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    std::is_signed

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

    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

    T - a type to check

    Helper variable template

    template< class T > constexpr bool is_signed_v = is_signed<T>::value;
    		 (since C++17)
    [edit]

    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 {};
}

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 // 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: 

    true

    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

    See also

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