[iterators.common] (original) (raw)

24 Iterators library [iterators]

24.5 Iterator adaptors [predef.iterators]

24.5.5 Common iterators [iterators.common]

24.5.5.1 Class template common_iterator [common.iterator]

Class template common_iterator is an iterator/sentinel adaptor that is capable of representing a non-common range of elements (where the types of the iterator and sentinel differ) as a common range (where they are the same).

It does this by holding either an iterator or a sentinel, and implementing the equality comparison operators appropriately.

[Note 1:

The common_iterator type is useful for interfacing with legacy code that expects the begin and end of a range to have the same type.

— _end note_]

[Example 1: template<class ForwardIterator> void fun(ForwardIterator begin, ForwardIterator end); list<int> s;using CI = common_iterator<counted_iterator<list<int>::iterator>, default_sentinel_t>; fun(CI(counted_iterator(s.begin(), 10)), CI(default_sentinel)); — _end example_]

namespace std { template<input_or_output_iterator I, sentinel_for<I> S> requires (<I, S> && copyable<I>) class common_iterator { public: constexpr common_iterator() requires default_initializable<I> = default;constexpr common_iterator(I i);constexpr common_iterator(S s);template<class I2, class S2> requires convertible_to<const I2&, I> && convertible_to<const S2&, S> constexpr common_iterator(const common_iterator<I2, S2>& x);template<class I2, class S2> requires convertible_to<const I2&, I> && convertible_to<const S2&, S> && assignable_from<I&, const I2&> && assignable_from<S&, const S2&> constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);constexpr decltype(auto) operator*();constexpr decltype(auto) operator*() const requires dereferenceable<const I>;constexpr auto operator->() const requires see below;constexpr common_iterator& operator++();constexpr decltype(auto) operator++(int);template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y);template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> && equality_comparable_with<I, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y);template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2> requires sized_sentinel_for<S, I2> friend constexpr iter_difference_t<I2> operator-( const common_iterator& x, const common_iterator<I2, S2>& y);friend constexpr decltype(auto) iter_move(const common_iterator& i) noexcept(noexcept(ranges::iter_move(declval<const I&>()))) requires input_iterator<I>;template<indirectly_swappable<I> I2, class S2> friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y) noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));private: variant<I, S> v_; };template<class I, class S> struct incrementable_traits<common_iterator<I, S>> { using difference_type = iter_difference_t<I>;};template<input_iterator I, class S> struct iterator_traits<common_iterator<I, S>> { using iterator_concept = see below;using iterator_category = see below; using value_type = iter_value_t<I>;using difference_type = iter_difference_t<I>;using pointer = see below;using reference = iter_reference_t<I>;};}

24.5.5.2 Associated types [common.iter.types]

The nested typedef-name iterator_category of the specialization of iterator_traits for common_iterator<I, S>is defined if and only if iter_difference_t<I> is an integral type.

In that case,iterator_category denotes forward_iterator_tag if the qualified-id iterator_traits<I>​::​iterator_categoryis valid and denotes a type that models derived_from<forward_iterator_tag>; otherwise it denotes input_iterator_tag.

The remaining nested typedef-names of the specialization ofiterator_traits for common_iterator<I, S> are defined as follows:

• iterator_concept denotes forward_iterator_tagif I models forward_iterator; otherwise it denotes input_iterator_tag.
• Let a denote an lvalue of type const common_iterator<I, S>.
  If the expression a.operator->() is well-formed, then pointer denotes decltype(a.operator->()).
  Otherwise, pointer denotes void.

24.5.5.3 Constructors and conversions [common.iter.const]

constexpr common_iterator(I i);

Effects: Initializes v_ as if by v_{in_place_type<I>, std​::​move(i)}.

constexpr common_iterator(S s);

Effects: Initializes v_ as if byv_{in_place_type<S>, std​::​move(s)}.

template<class I2, class S2> requires [convertible_to](concept.convertible#concept:convertible%5Fto "18.4.4 Concept convertible_­to [concept.convertible]")<const I2&, I> && [convertible_to](concept.convertible#concept:convertible%5Fto "18.4.4 Concept convertible_­to [concept.convertible]")<const S2&, S> constexpr common_iterator(const common_iterator<I2, S2>& x);

Preconditions: x.v_.valueless_by_exception() is false.

Effects: Initializes v_ as if byv_{in_place_index<i>, get<i>(x.v_)}, where i is x.v_.index().

Preconditions: x.v_.valueless_by_exception() is false.

Effects: Equivalent to:

• If v_.index() == x.v_.index(), thenget<i>(v_) = get<i>(x.v_).
• Otherwise, v_.emplace<i>(get<i>(x.v_)).

where i is x.v_.index().

24.5.5.4 Accessors [common.iter.access]

constexpr decltype(auto) operator*();constexpr decltype(auto) operator*() const requires [_dereferenceable_](iterator.synopsis#concept:dereferenceable "24.2 Header <iterator> synopsis [iterator.synopsis]")<const I>;

Preconditions: holds_alternative<I>(v_) is true.

Effects: Equivalent to: return *get<I>(v_);

constexpr auto operator->() const requires _see below_;

Preconditions: holds_alternative<I>(v_) is true.

Effects:

• If I is a pointer type or if the expressionget<I>(v_).operator->() is well-formed, equivalent to: return get<I>(v_);
• Otherwise, if iter_reference_t<I> is a reference type, equivalent to:auto&& tmp = *get<I>(v_);return addressof(tmp);
• Otherwise, equivalent to:return proxy(*get<I>(v_)); where_proxy_ is the exposition-only class:class proxy { iter_value_t<I> keep_;constexpr proxy(iter_reference_t<I>&& x) : keep_(std::move(x)) {} public: constexpr const iter_value_t<I>* operator->() const noexcept { return addressof(keep_);} };

24.5.5.5 Navigation [common.iter.nav]

constexpr common_iterator& operator++();

Preconditions: holds_alternative<I>(v_) is true.

Effects: Equivalent to ++get<I>(v_).

constexpr decltype(auto) operator++(int);

Preconditions: holds_alternative<I>(v_) is true.

Effects: If I models forward_iterator, equivalent to:common_iterator tmp = *this;++*this;return tmp;

Otherwise, ifrequires(I& i) { { *i++ } -> can-reference; }is true orindirectly_readable<I> && constructible_from<iter_value_t<I>, iter_reference_t<I>> && move_constructible<iter_value_t<I>> is false, equivalent to:return get<I>(v_)++;

Otherwise, equivalent to:postfix-proxy p(**this);++*this;return p;where postfix-proxy is the exposition-only class:class postfix-proxy { iter_value_t<I> keep_;constexpr postfix-proxy(iter_reference_t<I>&& x) : keep_(std::forward<iter_reference_t<I>>(x)) {} public: constexpr const iter_value_t<I>& operator*() const noexcept { return keep_;} };

24.5.5.6 Comparisons [common.iter.cmp]

template<class I2, [sentinel_for](iterator.concept.sentinel#concept:sentinel%5Ffor "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<I> S2> requires [sentinel_for](iterator.concept.sentinel#concept:sentinel%5Ffor "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<S, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y);

Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception()are each false.

Returns: true if i == j, and otherwise get<i>(x.v_) == get<j>(y.v_), where i is x.v_.index() and j is y.v_.index().

Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception()are each false.

Returns: true if i and j are each 1, and otherwiseget<i>(x.v_) == get<j>(y.v_), wherei is x.v_.index() and j is y.v_.index().

Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception()are each false.

Returns: 0 if i and j are each 1, and otherwiseget<i>(x.v_) - get<j>(y.v_), wherei is x.v_.index() and j is y.v_.index().

24.5.5.7 Customizations [common.iter.cust]

friend constexpr decltype(auto) iter_move(const common_iterator& i) noexcept(noexcept(ranges::iter_move(declval<const I&>()))) requires [input_iterator](iterator.concept.input#concept:input%5Fiterator "24.3.4.9 Concept input_­iterator [iterator.concept.input]")<I>;

Preconditions: holds_alternative<I>(i.v_) is true.

Effects: Equivalent to: return ranges​::​iter_move(get<I>(i.v_));

template<[indirectly_swappable](alg.req.ind.swap#concept:indirectly%5Fswappable "24.3.7.4 Concept indirectly_­swappable [alg.req.ind.swap]")<I> I2, class S2> friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y) noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));

Preconditions: holds_alternative<I>(x.v_) and holds_alternative<I2>(y.v_)are each true.

Effects: Equivalent to ranges​::​iter_swap(get<I>(x.v_), get<I2>(y.v_)).