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std::is_unsigned

From cppreference.com
< cpp‎ | types
 
 
Metaprogramming library
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(C++11)
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(C++14)
(C++11)(deprecated in C++26)
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is_unsigned
(C++11)
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Compile-time rational arithmetic
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Defined in header <type_traits>
template< class T >
struct is_unsigned;
(since C++11)

std::is_unsigned is a UnaryTypeTrait.

Checks whether T is an unsigned arithmetic type.

  • If std::is_arithmetic<T>::value is true, provides the member constant value equal to T(0) < T(-1).
  • Otherwise, provides the member constant value equal to false.

If the program adds specializations for std::is_unsigned or std::is_unsigned_v, the behavior is undefined.

Contents

[edit] Template parameters

T - a type to check

[edit] Helper variable template

template< class T >
constexpr bool is_unsigned_v = is_unsigned<T>::value;
(since C++17)

Inherited from std::integral_constant

Member constants

value
[static]
true if T is an unsigned integral 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>

[edit] Possible implementation

namespace detail
{
    template<typename T,bool = std::is_arithmetic<T>::value>
    struct is_unsigned : std::integral_constant<bool, T(0) < T(-1)> {};
 
    template<typename T>
    struct is_unsigned<T,false> : std::false_type {};
} // namespace detail
 
template<typename T>
struct is_unsigned : detail::is_unsigned<T>::type {};

[edit] Example

#include <iostream>
#include <type_traits>
 
class A {};
static_assert(std::is_unsigned_v<A> == false);
 
enum B : unsigned {};
static_assert(std::is_unsigned_v<B> == false);
 
enum class C : unsigned {};
static_assert(std::is_unsigned_v<C> == false);
 
struct S { unsigned p : 1; int q : 1; };
static_assert
(
    std::is_unsigned_v<decltype(S::p)> not_eq
    std::is_unsigned_v<decltype(S::q)>
);
 
static_assert
(
    std::is_unsigned_v<float> == false &&
    std::is_unsigned_v<signed int> == false &&
    std::is_unsigned_v<unsigned int> == true &&
    std::is_unsigned_v<bool> == true
);
 
int main() 
{
    // signedness of char is implementation-defined:
    std::cout << std::boolalpha << std::is_unsigned<char>::value << '\n';
}

Possible output:

false

[edit] 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

[edit] See also

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