std::is_signed

 

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Compile-time rational arithmetic
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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.

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 > constexprbool 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);   enumclass D :int{}; static_assert(std::is_signed_v<D>==false);   static_assert ( std::is_signed<signedint>::value==true and // C++11 std::is_signed<signedint>()==true and // C++11 std::is_signed<signedint>{}==true and // C++11 std::is_signed_v<signedint>==true and // C++17 std::is_signed_v<unsignedint>==false and std::is_signed_v<float>==true and std::is_signed_v<bool>==false and std::is_signed_v<signedchar>==true and std::is_signed_v<unsignedchar>==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

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)