Assignable wrapper (C++20)

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_trange_value_trange_reference_t	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)	elements_of (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Range factories
empty_viewviews::empty single_viewviews::single	iota_viewviews::iota repeat_viewviews::repeat (C++23)(C++23)	basic_istream_viewviews::istream
Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23) filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23) lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)
Range generators
std::generator (C++23)
Range adaptor objects
Range adaptor closure objects
range_adaptor_closure (C++23)
Helper items
copyable-box movable-box (until C++23)(C++23)	simple-view non-propagating-cache

 

template<class T>     requires std::copy_constructible<T> && std::is_object_v<T> class /*copyable-box*/;		(since C++20) (until C++23) (exposition only*)
template<class T>     requires std::move_constructible<T> && std::is_object_v<T> class /*movable-box*/;		(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 T is already copyable, or both std::is_nothrow_move_constructible_v<T> and std::is_nothrow_copy_constructible_v<T> are true, /*copyable-box*/<T> may store only a T object, because it always contains a value.	(until C++23)
If T is already copyable, or is copy_constructible and both std::is_nothrow_move_constructible_v<T> and std::is_nothrow_copy_constructible_v<T> are true, or does not satisfy copy_constructible but satisfies movable, or does not satisfy copy_constructible but std::is_nothrow_move_constructible_v<T> is true, /*movable-box*/<T> may store only a T object, because it always contains a value.	(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>     : /*copyable-box*/(std::in_place) { }		(since C++20) (until C++23)
constexpr /*movable-box*/() noexcept(std::is_nothrow_default_constructible_v<T>)     requires std::default_initializable<T>     : /*movable-box*/(std::in_place) { }		(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)

1) If std::copyable<T> is not modeled, the copy assignment operator is equivalently defined as:

constexpr /*copyable-box*/& operator=(const /*copyable-box*/& other)     noexcept(std::is_nothrow_copy_constructible_v<T>) {     if (this != std::addressof(other))         if (other)             emplace(*other);         else             reset();     return *this; }	(until C++23)
constexpr /*movable-box*/& operator=(const /*movable-box*/& other)     noexcept(std::is_nothrow_copy_constructible_v<T>)     requires std::copy_constructible<T> {     if (this != std::addressof(other))         if (other)             emplace(*other);         else             reset();     return *this; }	(since C++23)

Otherwise, it is identical to the copy assignment operator of std::optional.

2) If std::movable<T> is not modeled, the move assignment operator is equivalently defined as:

constexpr /*copyable-box*/& operator=(/*copyable-box*/&& other)     noexcept(std::is_nothrow_move_constructible_v<T>) {     if (this != std::addressof(other))         if (other)             emplace(std::move(*other));         else             reset();     return *this; }	(until C++23)
constexpr /*movable-box*/& operator=(/*movable-box*/&& other)     noexcept(std::is_nothrow_move_constructible_v<T>) {     if (this != std::addressof(other))         if (other)             emplace(std::move(*other));         else             reset();     return *this; }	(since C++23)

Otherwise, it is identical to the move assignment operator of std::optional.

Members identical to std::optional

Member functions

(constructor)	constructs the optional object (public member function of std::optional<T>)
(destructor)	destroys the contained value, if there is one (public member function of std::optional<T>)
operator=	assigns contents (public member function of std::optional<T>)
Observers
operator->operator*	accesses the contained value (public member function of std::optional<T>)
operator boolhas_value	checks whether the object contains a value (public member function of std::optional<T>)
Modifiers
reset	destroys any contained value (public member function of std::optional<T>)
emplace	constructs the contained value in-place (public member function of std::optional<T>)

Notes

A copyable-box (until C++23)movable-box (since C++23) does not contain a value only if

• T does not model movable or copyable, 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	Comment
__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 constructor constructs 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

ranges::single_viewviews::single (C++20)	a view that contains a single element of a specified value (class template) (customization point object)
ranges::repeat_viewviews::repeat (C++23)	a view consisting of a generated sequence by repeatedly producing the same value (class template) (customization point object)
ranges::filter_viewviews::filter (C++20)	a view that consists of the elements of a range that satisfies a predicate (class template) (range adaptor object)
ranges::transform_viewviews::transform (C++20)	a view of a sequence that applies a transformation function to each element (class template) (range adaptor object)
ranges::take_while_viewviews::take_while (C++20)	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)
ranges::drop_while_viewviews::drop_while (C++20)	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)
ranges::zip_transform_viewviews::zip_transform (C++23)	a view consisting of tuples of results of application of a transformation function to corresponding elements of the adapted views (class template) (customization point object)
ranges::adjacent_transform_viewviews::adjacent_transform (C++23)	a view consisting of tuples of results of application of a transformation function to adjacent elements of the adapted view (class template) (range adaptor object)