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

N^{th}

element of the adapted view's value-type.

The name views::elements<N> denotes a range adaptor object ([range.adaptor.object]).

Given a subexpression E and constant expression N, the expression views::elements<N>(E) is expression-equivalent to elements_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 << ' '; // prints Babbage Hamilton Lovelace Turing } auto birth_years = historical_figures | views::elements<1>; for (auto&& born : birth_years) { cout << born << ' '; // prints 1791 1936 1815 1912 } — 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 << ' '; // prints Babbage Hamilton Lovelace Turing } — 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); // prints 2 — end example]

25.7.23.2 Class template elements_view [range.elements.view]

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namespace std::ranges { template<class T, size_t N> concept has-tuple-element = // exposition only tuple-like<T> && N < tuple_size_v<T>; template<class T, size_t N> concept returnable-element = // exposition only 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 (!simple-view<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_); } private: // [range.elements.iterator], class template elements_view::iterator template<bool> class iterator; // exposition only // [range.elements.sentinel], class template elements_view::sentinel template<bool> class sentinel; // exposition only V base_ = V(); // exposition only }; }

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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>; // exposition only iterator_t<Base> current_ = iterator_t<Base>(); // exposition only static constexpr decltype(auto) get-element(const iterator_t<Base>& i); // exposition only public: using iterator_concept = see below; using iterator_category = see below; // not always present 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_concept is defined as follows:

(1.1)
If Base models random_access_range, then iterator_concept denotes random_access_iterator_tag.
(1.2)
Otherwise, if Base models bidirectional_range, then iterator_concept denotes bidirectional_iterator_tag.
(1.3)
Otherwise, if Base models forward_range, then iterator_concept denotes forward_iterator_tag.
(1.4)
Otherwise, iterator_concept denotes input_iterator_tag.

The member typedef-name iterator_category is defined if and only if Base models forward_range .

In that case, iterator_category is defined as follows: Let C denote the type iterator_traits<iterator_t<Base>>::iterator_category.

(2.1)
If std::get<N>(*current_) is an rvalue, iterator_category denotes input_iterator_tag.
(2.2)
Otherwise, if C models derived_from<random_access_iterator_tag>, iterator_category denotes random_access_iterator_tag.
(2.3)
Otherwise, iterator_category denotes C.

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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)); }

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constexpr explicit iterator(iterator_t<Base> current);

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

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

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

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

Effects: Equivalent to: return current_;

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constexpr iterator_t<Base> base() &&;

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

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constexpr iterator& operator++();

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

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constexpr void operator++(int);

Effects: Equivalent to: ++current_.

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constexpr iterator operator++(int) requires forward_range<Base>;

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

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constexpr iterator& operator--() requires bidirectional_range<Base>;

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

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constexpr iterator operator--(int) requires bidirectional_range<Base>;

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

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constexpr iterator& operator+=(difference_type n)
  requires random_access_range<Base>;

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

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constexpr iterator& operator-=(difference_type n)
  requires random_access_range<Base>;

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

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friend constexpr bool operator==(const iterator& x, const iterator& y)
  requires equality_comparable<Base>;

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

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friend constexpr bool operator<(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

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

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friend constexpr bool operator>(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

Effects: Equivalent to: return y < x;

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friend constexpr bool operator<=(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

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

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friend constexpr bool operator>=(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

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

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friend constexpr auto operator<=>(const iterator& x, const iterator& y)
  requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;

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

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friend constexpr iterator operator+(const iterator& x, difference_type y)
  requires random_access_range<Base>;

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

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friend constexpr iterator operator+(difference_type x, const iterator& y)
  requires random_access_range<Base>;

Effects: Equivalent to: return y + x;

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friend constexpr iterator operator-(const iterator& x, difference_type y)
  requires random_access_range<Base>;

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

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friend constexpr difference_type operator-(const iterator& x, const iterator& y)
  requires sized_sentinel_for<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]

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constexpr explicit sentinel(sentinel_t<Base> end);

Effects: Initializes end_ with end.

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

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

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constexpr sentinel_t<Base> base() const;

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

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template<bool OtherConst>
  requires sized_sentinel_for<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_;

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

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