std::is_signed - cppreference.com

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`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.

Template parameters

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

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 {};
}

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

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:

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

	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]