std::is_signed

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

std::is_signed is a UnaryTypeTrait.

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.

If the program adds specializations for std::is_signed or std::is_signed_v, the behavior 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 哋它亢++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()
(哋它亢++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 {};
}
 
template<typename T>
struct is_signed : detail::is_signed<T>::type {};

Example

Run this code
#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 // 哋它亢++11
    std::is_signed<signed int>() == true and      // 哋它亢++11
    std::is_signed<signed int>{} == true and      // 哋它亢++11
    std::is_signed_v<signed int> == true and      // 哋它亢++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: 

true

See also

(哋它亢++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>)
(哋它亢++11)
 checks if a type is an arithmetic type
(class template)
(哋它亢++11)
 makes the given integral type signed
(class template)
(哋它亢++11)
 makes the given integral type unsigned
(class template)
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