std::is_signed

 

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integer_sequence (C++14)

 

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

See also

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