cppdraft_translate/cppdraft/range/elements.md

[range.elements]

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]

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

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

Given a subexpression E and constant expression N, the expression views​::​elements(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 << ' '; // 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]

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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]

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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 onlytuple-like && N < tuple_size_v; template<class T, size_t N>concept returnable-element = // exposition only is_reference_v || move_constructible<tuple_element_t<N, T>>; template<input_range V, size_t N>requires view && has-tuple-element<range_value_t, N> &&has-tuple-element<remove_reference_t<range_reference_t>, N> &&returnable-element<range_reference_t, N>class elements_view : public view_interface<elements_view<V, N>> {public: elements_view() requires default_initializable = default; constexpr explicit elements_view(V base); constexpr V base() const & requires copy_constructible { return base_; }constexpr V base() && { return std::move(base_); }constexpr auto begin() requires (){ return iterator(ranges::begin(base_)); }constexpr auto begin() const requires range{ return iterator(ranges::begin(base_)); }constexpr auto end() requires ( && ){ return sentinel{ranges::end(base_)}; }constexpr auto end() requires ( && common_range){ return iterator{ranges::end(base_)}; }constexpr auto end() const requires range{ return sentinel{ranges::end(base_)}; }constexpr auto end() const requires common_range{ return iterator{ranges::end(base_)}; }constexpr auto size() requires sized_range{ return ranges::size(base_); }constexpr auto size() const requires sized_range{ return ranges::size(base_); }constexpr auto reserve_hint() requires approximately_sized_range{ return ranges::reserve_hint(base_); }constexpr auto reserve_hint() const requires approximately_sized_range{ return ranges::reserve_hint(base_); }private:// [range.elements.iterator], class template elements_view​::​iteratortemplate class iterator; // exposition only// [range.elements.sentinel], class template elements_view​::​sentineltemplate class sentinel; // exposition only V base_ = V(); // exposition only};}

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constexpr explicit elements_view(V base);

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

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

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namespace std::ranges {template<input_range V, size_t N>requires view && has-tuple-element<range_value_t, N> &&has-tuple-element<remove_reference_t<range_reference_t>, N> &&returnable-element<range_reference_t, N>templateclass elements_view<V, N>::iterator {using Base = maybe-const<Const, V>; // exposition only iterator_t<Base> current_ = iterator_t<Base>(); // exposition onlystatic constexpr decltype(auto) get-element(const iterator_t<Base>& i); // exposition onlypublic:using iterator_concept = see below; using iterator_category = see below; // not always presentusing 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 i)requires Const && convertible_to<iterator_t, 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) constrequires 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>>; };}

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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.

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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 typeiterator_traits<iterator_t<Base>>​::​iterator_category.

• (2.1)

  If std​::​get(**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);

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

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constexpr explicit iterator(iterator_t<Base> current);

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

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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>, iterator_t<Base>>;

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

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

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Effects: Equivalent to: return current_;

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constexpr iterator_t<Base> base() &&;

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

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constexpr iterator& operator++();

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

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constexpr void operator++(int);

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Effects: Equivalent to: ++current_.

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constexpr iterator operator++(int) requires [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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constexpr iterator& operator--() requires [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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constexpr iterator operator--(int) requires [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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constexpr iterator& operator+=(difference_type n) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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constexpr iterator& operator-=(difference_type n) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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friend constexpr bool operator==(const iterator& x, const iterator& y) requires [equality_comparable](concept.equalitycomparable#concept:equality_comparable "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")<Base>;

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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](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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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](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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Effects: Equivalent to: return y < x;

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friend constexpr bool operator<=(const iterator& x, const iterator& y) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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friend constexpr bool operator>=(const iterator& x, const iterator& y) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base> && [three_way_comparable](cmp.concept#concept:three_way_comparable "17.12.4 Concept three_­way_­comparable [cmp.concept]")<iterator_t<Base>>;

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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](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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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](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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Effects: Equivalent to: return y + x;

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friend constexpr iterator operator-(const iterator& x, difference_type y) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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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.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<iterator_t<Base>, iterator_t<Base>>;

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

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

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namespace std::ranges {template<input_range V, size_t N>requires view && has-tuple-element<range_value_t, N> &&has-tuple-element<remove_reference_t<range_reference_t>, N> &&returnable-element<range_reference_t, N>templateclass elements_view<V, N>::sentinel {private:using Base = maybe-const<Const, V>; // exposition only sentinel_t<Base> end_ = sentinel_t<Base>(); // exposition onlypublic:sentinel() = default; constexpr explicit sentinel(sentinel_t<Base> end); constexpr sentinel(sentinel other)requires Const && convertible_to<sentinel_t, sentinel_t<Base>>; constexpr sentinel_t<Base> base() const; templaterequires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>friend constexpr bool operator==(const iterator& x, const sentinel& y); templaterequires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>friend constexpr range_difference_t<maybe-const<OtherConst, V>>operator-(const iterator& x, const sentinel& y); templaterequires 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& y); };}

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constexpr explicit sentinel(sentinel_t<Base> end);

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Effects: Initializes end_ with end.

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constexpr sentinel(sentinel<!Const> other) requires Const && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<sentinel_t<V>, sentinel_t<Base>>;

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

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constexpr sentinel_t<Base> base() const;

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Effects: Equivalent to: return end_;

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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);

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

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template<bool OtherConst> requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "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);

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

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template<bool OtherConst> requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "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 sentinel& x, const iterator<OtherConst>& y);

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