::is_signed_v<C> == false);
 
enum class D : int {};
static_assert(std::is_signed_v<D> == false);
 
static_assert
(
    std::is_signed<signed int>::value == true and // C++11
    std::is_signed<signed int>() == true and      // C++11
    std::is_signed<signed int>{} == true and      // C++11
    std::is_signed_v<signed int> == true and      // C++17
    std::is_signed_v<unsigned int> == false and
    std::is_signed_v<float> == true and
    std::is_signed_v<bool> == false and
    std::is_signed_v<signed char> == true and
    std::is_signed_v<unsigned char> == false
);
 
int main()
{
    // signedness of char is implementation-defined:
    std::cout << std::boolalpha << std::is_signed_v<char> << '\n';
}

Possible output:

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 2197 C++11 value could be true even if T is not an arithmetic type can only be false in this case [edit] See also checks if a type is an unsigned arithmetic type
(class template) [edit] identifies signed types
(public static member constant of std::numeric_limits<T>) [edit] checks if a type is an arithmetic type
(class template) [edit] obtains the corresponding signed type for the given integral type
(class template) [edit] obtains the corresponding signed type for the given integral type
(class template) [edit]

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