Copyable wrapper (C++20)

 

C++

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Freestanding and hosted

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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_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (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
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

template<class T>     requires std::copy_constructible<T> && std::is_object_v<T> class /*copyable-box*/;		(since C++20)

ranges::single_view and range adaptors that store an invocable object are specified in terms of an exposition-only class template copyable-box. The name copyable-box is for exposition purposes only.

/*copyable-box*/<T> 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 makes it always satisfy copyable.

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.

Template parameters

T

-

the type of the contained value, must be a object type that models copy_constructible

Member functions

copyable-box::copyable-box

constexpr /*copyable-box*/() noexcept(std::is_nothrow_default_constructible_v<T>)     requires std::default_initializable<T>     : /*copyable-box*/(std::in_place) { }		(since C++20)

The default constructor is provided if and only if T models default_initializable.

A default-constructed /*copyable-box*/<T> contains a value-initialized T object.

copyable-box::operator=

constexpr /*copyable-box*/& operator=(const /*copyable-box*/& other);     noexcept(/* see below */);	(1)	(since C++20)
constexpr /*copyable-box*/& operator=(/*copyable-box*/&& other)     noexcept(std::is_nothrow_move_constructible_v<T>);	(2)	(since C++20)

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;
}

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;
}

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 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).

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::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)