25 Ranges library [ranges]

25.7 Range adaptors [range.adaptors]

25.7.14 Join view [range.join]

25.7.14.1 Overview [range.join.overview]

join_view flattens a view of ranges into a view.

The name views::join denotes a range adaptor object ([range.adaptor.object]).

Given a subexpression E, the expression views::join(E) is expression-equivalent to join_view<views::all_t<decltype((E))>>{E}.

[Example 1: vector<string> ss{"hello", " ", "world", "!"}; for (char ch : ss | views::join) cout << ch; // prints hello world! — end example]

25.7.14.2 Class template join_view [range.join.view]

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namespace std::ranges { template<input_range V> requires view<V> && input_range<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], class template join_view::iterator template<bool Const> struct iterator; // exposition only // [range.join.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<V> non-propagating-cache<remove_cv_t<InnerRng>> inner_; // exposition only, present only // if is_reference_v<InnerRng> is false public: join_view() requires default_initializable<V> = default; constexpr explicit join_view(V base); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr auto begin() { if constexpr (forward_range<V>) { constexpr bool use_const = simple-view<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() const requires forward_range<const V> && is_reference_v<range_reference_t<const V>> && input_range<range_reference_t<const V>> { return iterator<true>{*this, ranges::begin(base_)}; } constexpr auto end() { if constexpr (forward_range<V> && is_reference_v<InnerRng> && forward_range<InnerRng> && common_range<V> && common_range<InnerRng>) return iterator<simple-view<V>>{*this, ranges::end(base_)}; else return sentinel<simple-view<V>>{*this}; } constexpr auto end() const requires forward_range<const V> && is_reference_v<range_reference_t<const V>> && input_range<range_reference_t<const V>> { if constexpr (forward_range<range_reference_t<const V>> && common_range<const V> && common_range<range_reference_t<const V>>) return iterator<true>{*this, ranges::end(base_)}; else return sentinel<true>{*this}; } }; template<class R> explicit join_view(R&&) -> join_view<views::all_t<R>>; }

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constexpr explicit join_view(V base);

Effects: Initializes base_ with std::move(base).

25.7.14.3 Class template join_view::iterator [range.join.iterator]

namespace std::ranges { template<input_range V> requires view<V> && input_range<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 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<Base>; // exposition only constexpr explicit iterator(Parent& parent) requires (!forward_range<Base>); // exposition only public: using iterator_concept = see below; using iterator_category = see below; // not always present using 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<iterator_t<V>, OuterIter> && convertible_to<iterator_t<InnerRng>, InnerIter>; constexpr decltype(auto) operator*() const { return **inner_; } constexpr InnerIter operator->() const requires has-arrow<InnerIter> && copyable<InnerIter>; constexpr iterator& operator++(); constexpr void operator++(int); constexpr iterator operator++(int) requires ref-is-glvalue && forward_range<Base> && forward_range<range_reference_t<Base>>; constexpr iterator& operator--() requires ref-is-glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>> && common_range<range_reference_t<Base>>; constexpr iterator operator--(int) requires ref-is-glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>> && common_range<range_reference_t<Base>>; friend constexpr bool operator==(const iterator& x, const iterator& y) requires ref-is-glvalue && forward_range<Base> && equality_comparable<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<InnerIter>; }; }

iterator::iterator_concept is defined as follows:

(1.1)
If ref-is-glvalue is true, Base models bidirectional_range, and range_reference_t<Base> models both bidirectional_range and common_range, then iterator_concept denotes bidirectional_iterator_tag.
(1.2)
Otherwise, if ref-is-glvalue is true and Base and range_reference_t<Base> each model forward_range, then iterator_concept denotes forward_iterator_tag.
(1.3)
Otherwise, iterator_concept denotes input_iterator_tag.

The member typedef-name iterator_category is defined if and only if ref-is-glvalue is true, Base models forward_range, and range_reference_t<Base> models forward_range .

In that case, iterator::iterator_category is defined as follows:

(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.
(2.2)
If OUTERC and INNERC each model derived_from<bidirectional_iterator_tag> and range_reference_t<Base> models common_range, iterator_category denotes bidirectional_iterator_tag.
(2.3)
Otherwise, if OUTERC and INNERC each model derived_from<forward_iterator_tag>, iterator_category denotes forward_iterator_tag.
(2.4)
Otherwise, iterator_category denotes input_iterator_tag.

iterator::difference_type denotes the type: common_type_t< range_difference_t<Base>, range_difference_t<range_reference_t<Base>>>

join_view iterators use the satisfy function to skip over empty inner ranges.

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constexpr OuterIter& outer();
constexpr const OuterIter& outer() const;

Returns: outer_ if Base models forward_range; otherwise, *parent_->outer_.

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constexpr void satisfy();

Effects: Equivalent to: auto update_inner = [this](const iterator_t<Base>& x) -> auto&& { if constexpr (ref-is-glvalue) // *x is a reference return *x; else return 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();

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constexpr iterator(Parent& parent, OuterIter outer)
  requires forward_range<Base>;

Effects: Initializes outer_ with std::move(outer) and parent_ with addressof(parent); then calls satisfy().

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constexpr explicit iterator(Parent& parent)
  requires (!forward_range<Base>);

Effects: Initializes parent_ with addressof(parent); then calls satisfy().

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constexpr iterator(iterator<!Const> i)
  requires Const &&
           convertible_to<iterator_t<V>, OuterIter> &&
           convertible_to<iterator_t<InnerRng>, InnerIter>;

Effects: Initializes outer_ with std::move(i.outer_), inner_ with std::move(i.inner_), and parent_ with i.parent_.

[Note 1:

Const can only be true when Base models forward_range .

— end note]

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constexpr InnerIter operator->() const
  requires has-arrow<InnerIter> && copyable<InnerIter>;

Effects: Equivalent to: return *inner_;

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constexpr iterator& operator++();

Let inner-range be:

(12.1)
If ref-is-glvalue is true, *outer().
(12.2)
Otherwise, *parent_->inner_.

Effects: Equivalent to: if (++*inner_ == ranges::end(as-lvalue(inner-range))) { ++outer(); satisfy(); } return *this;

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constexpr void operator++(int);

Effects: Equivalent to: ++*this.

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constexpr iterator operator++(int)
  requires ref-is-glvalue && forward_range<Base> &&
           forward_range<range_reference_t<Base>>;

Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;

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constexpr iterator& operator--()
  requires ref-is-glvalue && bidirectional_range<Base> &&
           bidirectional_range<range_reference_t<Base>> &&
           common_range<range_reference_t<Base>>;

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;

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constexpr iterator operator--(int)
  requires ref-is-glvalue && bidirectional_range<Base> &&
           bidirectional_range<range_reference_t<Base>> &&
           common_range<range_reference_t<Base>>;

Effects: Equivalent to: auto tmp = *this; --*this; return tmp;

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friend constexpr bool operator==(const iterator& x, const iterator& y)
  requires ref-is-glvalue && forward_range<Base> &&
           equality_comparable<iterator_t<range_reference_t<Base>>>;

Effects: Equivalent to: return x.outer_ == y.outer_ && x.inner_ == y.inner_;

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friend constexpr void iter_swap(const iterator& x, const iterator& y)
  noexcept(noexcept(ranges::iter_swap(*x.inner_, *y.inner_)))
  requires indirectly_swappable<InnerIter>;

Effects: Equivalent to: return ranges::iter_swap(*x.inner_, *y.inner_);

25.7.14.4 Class template join_view::sentinel [range.join.sentinel]

namespace std::ranges { template<input_range V> requires view<V> && input_range<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<sentinel_t<V>, sentinel_t<Base>>; template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y); }; }

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constexpr explicit sentinel(Parent& parent);

Effects: Initializes end_ with ranges::end(parent.base_).

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constexpr sentinel(sentinel<!Const> s)
  requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;

Effects: Initializes end_ with std::move(s.end_).

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template<bool OtherConst>
  requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

Effects: Equivalent to: return x.outer() == y.end_;