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std::is_signed

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

If T is an arithmetic type, provides the member constant value equal 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.

Template parameters

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

Example

#include <iostream>
#include <type_traits>
 
class A {};
enum B : int {};
enum class C : int {};
 
int main() 
{
    std::cout << std::boolalpha;
    std::cout << std::is_signed<A>::value << '\n';
    std::cout << std::is_signed<float>::value << '\n';
    std::cout << std::is_signed<signed int>::value << '\n';
    std::cout << std::is_signed<unsigned int>::value << '\n';
    std::cout << std::is_signed<B>::value << '\n';
    std::cout << std::is_signed<C>::value << '\n';
 
    // shorter:
    std::cout << std::is_signed<signed int>() << '\n';
    std::cout << std::is_signed<unsigned int>() << '\n';
}

Output:

false
true
true
false
false
false
true
false

is_unsigned (C++11)	checks if a type is unsigned arithmetic type (class template)
is_signed [static]	identifies signed types (public static member constant of `std::numeric_limits`)
is_arithmetic (C++11)	checks if a type is arithmetic type (class template)
make_signed (C++11)	makes the given integral type signed (class template)
make_unsigned (C++11)	makes the given integral type unsigned (class template)
std::experimental::is_signed_v (library fundamentals TS)	variable template alias of `std::is_signed::value` (variable template)

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