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[range.join]
# 25 Ranges library [[ranges]](./#ranges)
## 25.7 Range adaptors [[range.adaptors]](range.adaptors#range.join)
### 25.7.14 Join view [range.join]
#### [25.7.14.1](#overview) Overview [[range.join.overview]](range.join.overview)
[1](#overview-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6650)
join_view flattens a view of ranges into a view[.](#overview-1.sentence-1)
[2](#overview-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6653)
The name views​::​join denotes a
range adaptor object ([[range.adaptor.object]](range.adaptor.object "25.7.2 Range adaptor objects"))[.](#overview-2.sentence-1)
Given a subexpression E, the expressionviews​::​join(E) is expression-equivalent tojoin_view<views​::​all_t<decltype((E))>>{E}[.](#overview-2.sentence-2)
[3](#overview-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6661)
[*Example [1](#overview-example-1)*: vector<string> ss{"hello", " ", "world", "!"};for (char ch : ss | views::join) cout << ch; // prints hello world! — *end example*]
#### [25.7.14.2](#view) Class template join_view [[range.join.view]](range.join.view)
[🔗](#lib:join_view)
namespace std::ranges {template<[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]") V>requires [view](range.view#concept:view "25.4.5 Views [range.view]")<V> && [input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<V>>class join_view : public view_interface<join_view<V>> {private:using *InnerRng* = range_reference_t<V>; // *exposition only*// [[range.join.iterator]](#iterator "25.7.14.3 Class template join_­view​::​iterator"), class template join_view​::​*iterator*template<bool Const>struct *iterator*; // *exposition only*// [[range.join.sentinel]](#sentinel "25.7.14.4 Class template join_­view​::​sentinel"), class template join_view​::​*sentinel*template<bool Const>struct *sentinel*; // *exposition only* V *base_* = V(); // *exposition only**non-propagating-cache*<iterator_t<V>> *outer_*; // *exposition only*, present only// when ![forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<V>*non-propagating-cache*<remove_cv_t<*InnerRng*>> *inner_*; // *exposition only*, present only// if is_reference_v<*InnerRng*> is falsepublic: join_view() requires [default_initializable](concept.default.init#concept:default_initializable "18.4.12 Concept default_­initializable [concept.default.init]")<V> = default; constexpr explicit join_view(V base); constexpr V base() const & requires [copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")<V> { return *base_*; }constexpr V base() && { return std::move(*base_*); }constexpr auto begin() {if constexpr ([forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<V>) {constexpr bool use_const = [*simple-view*](range.utility.helpers#concept:simple-view "25.5.2 Helper concepts [range.utility.helpers]")<V> && is_reference_v<*InnerRng*>; return *iterator*<use_const>{*this, ranges::begin(*base_*)}; } else {*outer_* = ranges::begin(*base_*); return *iterator*<false>{*this}; }}constexpr auto begin() constrequires [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<const V> && is_reference_v<range_reference_t<const V>> &&[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<const V>>{ return *iterator*<true>{*this, ranges::begin(*base_*)}; }constexpr auto end() {if constexpr ([forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<V> && is_reference_v<*InnerRng*> && [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<*InnerRng*> &&[common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<V> && [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<*InnerRng*>)return *iterator*<[*simple-view*](range.utility.helpers#concept:simple-view "25.5.2 Helper concepts [range.utility.helpers]")<V>>{*this, ranges::end(*base_*)}; elsereturn *sentinel*<[*simple-view*](range.utility.helpers#concept:simple-view "25.5.2 Helper concepts [range.utility.helpers]")<V>>{*this}; }constexpr auto end() constrequires [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<const V> && is_reference_v<range_reference_t<const V>> &&[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<const V>> {if constexpr ([forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<const V>> &&[common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<const V> &&[common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<const V>>)return *iterator*<true>{*this, ranges::end(*base_*)}; elsereturn *sentinel*<true>{*this}; }}; template<class R>explicit join_view(R&&) -> join_view<views::all_t<R>>;}
[🔗](#lib:join_view,constructor)
`constexpr explicit join_view(V base);
`
[1](#view-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6756)
*Effects*: Initializes *base_* with std​::​move(base)[.](#view-1.sentence-1)
#### [25.7.14.3](#iterator) Class template join_view​::​*iterator* [[range.join.iterator]](range.join.iterator)
[🔗](#lib:join_view::iterator)
namespace std::ranges {template<[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]") V>requires [view](range.view#concept:view "25.4.5 Views [range.view]")<V> && [input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<V>>template<bool Const>struct join_view<V>::*iterator* {private:using *Parent* = *maybe-const*<Const, join_view>; // *exposition only*using *Base* = *maybe-const*<Const, V>; // *exposition only*using *OuterIter* = iterator_t<*Base*>; // *exposition only*using *InnerIter* = iterator_t<range_reference_t<*Base*>>; // *exposition only*static constexpr bool *ref-is-glvalue* = // *exposition only* is_reference_v<range_reference_t<*Base*>>; *OuterIter* *outer_* = *OuterIter*(); // *exposition only*, present only// if *Base* models [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]") optional<*InnerIter*> *inner_*; // *exposition only**Parent** *parent_* = nullptr; // *exposition only*constexpr void *satisfy*(); // *exposition only*constexpr *OuterIter*& *outer*(); // *exposition only*constexpr const *OuterIter*& *outer*() const; // *exposition only*constexpr *iterator*(*Parent*& parent, *OuterIter* outer)requires [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; // *exposition only*constexpr explicit *iterator*(*Parent*& parent)requires (![forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<*Base*>); // *exposition only*public:using iterator_concept = *see below*; using iterator_category = *see below*; // not always presentusing value_type = range_value_t<range_reference_t<*Base*>>; using difference_type = *see below*; *iterator*() = default; constexpr *iterator*(*iterator*<!Const> i)requires Const &&[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<iterator_t<V>, *OuterIter*> &&[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<iterator_t<*InnerRng*>, *InnerIter*>; constexpr decltype(auto) operator*() const { return ***inner_*; }constexpr *InnerIter* operator->() constrequires [*has-arrow*](range.utility.helpers#concept:has-arrow "25.5.2 Helper concepts [range.utility.helpers]")<*InnerIter*> && [copyable](concepts.object#concept:copyable "18.6 Object concepts [concepts.object]")<*InnerIter*>; constexpr *iterator*& operator++(); constexpr void operator++(int); constexpr *iterator* operator++(int)requires *ref-is-glvalue* && [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<*Base*> &&[forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<*Base*>>; constexpr *iterator*& operator--()requires *ref-is-glvalue* && [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<*Base*> &&[bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<*Base*>> &&[common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<*Base*>>; constexpr *iterator* operator--(int)requires *ref-is-glvalue* && [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<*Base*> &&[bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<*Base*>> &&[common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<*Base*>>; friend constexpr bool operator==(const *iterator*& x, const *iterator*& y)requires *ref-is-glvalue* && [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<*Base*> &&[equality_comparable](concept.equalitycomparable#concept:equality_comparable "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")<iterator_t<range_reference_t<*Base*>>>; friend constexpr decltype(auto) iter_move(const *iterator*& i)noexcept(noexcept(ranges::iter_move(*i.*inner_*))) {return ranges::iter_move(*i.*inner_*); }friend constexpr void iter_swap(const *iterator*& x, const *iterator*& y)noexcept(noexcept(ranges::iter_swap(*x.*inner_*, *y.*inner_*)))requires [indirectly_swappable](alg.req.ind.swap#concept:indirectly_swappable "24.3.7.4 Concept indirectly_­swappable [alg.req.ind.swap]")<*InnerIter*>; };}
[1](#iterator-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6843)
*iterator*​::​iterator_concept is defined as follows:
- [(1.1)](#iterator-1.1)
If *ref-is-glvalue* is true, *Base* models [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]"), and range_reference_t<*Base*> models
both [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]") and [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]"),
then iterator_concept denotes bidirectional_iterator_tag[.](#iterator-1.1.sentence-1)
- [(1.2)](#iterator-1.2)
Otherwise, if *ref-is-glvalue* is true and *Base* and range_reference_t<*Base*> each model [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]"), then iterator_concept denotes forward_iterator_tag[.](#iterator-1.2.sentence-1)
- [(1.3)](#iterator-1.3)
Otherwise, iterator_concept denotes input_iterator_tag[.](#iterator-1.3.sentence-1)
[2](#iterator-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6858)
The member [*typedef-name*](dcl.typedef#nt:typedef-name "9.2.4 The typedef specifier [dcl.typedef]") iterator_category is defined
if and only if *ref-is-glvalue* is true,*Base* models [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]"), andrange_reference_t<*Base*> models [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")[.](#iterator-2.sentence-1)
In that case,*iterator*​::​iterator_category is defined as follows:
- [(2.1)](#iterator-2.1)
Let *OUTERC* denote iterator_traits<iterator_t<*Base*>>​::​iterator_category, and
let *INNERC* denote iterator_traits<iterator_t<range_reference_t<*Base*>>>​::​iterator_category[.](#iterator-2.1.sentence-1)
- [(2.2)](#iterator-2.2)
If *OUTERC* and *INNERC* each model [derived_from](concept.derived#concept:derived_from "18.4.3 Concept derived_­from [concept.derived]")<bidirectional_iterator_tag> and range_reference_t<*Base*> models [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]"), iterator_category denotes bidirectional_iterator_tag[.](#iterator-2.2.sentence-1)
- [(2.3)](#iterator-2.3)
Otherwise, if *OUTERC* and *INNERC* each model [derived_from](concept.derived#concept:derived_from "18.4.3 Concept derived_­from [concept.derived]")<forward_iterator_tag>, iterator_category denotes forward_iterator_tag[.](#iterator-2.3.sentence-1)
- [(2.4)](#iterator-2.4)
Otherwise, iterator_category denotes input_iterator_tag[.](#iterator-2.4.sentence-1)
[3](#iterator-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6883)
*iterator*​::​difference_type denotes the type:common_type_t< range_difference_t<*Base*>,
range_difference_t<range_reference_t<*Base*>>>
[4](#iterator-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6891)
join_view iterators use the *satisfy* function to skip over
empty inner ranges[.](#iterator-4.sentence-1)
[🔗](#lib:outer,join_view::iterator)
`constexpr OuterIter& outer();
constexpr const OuterIter& outer() const;
`
[5](#iterator-5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6902)
*Returns*: *outer_* if *Base* models [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]");
otherwise, **parent_*->*outer_*[.](#iterator-5.sentence-1)
[🔗](#lib:satisfy,join_view::iterator)
`constexpr void satisfy();
`
[6](#iterator-6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6914)
*Effects*: Equivalent to:auto update_inner = [this](const iterator_t<*Base*>& x) -> auto&& {if constexpr (*ref-is-glvalue*) // *x is a referencereturn *x; elsereturn *parent_*->*inner_*.*emplace-deref*(x);};
for (; *outer*() != ranges::end(*parent_*->*base_*); ++*outer*()) {auto&& inner = update_inner(*outer*()); *inner_* = ranges::begin(inner); if (**inner_* != ranges::end(inner))return;}if constexpr (*ref-is-glvalue*)*inner_*.reset();
[🔗](#lib:join_view::iterator,constructor)
`constexpr iterator(Parent& parent, OuterIter outer)
requires [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`
[7](#iterator-7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6943)
*Effects*: Initializes *outer_* with std​::​move(outer) and*parent_* with addressof(parent); then calls *satisfy*()[.](#iterator-7.sentence-1)
[🔗](#lib:join_view::iterator,constructor_)
`constexpr explicit iterator(Parent& parent)
requires (![forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<Base>);
`
[8](#iterator-8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6956)
*Effects*: Initializes *parent_* with addressof(parent);
then calls *satisfy*()[.](#iterator-8.sentence-1)
[🔗](#lib:join_view::iterator,constructor__)
`constexpr iterator(iterator<!Const> i)
requires Const &&
[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<iterator_t<V>, OuterIter> &&
[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<iterator_t<InnerRng>, InnerIter>;
`
[9](#iterator-9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6971)
*Effects*: Initializes *outer_* with std​::​move(i.*outer_*),*inner_* with std​::​move(i.*inner_*), and*parent_* with i.*parent_*[.](#iterator-9.sentence-1)
[10](#iterator-10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6977)
[*Note [1](#iterator-note-1)*:
Const can only be true when *Base* models [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")[.](#iterator-10.sentence-1)
— *end note*]
[🔗](#lib:operator-%3e,join_view::iterator)
`constexpr InnerIter operator->() const
requires [has-arrow](range.utility.helpers#concept:has-arrow "25.5.2 Helper concepts [range.utility.helpers]")<InnerIter> && [copyable](concepts.object#concept:copyable "18.6 Object concepts [concepts.object]")<InnerIter>;
`
[11](#iterator-11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6991)
*Effects*: Equivalent to: return **inner_*;
[🔗](#lib:operator++,join_view::iterator)
`constexpr iterator& operator++();
`
[12](#iterator-12)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7002)
Let *inner-range* be:
- [(12.1)](#iterator-12.1)
If *ref-is-glvalue* is true, **outer*()[.](#iterator-12.1.sentence-1)
- [(12.2)](#iterator-12.2)
Otherwise, **parent_*->*inner_*[.](#iterator-12.2.sentence-1)
[13](#iterator-13)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7009)
*Effects*: Equivalent to:if (++**inner_* == ranges::end(*as-lvalue*(*inner-range*))) {++*outer*(); *satisfy*();}return *this;
[🔗](#lib:operator++,join_view::iterator_)
`constexpr void operator++(int);
`
[14](#iterator-14)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7027)
*Effects*: Equivalent to: ++*this[.](#iterator-14.sentence-1)
[🔗](#lib:operator++,join_view::iterator__)
`constexpr iterator operator++(int)
requires ref-is-glvalue && [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<Base> &&
[forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<Base>>;
`
[15](#iterator-15)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7040)
*Effects*: Equivalent to:auto tmp = *this;++*this;return tmp;
[🔗](#lib:operator--,join_view::iterator)
`constexpr iterator& operator--()
requires ref-is-glvalue && [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<Base> &&
[bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<Base>> &&
[common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<Base>>;
`
[16](#iterator-16)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7059)
*Effects*: Equivalent to:if (*outer_* == ranges::end(*parent_*->*base_*))*inner_* = ranges::end(*as-lvalue*(*--*outer_*));while (**inner_* == ranges::begin(*as-lvalue*(**outer_*)))**inner_* = ranges::end(*as-lvalue*(*--*outer_*));--**inner_*;return *this;
[🔗](#lib:operator--,join_view::iterator_)
`constexpr iterator operator--(int)
requires ref-is-glvalue && [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<Base> &&
[bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<Base>> &&
[common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<Base>>;
`
[17](#iterator-17)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7081)
*Effects*: Equivalent to:auto tmp = *this;--*this;return tmp;
[🔗](#lib:operator==,join_view::iterator)
`friend constexpr bool operator==(const iterator& x, const iterator& y)
requires ref-is-glvalue && [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<Base> &&
[equality_comparable](concept.equalitycomparable#concept:equality_comparable "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")<iterator_t<range_reference_t<Base>>>;
`
[18](#iterator-18)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7099)
*Effects*: Equivalent to:return x.*outer_* == y.*outer_* && x.*inner_* == y.*inner_*;
[🔗](#lib:iter_swap,join_view::iterator)
`friend constexpr void iter_swap(const iterator& x, const iterator& y)
noexcept(noexcept(ranges::iter_swap(*x.inner_, *y.inner_)))
requires [indirectly_swappable](alg.req.ind.swap#concept:indirectly_swappable "24.3.7.4 Concept indirectly_­swappable [alg.req.ind.swap]")<InnerIter>;
`
[19](#iterator-19)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7113)
*Effects*: Equivalent to: ranges​::​iter_swap(*x.*inner_*, *y.*inner_*);
#### [25.7.14.4](#sentinel) Class template join_view​::​*sentinel* [[range.join.sentinel]](range.join.sentinel)
[🔗](#lib:join_view::sentinel)
namespace std::ranges {template<[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]") V>requires [view](range.view#concept:view "25.4.5 Views [range.view]")<V> && [input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]")<range_reference_t<V>>template<bool Const>struct join_view<V>::*sentinel* {private:using *Parent* = *maybe-const*<Const, join_view>; // *exposition only*using *Base* = *maybe-const*<Const, V>; // *exposition only* sentinel_t<*Base*> *end_* = sentinel_t<*Base*>(); // *exposition only*public:*sentinel*() = default; constexpr explicit *sentinel*(*Parent*& parent); constexpr *sentinel*(*sentinel*<!Const> s)requires Const && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<sentinel_t<V>, sentinel_t<*Base*>>; template<bool OtherConst>requires [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<sentinel_t<*Base*>, iterator_t<*maybe-const*<OtherConst, V>>>friend constexpr bool operator==(const *iterator*<OtherConst>& x, const *sentinel*& y); };}
[🔗](#lib:join_view::sentinel,constructor)
`constexpr explicit sentinel(Parent& parent);
`
[1](#sentinel-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7152)
*Effects*: Initializes *end_* with ranges​::​end(parent.*base_*)[.](#sentinel-1.sentence-1)
[🔗](#lib:join_view::sentinel,constructor_)
`constexpr sentinel(sentinel<!Const> s)
requires Const && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<sentinel_t<V>, sentinel_t<Base>>;
`
[2](#sentinel-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7164)
*Effects*: Initializes *end_* with std​::​move(s.*end_*)[.](#sentinel-2.sentence-1)
[🔗](#lib:operator==,join_view::sentinel)
`template<bool OtherConst>
requires [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
`
[3](#sentinel-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L7177)
*Effects*: Equivalent to: return x.*outer*() == y.*end_*;