[range.elements] (original) (raw)

25 Ranges library [ranges]

25.7 Range adaptors [range.adaptors]

25.7.23 Elements view [range.elements]

25.7.23.1 Overview [range.elements.overview]

elements_view takes a view of tuple-like values and a size_t, and produces a view with a value-type of the element of the adapted view's value-type.

Given a subexpression E and constant expression N, the expression views::elements<N>(E) is expression-equivalent toelements_view<views::all_t<decltype((E))>, N>{E}.

[Example 1: auto historical_figures = map{ pair{"Lovelace"sv, 1815},{"Turing"sv, 1912},{"Babbage"sv, 1791},{"Hamilton"sv, 1936} };auto names = historical_figures | views::elements<0>;for (auto&& name : names) { cout << name << ' '; } auto birth_years = historical_figures | views::elements<1>;for (auto&& born : birth_years) { cout << born << ' '; } — _end example_]

keys_view is an alias for elements_view<R, 0>, and is useful for extracting keys from associative containers.

[Example 2: auto names = historical_figures | views::keys;for (auto&& name : names) { cout << name << ' '; } — _end example_]

values_view is an alias for elements_view<R, 1>, and is useful for extracting values from associative containers.

[Example 3: auto is_even = [](const auto x) { return x % 2 == 0; }; cout << ranges::count_if(historical_figures | views::values, is_even); — _end example_]

25.7.23.2 Class template elements_view [range.elements.view]

namespace std::ranges { template<class T, size_t N> concept has-tuple-element = tuple-like<T> && N < tuple_size_v<T>;template<class T, size_t N> concept returnable-element = is_reference_v<T> || move_constructible<tuple_element_t<N, T>>;template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> && returnable-element<range_reference_t<V>, N> class elements_view : public view_interface<elements_view<V, N>> { public: elements_view() requires default_initializable<V> = default;constexpr explicit elements_view(V base);constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr auto begin() requires ( _simple-view_ <V>) { return iterator<false>(ranges::begin(base_)); } constexpr auto begin() const requires range<const V> { return iterator<true>(ranges::begin(base_)); } constexpr auto end() requires (<V> && $common\_range$ <V>) { return sentinel<false>{ranges::end(base_)}; } constexpr auto end() requires ( _simple-view_ <V> && common_range<V>) { return iterator<false>{ranges::end(base_)}; } constexpr auto end() const requires range<const V> { return sentinel<true>{ranges::end(base_)}; } constexpr auto end() const requires common_range<const V> { return iterator<true>{ranges::end(base_)}; } constexpr auto size() requires sized_range<V> { return ranges::size(base_); } constexpr auto size() const requires sized_range<const V> { return ranges::size(base_); } constexpr auto reserve_hint() requires approximately_sized_range<V> { return ranges::reserve_hint(base_); } constexpr auto reserve_hint() const requires approximately_sized_range<const V> { return ranges::reserve_hint(base_); } private: template<bool> class iterator; template<bool> class sentinel; V base_ = V(); };}

constexpr explicit elements_view(V base);

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

25.7.23.3 Class template elements_view::iterator [range.elements.iterator]

namespace std::ranges { template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> && returnable-element<range_reference_t<V>, N> template<bool Const> class elements_view<V, N>::iterator { using Base = maybe-const<Const, V>; iterator_t<_Base_> current_ = iterator_t<_Base_>(); static constexpr decltype(auto) get-element(const iterator_t<_Base_>& i); public: using iterator_concept = see below;using iterator_category = see below; using value_type = remove_cvref_t<tuple_element_t<N, range_value_t<_Base_>>>;using difference_type = range_difference_t<_Base_>;iterator() requires default_initializable<iterator_t<_Base_>> = default;constexpr explicit iterator(iterator_t<_Base_> current);constexpr iterator(iterator<!Const> i) requires Const && convertible_to<iterator_t<V>, iterator_t<_Base_>>;constexpr const iterator_t<_Base_>& base() const & noexcept;constexpr iterator_t<_Base_> base() &&;constexpr decltype(auto) operator*() const { return get-element(current_); } constexpr iterator& operator++();constexpr void operator++(int);constexpr iterator operator++(int) requires forward_range<_Base_>;constexpr iterator& operator--() requires bidirectional_range<_Base_>;constexpr iterator operator--(int) requires bidirectional_range<_Base_>;constexpr iterator& operator+=(difference_type x) requires random_access_range<_Base_>;constexpr iterator& operator-=(difference_type x) requires random_access_range<_Base_>;constexpr decltype(auto) operator[](difference_type n) const requires random_access_range<_Base_> { return get-element(current_ + n); } friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<_Base_>>;friend constexpr bool operator<(const _iterator_& x, const _iterator_& y) requires random_access_range<_Base_>;friend constexpr bool operator>(const iterator& x, const iterator& y) requires random_access_range<_Base_>;friend constexpr bool operator<=(const _iterator_& x, const _iterator_& y) requires random_access_range<_Base_>;friend constexpr bool operator>=(const iterator& x, const iterator& y) requires random_access_range<_Base_>;friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<_Base_> && three_way_comparable<iterator_t<_Base_>>;friend constexpr iterator operator+(const iterator& x, difference_type y) requires random_access_range<_Base_>;friend constexpr iterator operator+(difference_type x, const iterator& y) requires random_access_range<_Base_>;friend constexpr iterator operator-(const iterator& x, difference_type y) requires random_access_range<_Base_>;friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires sized_sentinel_for<iterator_t<_Base_>, iterator_t<_Base_>>;};}

The member typedef-name iterator_conceptis defined as follows:

Otherwise, if Base models bidirectional_range, then iterator_concept denotes bidirectional_iterator_tag.
Otherwise, if Base models forward_range, then iterator_concept denotes forward_iterator_tag.
Otherwise, iterator_concept denotes input_iterator_tag.

In that case, iterator_category is defined as follows: Let C denote the typeiterator_traits<iterator_t<_Base_>>::iterator_category.

If std::get<N>(*current_) is an rvalue,iterator_category denotes input_iterator_tag.
Otherwise, if C models derived_from<random_access_iterator_tag>,iterator_category denotes random_access_iterator_tag.
Otherwise, iterator_category denotes C.

static constexpr decltype(auto) _get-element_(const iterator_t<_Base_>& i);

Effects: Equivalent to:if constexpr (is_reference_v<range_reference_t<_Base_>>) { return std::get<N>(*i);} else { using E = remove_cv_t<tuple_element_t<N, range_reference_t<_Base_>>>;return static_cast<E>(std::get<N>(*i));}

constexpr explicit _iterator_(iterator_t<_Base_> current);

Effects: Initializes current_ with std::move(current).

constexpr _iterator_(_iterator_<!Const> i) requires Const && [convertible_to](concept.convertible#concept:convertible%5Fto "18.4.4 Concept convertible_to [concept.convertible]")<iterator_t<V>, iterator_t<_Base_>>;

Effects: Initializes current_ with std::move(i.current_).

constexpr const iterator_t<_Base_>& base() const & noexcept;

Effects: Equivalent to: return current_;

constexpr iterator_t<_Base_> base() &&;

Effects: Equivalent to: return std::move(current_);

constexpr _iterator_& operator++();

Effects: Equivalent to:++current_;return *this;

constexpr void operator++(int);

Effects: Equivalent to: ++current_.

Effects: Equivalent to:auto temp = *this;++current_;return temp;

Effects: Equivalent to:--current_;return *this;

Effects: Equivalent to:auto temp = *this;--current_;return temp;

Effects: Equivalent to:current_ += n;return *this;

Effects: Equivalent to:current_ -= n;return *this;

friend constexpr bool operator==(const _iterator_& x, const _iterator_& y) requires [equality_comparable](concept.equalitycomparable#concept:equality%5Fcomparable "18.5.4 Concept equality_comparable [concept.equalitycomparable]")<_Base_>;

Effects: Equivalent to: return x.current_ == y.current_;

friend constexpr bool operator<(const _iterator_& x, const _iterator_& y) requires [random_access_range](range.refinements#concept:random%5Faccess%5Frange "25.4.5 Other range refinements [range.refinements]")<_Base_>;

Effects: Equivalent to: return x.current_ < y.current_;

friend constexpr bool operator>(const _iterator_& x, const _iterator_& y) requires [random_access_range](range.refinements#concept:random%5Faccess%5Frange "25.4.5 Other range refinements [range.refinements]")<_Base_>;

Effects: Equivalent to: return y < x;

friend constexpr bool operator<=(const _iterator_& x, const _iterator_& y) requires [random_access_range](range.refinements#concept:random%5Faccess%5Frange "25.4.5 Other range refinements [range.refinements]")<_Base_>;

Effects: Equivalent to: return !(y < x);

friend constexpr bool operator>=(const _iterator_& x, const _iterator_& y) requires [random_access_range](range.refinements#concept:random%5Faccess%5Frange "25.4.5 Other range refinements [range.refinements]")<_Base_>;

Effects: Equivalent to: return !(x < y);

Effects: Equivalent to: return x.current_ <=> y.current_;

friend constexpr _iterator_ operator+(const _iterator_& x, difference_type y) requires [random_access_range](range.refinements#concept:random%5Faccess%5Frange "25.4.5 Other range refinements [range.refinements]")<_Base_>;

Effects: Equivalent to: return iterator{x} += y;

friend constexpr _iterator_ operator+(difference_type x, const _iterator_& y) requires [random_access_range](range.refinements#concept:random%5Faccess%5Frange "25.4.5 Other range refinements [range.refinements]")<_Base_>;

Effects: Equivalent to: return y + x;

friend constexpr _iterator_ operator-(const _iterator_& x, difference_type y) requires [random_access_range](range.refinements#concept:random%5Faccess%5Frange "25.4.5 Other range refinements [range.refinements]")<_Base_>;

Effects: Equivalent to: return iterator{x} -= y;

friend constexpr difference_type operator-(const _iterator_& x, const _iterator_& y) requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized%5Fsentinel%5Ffor "24.3.4.8 Concept sized_sentinel_for [iterator.concept.sizedsentinel]")<iterator_t<_Base_>, iterator_t<_Base_>>;

Effects: Equivalent to: return x.current_ - y.current_;

25.7.23.4 Class template elements_view::sentinel [range.elements.sentinel]

constexpr explicit _sentinel_(sentinel_t<_Base_> end);

Effects: Initializes end_ with end.

constexpr _sentinel_(_sentinel_<!Const> other) requires Const && [convertible_to](concept.convertible#concept:convertible%5Fto "18.4.4 Concept convertible_to [concept.convertible]")<sentinel_t<V>, sentinel_t<_Base_>>;

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

constexpr sentinel_t<_Base_> base() const;

Effects: Equivalent to: return end_;

template<bool OtherConst> requires [sentinel_for](iterator.concept.sentinel#concept:sentinel%5Ffor "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);

Effects: Equivalent to: return x.current_ == y.end_;

template<bool OtherConst> requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized%5Fsentinel%5Ffor "24.3.4.8 Concept sized_sentinel_for [iterator.concept.sizedsentinel]")<sentinel_t<_Base_>, iterator_t<_maybe-const_<OtherConst, V>>> friend constexpr range_difference_t<_maybe-const_<OtherConst, V>> operator-(const _iterator_<OtherConst>& x, const _sentinel_& y);

Effects: Equivalent to: return x.current_ - y.end_;

Effects: Equivalent to: return x.end_ - y.current_;