Assignable wrapper (C++20)
template< class T > requires std::copy_constructible<T> && std::is_object_v<T> |
(since C++20) (until C++23) (exposition only*) |
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| template< class T > requires std::move_constructible<T> && std::is_object_v<T> |
(since C++23) (exposition only*) |
|
ranges::single_view, ranges::repeat_view,(since C++23) and range adaptors that store an invocable object are specified in terms of an exposition-only class template copyable-box(until C++23)movable-box(since C++23). The name shown here is for exposition purposes only.
The wrapper behaves exactly like std::optional<T>, except that the default constructor, copy assignment operator, and move assignment operator are (conditionally) different from those of std::optional, which augments T with assignability when needed and makes it always satisfy copyableor movable(since C++23).
|
If |
(until C++23) |
|
If
/*movable-box*/<T> may store only a |
(since C++23) |
Template parameters
| T | - | the type of the contained value, must be an object type that models copy_constructible(until C++23)move_constructible(since C++23)
|
Member functions
Default constructor
constexpr /*copyable-box*/() noexcept(std::is_nothrow_default_constructible_v<T>) requires std::default_initializable<T> |
(since C++20) (until C++23) |
|
| constexpr /*movable-box*/() noexcept(std::is_nothrow_default_constructible_v<T>) requires std::default_initializable<T> |
(since C++23) | |
The default constructor is provided if and only if T models default_initializable.
A default-constructed wrapper contains a value-initialized T object.
Assignment operators
| (1) | ||
constexpr /*copyable-box*/& operator=(const /*copyable-box*/& other); noexcept(/* see below */); |
(since C++20) (until C++23) |
|
| constexpr /*movable-box*/& operator=(const /*movable-box*/& other); noexcept(/* see below */) requires std::copy_constructible<T>; |
(since C++23) | |
| (2) | ||
constexpr /*copyable-box*/& operator=(/*copyable-box*/&& other) noexcept(std::is_nothrow_move_constructible_v<T>); |
(since C++20) (until C++23) |
|
| constexpr /*movable-box*/& operator=(/*movable-box*/&& other) noexcept(std::is_nothrow_move_constructible_v<T>); |
(since C++23) | |
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constexpr /*copyable-box*/& operator=(const /*copyable-box*/& other) |
(until C++23) |
|
constexpr /*movable-box*/& operator=(const /*movable-box*/& other) |
(since C++23) |
std::optional.|
constexpr /*copyable-box*/& operator=(/*copyable-box*/&& other) |
(until C++23) |
|
constexpr /*movable-box*/& operator=(/*movable-box*/&& other) |
(since C++23) |
std::optional.Notes
A copyable-box(until C++23)movable-box(since C++23) does not contain a value only if
-
Tdoes not modelmovableorcopyable, and an exception is thrown on move assignment or copy assignment respectively, or - it is initialized/assigned from another valueless wrapper.
Before P2325R3, the wrapper was called semiregular-box in the standard and always satisfied semiregular, as the default constructor was always provided (which might construct a valueless wrapper).
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_ranges |
201911L |
(C++20) | Ranges library and constrained algorithms |
202106L |
(C++20) (DR) |
Non-default-initializable views | |
202207L |
(C++23) | Relaxing range adaptors to allow for move-only types |
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| P2325R3 | C++20 | if T is not default_initializable, the default constructorconstructs a wrapper which does not contain a value |
the wrapper is also not default_initializable
|
| LWG 3572 | C++20 | conditionally different assignment operators were not constexpr | made constexpr |
See also
a view that contains a single element of a specified value(class template) (customization point object) | |
a view consisting of a generated sequence by repeatedly producing the same value(class template) (customization point object) | |
a view that consists of the elements of a range that satisfies a predicate(class template) (range adaptor object) | |
a view of a sequence that applies a transformation function to each element(class template) (range adaptor object) | |
a view consisting of the initial elements of another view, until the first element on which a predicate returns false(class template) (range adaptor object) | |
a view consisting of the elements of another view, skipping the initial subsequence of elements until the first element where the predicate returns false(class template) (range adaptor object) | |
a view consisting of results of application of a transformation function to corresponding elements of the adapted views(class template) (customization point object) | |
a view consisting of results of application of a transformation function to adjacent elements of the adapted view(class template) (range adaptor object) |