cppdraft_translate/cppdraft/predef/iterators.md

[predef.iterators]

24 Iterators library [iterators]

24.5 Iterator adaptors [predef.iterators]

24.5.1 Reverse iterators [reverse.iterators]

24.5.1.1 General [reverse.iterators.general]

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Class template reverse_iterator is an iterator adaptor that iterates from the end of the sequence defined by its underlying iterator to the beginning of that sequence.

24.5.1.2 Class template reverse_iterator [reverse.iterator]

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namespace std {templateclass reverse_iterator {public:using iterator_type = Iterator; using iterator_concept = see below; using iterator_category = see below; using value_type = iter_value_t; using difference_type = iter_difference_t; using pointer = typename iterator_traits::pointer; using reference = iter_reference_t; constexpr reverse_iterator(); constexpr explicit reverse_iterator(Iterator x); template constexpr reverse_iterator(const reverse_iterator& u); template constexpr reverse_iterator& operator=(const reverse_iterator& u); constexpr Iterator base() const; constexpr reference operator*() const; constexpr pointer operator->() const requires see below; constexpr reverse_iterator& operator++(); constexpr reverse_iterator operator++(int); constexpr reverse_iterator& operator--(); constexpr reverse_iterator operator--(int); constexpr reverse_iterator operator+ (difference_type n) const; constexpr reverse_iterator& operator+=(difference_type n); constexpr reverse_iterator operator- (difference_type n) const; constexpr reverse_iterator& operator-=(difference_type n); constexpr unspecified operator[](difference_type n) const; friend constexpr iter_rvalue_reference_t iter_move(const reverse_iterator& i) noexcept(see below); template<indirectly_swappable Iterator2>friend constexpr void iter_swap(const reverse_iterator& x, const reverse_iterator& y) noexcept(see below); protected: Iterator current; };}

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The member typedef-name iterator_concept denotes

• (1.1)

random_access_iterator_tag if Iterator modelsrandom_access_iterator, and

• (1.2)

bidirectional_iterator_tag otherwise.

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The member typedef-name iterator_category denotes

• (2.1)

random_access_iterator_tag if the typeiterator_traits<​Iterator>​::​iterator_category modelsderived_from<random_access_iterator_tag>, and

• (2.2)

iterator_traits<​Iterator>​::​iterator_category otherwise.

24.5.1.3 Requirements [reverse.iter.requirements]

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The template parameterIterator shall either meet the requirements of aCpp17BidirectionalIterator ([bidirectional.iterators]) or modelbidirectional_iterator ([iterator.concept.bidir]).

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Additionally,Iterator shall either meet the requirements of aCpp17RandomAccessIterator ([random.access.iterators]) or modelrandom_access_iterator ([iterator.concept.random.access]) if the definitions of any of the members

• (2.1)

operator+,operator-,operator+=,operator-= ([reverse.iter.nav]), or

• (2.2)

operator[] ([reverse.iter.elem]),

or the non-member operators ([reverse.iter.cmp])

• (2.3)

operator<,operator>,operator<=,operator>=,operator-, oroperator+ ([reverse.iter.nonmember])

are instantiated ([temp.inst]).

24.5.1.4 Construction and assignment [reverse.iter.cons]

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constexpr reverse_iterator();

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Effects: Value-initializescurrent.

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constexpr explicit reverse_iterator(Iterator x);

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Effects: Initializescurrent with x.

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template<class U> constexpr reverse_iterator(const reverse_iterator<U>& u);

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Constraints: is_same_v<U, Iterator> is false andconst U& models convertible_to.

4

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Effects: Initializescurrent withu.current.

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template<class U> constexpr reverse_iterator& operator=(const reverse_iterator<U>& u);

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Constraints: is_same_v<U, Iterator> is false,const U& models convertible_to, andassignable_from<Iterator&, const U&> is modeled.

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Effects: Assigns u.current to current.

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Returns: *this.

24.5.1.5 Conversion [reverse.iter.conv]

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constexpr Iterator base() const;

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Returns: current.

24.5.1.6 Element access [reverse.iter.elem]

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constexpr reference operator*() const;

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Effects: As if by:Iterator tmp = current;return *--tmp;

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constexpr pointer operator->() const requires (is_pointer_v<Iterator> || requires(const Iterator i) { i.operator->(); });

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Effects:

• (2.1)

If Iterator is a pointer type, equivalent to:return prev(current);

• (2.2)

Otherwise, equivalent to:return prev(current).operator->();

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constexpr unspecified operator[](difference_type n) const;

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Returns: current[-n - 1].

24.5.1.7 Navigation [reverse.iter.nav]

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constexpr reverse_iterator operator+(difference_type n) const;

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Returns: reverse_iterator(current - n).

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constexpr reverse_iterator operator-(difference_type n) const;

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Returns: reverse_iterator(current + n).

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constexpr reverse_iterator& operator++();

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Effects: As if by: --current;

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Returns: *this.

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constexpr reverse_iterator operator++(int);

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Effects: As if by:reverse_iterator tmp = *this;--current;return tmp;

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constexpr reverse_iterator& operator--();

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Effects: As if by ++current.

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Returns: *this.

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constexpr reverse_iterator operator--(int);

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Effects: As if by:reverse_iterator tmp = *this;++current;return tmp;

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constexpr reverse_iterator& operator+=(difference_type n);

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Effects: As if by: current -= n;

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Returns: *this.

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constexpr reverse_iterator& operator-=(difference_type n);

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Effects: As if by: current += n;

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Returns: *this.

24.5.1.8 Comparisons [reverse.iter.cmp]

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template<class Iterator1, class Iterator2> constexpr bool operator==( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y);

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Constraints: x.base() == y.base() is well-formed and convertible to bool.

2

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Returns: x.base() == y.base().

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template<class Iterator1, class Iterator2> constexpr bool operator!=( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y);

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Constraints: x.base() != y.base() is well-formed and convertible to bool.

4

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Returns: x.base() != y.base().

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template<class Iterator1, class Iterator2> constexpr bool operator<( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y);

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Constraints: x.base() > y.base() is well-formed and convertible to bool.

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Returns: x.base() > y.base().

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template<class Iterator1, class Iterator2> constexpr bool operator>( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y);

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Constraints: x.base() < y.base() is well-formed and convertible to bool.

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Returns: x.base() < y.base().

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template<class Iterator1, class Iterator2> constexpr bool operator<=( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y);

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Constraints: x.base() >= y.base() is well-formed and convertible to bool.

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Returns: x.base() >= y.base().

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template<class Iterator1, class Iterator2> constexpr bool operator>=( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y);

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Constraints: x.base() <= y.base() is well-formed and convertible to bool.

12

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Returns: x.base() <= y.base().

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template<class Iterator1, [three_way_comparable_with](cmp.concept#concept:three_way_comparable_with "17.12.4 Concept three_­way_­comparable [cmp.concept]")<Iterator1> Iterator2> constexpr compare_three_way_result_t<Iterator1, Iterator2> operator<=>(const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y);

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Returns: y.base() <=> x.base().

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[Note 1:

The argument order in the Returns: element is reversed because this is a reverse iterator.

— end note]

24.5.1.9 Non-member functions [reverse.iter.nonmember]

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template<class Iterator1, class Iterator2> constexpr auto operator-( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y) -> decltype(y.base() - x.base());

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Returns: y.base() - x.base().

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template<class Iterator> constexpr reverse_iterator<Iterator> operator+( iter_difference_t<Iterator> n, const reverse_iterator<Iterator>& x);

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Returns: reverse_iterator(x.base() - n).

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friend constexpr iter_rvalue_reference_t<Iterator> iter_move(const reverse_iterator& i) noexcept(see below);

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Effects: Equivalent to:auto tmp = i.base();return ranges::iter_move(--tmp);

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Remarks: The exception specification is equivalent to:is_nothrow_copy_constructible_v &&noexcept(ranges::iter_move(--declval<Iterator&>()))

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template<[indirectly_swappable](alg.req.ind.swap#concept:indirectly_swappable "24.3.7.4 Concept indirectly_­swappable [alg.req.ind.swap]")<Iterator> Iterator2> friend constexpr void iter_swap(const reverse_iterator& x, const reverse_iterator<Iterator2>& y) noexcept(see below);

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Effects: Equivalent to:auto xtmp = x.base();auto ytmp = y.base(); ranges::iter_swap(--xtmp, --ytmp);

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Remarks: The exception specification is equivalent to:is_nothrow_copy_constructible_v && is_nothrow_copy_constructible_v &&noexcept(ranges::iter_swap(--declval<Iterator&>(), --declval<Iterator2&>()))

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template<class Iterator> constexpr reverse_iterator<Iterator> make_reverse_iterator(Iterator i);

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Returns: reverse_iterator(i).

24.5.2 Insert iterators [insert.iterators]

24.5.2.1 General [insert.iterators.general]

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To make it possible to deal with insertion in the same way as writing into an array, a special kind of iterator adaptors, calledinsert iterators, are provided in the library.

With regular iterator classes,while (first != last) *result++ = *first++; causes a range [first, last) to be copied into a range starting with result.

The same code withresult being an insert iterator will insert corresponding elements into the container.

This device allows all of the copying algorithms in the library to work in theinsert mode instead of the regular overwrite mode.

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An insert iterator is constructed from a container and possibly one of its iterators pointing to where insertion takes place if it is neither at the beginning nor at the end of the container.

Insert iterators meet the requirements of output iterators.

operator* returns the insert iterator itself.

The assignmentoperator=(const T& x) is defined on insert iterators to allow writing into them, it insertsx right before where the insert iterator is pointing.

In other words, an insert iterator is like a cursor pointing into the container where the insertion takes place.

back_insert_iterator inserts elements at the end of a container,front_insert_iterator inserts elements at the beginning of a container, andinsert_iterator inserts elements where the iterator points to in a container.

back_inserter,front_inserter, andinserter are three functions making the insert iterators out of a container.

24.5.2.2 Class template back_insert_iterator [back.insert.iterator]

24.5.2.2.1 General [back.insert.iter.general]

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namespace std {templateclass back_insert_iterator {protected: Container* container; public:using iterator_category = output_iterator_tag; using value_type = void; using difference_type = ptrdiff_t; using pointer = void; using reference = void; using container_type = Container; constexpr explicit back_insert_iterator(Container& x); constexpr back_insert_iterator& operator=(const typename Container::value_type& value); constexpr back_insert_iterator& operator=(typename Container::value_type&& value); constexpr back_insert_iterator& operator*(); constexpr back_insert_iterator& operator++(); constexpr back_insert_iterator operator++(int); };}

24.5.2.2.2 Operations [back.insert.iter.ops]

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constexpr explicit back_insert_iterator(Container& x);

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Effects: Initializescontainer with addressof(x).

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constexpr back_insert_iterator& operator=(const typename Container::value_type& value);

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Effects: As if by: container->push_back(value);

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Returns: *this.

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constexpr back_insert_iterator& operator=(typename Container::value_type&& value);

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Effects: As if by: container->push_back(std​::​move(value));

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Returns: *this.

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constexpr back_insert_iterator& operator*();

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Returns: *this.

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constexpr back_insert_iterator& operator++(); constexpr back_insert_iterator operator++(int);

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Returns: *this.

24.5.2.2.3 back_inserter [back.inserter]

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template<class Container> constexpr back_insert_iterator<Container> back_inserter(Container& x);

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Returns: back_insert_iterator(x).

24.5.2.3 Class template front_insert_iterator [front.insert.iterator]

24.5.2.3.1 General [front.insert.iter.general]

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namespace std {templateclass front_insert_iterator {protected: Container* container; public:using iterator_category = output_iterator_tag; using value_type = void; using difference_type = ptrdiff_t; using pointer = void; using reference = void; using container_type = Container; constexpr explicit front_insert_iterator(Container& x); constexpr front_insert_iterator& operator=(const typename Container::value_type& value); constexpr front_insert_iterator& operator=(typename Container::value_type&& value); constexpr front_insert_iterator& operator*(); constexpr front_insert_iterator& operator++(); constexpr front_insert_iterator operator++(int); };}

24.5.2.3.2 Operations [front.insert.iter.ops]

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constexpr explicit front_insert_iterator(Container& x);

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Effects: Initializescontainer with addressof(x).

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constexpr front_insert_iterator& operator=(const typename Container::value_type& value);

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Effects: As if by: container->push_front(value);

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Returns: *this.

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constexpr front_insert_iterator& operator=(typename Container::value_type&& value);

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Effects: As if by: container->push_front(std​::​move(value));

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Returns: *this.

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constexpr front_insert_iterator& operator*();

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Returns: *this.

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constexpr front_insert_iterator& operator++(); constexpr front_insert_iterator operator++(int);

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Returns: *this.

24.5.2.3.3 front_inserter [front.inserter]

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template<class Container> constexpr front_insert_iterator<Container> front_inserter(Container& x);

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Returns: front_insert_iterator(x).

24.5.2.4 Class template insert_iterator [insert.iterator]

24.5.2.4.1 General [insert.iter.general]

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namespace std {templateclass insert_iterator {protected: Container* container; ranges::iterator_t iter; public:using iterator_category = output_iterator_tag; using value_type = void; using difference_type = ptrdiff_t; using pointer = void; using reference = void; using container_type = Container; constexpr insert_iterator(Container& x, ranges::iterator_t i); constexpr insert_iterator& operator=(const typename Container::value_type& value); constexpr insert_iterator& operator=(typename Container::value_type&& value); constexpr insert_iterator& operator*(); constexpr insert_iterator& operator++(); constexpr insert_iterator& operator++(int); };}

24.5.2.4.2 Operations [insert.iter.ops]

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constexpr insert_iterator(Container& x, ranges::iterator_t<Container> i);

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Effects: Initializescontainer with addressof(x) anditer with i.

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constexpr insert_iterator& operator=(const typename Container::value_type& value);

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Effects: As if by:iter = container->insert(iter, value);++iter;

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Returns: *this.

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constexpr insert_iterator& operator=(typename Container::value_type&& value);

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Effects: As if by:iter = container->insert(iter, std::move(value));++iter;

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Returns: *this.

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constexpr insert_iterator& operator*();

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Returns: *this.

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constexpr insert_iterator& operator++(); constexpr insert_iterator& operator++(int);

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Returns: *this.

24.5.2.4.3 inserter [inserter]

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template<class Container> constexpr insert_iterator<Container> inserter(Container& x, ranges::iterator_t<Container> i);

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Returns: insert_iterator(x, i).

24.5.3 Constant iterators and sentinels [const.iterators]

24.5.3.1 General [const.iterators.general]

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Class template basic_const_iterator is an iterator adaptor with the same behavior as the underlying iterator except that its indirection operator implicitly converts the value returned by the underlying iterator's indirection operator to a type such that the adapted iterator is a constant iterator ([iterator.requirements]).

Some generic algorithms can be called with constant iterators to avoid mutation.

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Specializations of basic_const_iterator are constant iterators.

24.5.3.2 Alias templates [const.iterators.alias]

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`template<indirectly_readable It> using iter_const_reference_t = common_reference_t<const iter_value_t&&, iter_reference_t>;

template concept constant-iterator = // exposition only input_iterator && same_as<iter_const_reference_t, iter_reference_t>;

template<input_iterator I> using const_iterator = see below; `

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Result: If I models constant-iterator, I.

Otherwise, basic_const_iterator.

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template<[semiregular](concepts.object#concept:semiregular "18.6 Object concepts [concepts.object]") S> using [const_sentinel](#lib:const_sentinel "24.5.3.2 Alias templates [const.iterators.alias]") = see below;

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Result: If S models input_iterator, const_iterator.

Otherwise, S.

24.5.3.3 Class template basic_const_iterator [const.iterators.iterator]

namespace std {templateconcept not-a-const-iterator = see below; // exposition onlytemplate<indirectly_readable I>using iter-const-rvalue-reference-t = // exposition only common_reference_t<const iter_value_t&&, iter_rvalue_reference_t>; template<input_iterator Iterator>class basic_const_iterator { Iterator current_ = Iterator(); // exposition onlyusing reference = iter_const_reference_t; // exposition onlyusing rvalue-reference = // exposition only**iter-const-rvalue-reference-t; public:using iterator_concept = see below; using iterator_category = see below; // not always presentusing value_type = iter_value_t; using difference_type = iter_difference_t;

basic_const_iterator() requires default_initializable = default; constexpr basic_const_iterator(Iterator current); template<convertible_to U>constexpr basic_const_iterator(basic_const_iterator current); template<different-from<basic_const_iterator> T>requires convertible_to<T, Iterator>constexpr basic_const_iterator(T&& current); constexpr const Iterator& base() const & noexcept; constexpr Iterator base() &&; constexpr reference operator*() const; constexpr const auto* operator->() constrequires is_lvalue_reference_v<iter_reference_t> &&same_as<remove_cvref_t<iter_reference_t>, value_type>; constexpr basic_const_iterator& operator++(); constexpr void operator++(int); constexpr basic_const_iterator operator++(int) requires forward_iterator; constexpr basic_const_iterator& operator--() requires bidirectional_iterator; constexpr basic_const_iterator operator--(int) requires bidirectional_iterator; constexpr basic_const_iterator& operator+=(difference_type n)requires random_access_iterator; constexpr basic_const_iterator& operator-=(difference_type n)requires random_access_iterator; constexpr reference operator[](difference_type n) constrequires random_access_iterator; template<sentinel_for S>constexpr bool operator==(const S& s) const; template<not-a-const-iterator CI>requires constant-iterator && convertible_to<Iterator const&, CI>constexpr operator CI() const &; template<not-a-const-iterator CI>requires constant-iterator && convertible_to<Iterator, CI>constexpr operator CI() &&; constexpr bool operator<(const basic_const_iterator& y) constrequires random_access_iterator; constexpr bool operator>(const basic_const_iterator& y) constrequires random_access_iterator; constexpr bool operator<=(const basic_const_iterator& y) constrequires random_access_iterator; constexpr bool operator>=(const basic_const_iterator& y) constrequires random_access_iterator; constexpr auto operator<=>(const basic_const_iterator& y) constrequires random_access_iterator && three_way_comparable; template<different-from<basic_const_iterator> I>constexpr bool operator<(const I& y) constrequires random_access_iterator && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>constexpr bool operator>(const I& y) constrequires random_access_iterator && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>constexpr bool operator<=(const I& y) constrequires random_access_iterator && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>constexpr bool operator>=(const I& y) constrequires random_access_iterator && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>constexpr auto operator<=>(const I& y) constrequires random_access_iterator && totally_ordered_with<Iterator, I> &&three_way_comparable_with<Iterator, I>; template<not-a-const-iterator I>friend constexpr bool operator<(const I& x, const basic_const_iterator& y)requires random_access_iterator && totally_ordered_with<Iterator, I>; template<not-a-const-iterator I>friend constexpr bool operator>(const I& x, const basic_const_iterator& y)requires random_access_iterator && totally_ordered_with<Iterator, I>; template<not-a-const-iterator I>friend constexpr bool operator<=(const I& x, const basic_const_iterator& y)requires random_access_iterator && totally_ordered_with<Iterator, I>; template<not-a-const-iterator I>friend constexpr bool operator>=(const I& x, const basic_const_iterator& y)requires random_access_iterator && totally_ordered_with<Iterator, I>; friend constexpr basic_const_iterator operator+(const basic_const_iterator& i, difference_type n)requires random_access_iterator; friend constexpr basic_const_iterator operator+(difference_type n, const basic_const_iterator& i)requires random_access_iterator; friend constexpr basic_const_iterator operator-(const basic_const_iterator& i, difference_type n)requires random_access_iterator; template<sized_sentinel_for S>constexpr difference_type operator-(const S& y) const; template<not-a-const-iterator S>requires sized_sentinel_for<S, Iterator>friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y); friend constexpr rvalue-reference iter_move(const basic_const_iterator& i)noexcept(noexcept(static_cast<rvalue-reference>(ranges::iter_move(i.current_)))){return static_cast<rvalue-reference>(ranges::iter_move(i.current_)); }};}

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Given some type I, the concept not-a-const-iterator is defined as false if I is a specialization of basic_const_iterator andtrue otherwise.

24.5.3.4 Member types [const.iterators.types]

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basic_const_iterator​::​iterator_concept is defined as follows:

• (1.1)

  If Iterator models contiguous_iterator, then iterator_concept denotes contiguous_iterator_tag.

• (1.2)

  Otherwise, if Iterator models random_access_iterator, then iterator_concept denotes random_access_iterator_tag.

• (1.3)

  Otherwise, if Iterator models bidirectional_iterator, then iterator_concept denotes bidirectional_iterator_tag.

• (1.4)

  Otherwise, if Iterator models forward_iterator, then iterator_concept denotes forward_iterator_tag.

• (1.5)

  Otherwise, iterator_concept denotes input_iterator_tag.

2

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The member typedef-name iterator_category is defined if and only if Iterator models forward_iterator.

In that case,basic_const_iterator​::​iterator_category denotes the type iterator_traits<​Iterator>​::​iterator_category.

24.5.3.5 Operations [const.iterators.ops]

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constexpr basic_const_iterator(Iterator current);

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

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template<[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<Iterator> U> constexpr basic_const_iterator(basic_const_iterator<U> current);

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

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template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> T> requires [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<T, Iterator> constexpr basic_const_iterator(T&& current);

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

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constexpr const Iterator& base() const & noexcept;

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Effects: Equivalent to: return current_;

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constexpr Iterator base() &&;

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

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constexpr reference operator*() const;

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Effects: Equivalent to: return static_cast<reference>(**current_*);

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constexpr const auto* operator->() const requires is_lvalue_reference_v<iter_reference_t<Iterator>> && [same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<remove_cvref_t<iter_reference_t<Iterator>>, value_type>;

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Returns: If Iterator models contiguous_iterator,to_address(current_); otherwise, addressof(**current_*).

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constexpr basic_const_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 basic_const_iterator operator++(int) requires [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11 Concept forward_­iterator [iterator.concept.forward]")<Iterator>;

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

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constexpr basic_const_iterator& operator--() requires [bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]")<Iterator>;

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

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constexpr basic_const_iterator operator--(int) requires [bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]")<Iterator>;

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

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constexpr basic_const_iterator& operator+=(difference_type n) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr basic_const_iterator& operator-=(difference_type n) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;

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Let op be the operator.

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Effects: Equivalent to:current_ op n;return *this;

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constexpr reference operator[](difference_type n) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>

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Effects: Equivalent to: return static_cast<reference>(current_[n]);

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template<[sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<Iterator> S> constexpr bool operator==(const S& s) const;

16

#

Effects: Equivalent to: return current_ == s;

🔗

template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") CI> requires [constant-iterator](#concept:constant-iterator "24.5.3.2 Alias templates [const.iterators.alias]")<CI> && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<Iterator const&, CI> constexpr operator CI() const &;

17

#

Returns: current_.

🔗

template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") CI> requires [constant-iterator](#concept:constant-iterator "24.5.3.2 Alias templates [const.iterators.alias]")<CI> && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<Iterator, CI> constexpr operator CI() &&;

18

#

Returns: std​::​move(current_).

🔗

constexpr bool operator<(const basic_const_iterator& y) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr bool operator>(const basic_const_iterator& y) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr bool operator<=(const basic_const_iterator& y) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr bool operator>=(const basic_const_iterator& y) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr auto operator<=>(const basic_const_iterator& y) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [three_way_comparable](cmp.concept#concept:three_way_comparable "17.12.4 Concept three_­way_­comparable [cmp.concept]")<Iterator>;

19

#

Let op be the operator.

20

#

Effects: Equivalent to:return current_ op y.current_;

🔗

template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I> constexpr bool operator<(const I& y) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I> constexpr bool operator>(const I& y) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I> constexpr bool operator<=(const I& y) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I> constexpr bool operator>=(const I& y) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I> constexpr auto operator<=>(const I& y) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I> && [three_way_comparable_with](cmp.concept#concept:three_way_comparable_with "17.12.4 Concept three_­way_­comparable [cmp.concept]")<Iterator, I>;

21

#

Let op be the operator.

22

#

Effects: Equivalent to: return current_ op y;

🔗

template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I> friend constexpr bool operator<(const I& x, const basic_const_iterator& y) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I> friend constexpr bool operator>(const I& x, const basic_const_iterator& y) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I> friend constexpr bool operator<=(const I& x, const basic_const_iterator& y) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I> friend constexpr bool operator>=(const I& x, const basic_const_iterator& y) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>;

23

#

Let op be the operator.

24

#

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

🔗

friend constexpr basic_const_iterator operator+(const basic_const_iterator& i, difference_type n) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; friend constexpr basic_const_iterator operator+(difference_type n, const basic_const_iterator& i) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;

25

#

Effects: Equivalent to: return basic_const_iterator(i.current_ + n);

🔗

friend constexpr basic_const_iterator operator-(const basic_const_iterator& i, difference_type n) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;

26

#

Effects: Equivalent to: return basic_const_iterator(i.current_ - n);

🔗

template<[sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<Iterator> S> constexpr difference_type operator-(const S& y) const;

27

#

Effects: Equivalent to: return current_ - y;

🔗

template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") S> requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<S, Iterator> friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y);

28

#

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

24.5.4 Move iterators and sentinels [move.iterators]

24.5.4.1 General [move.iterators.general]

1

#

Class template move_iterator is an iterator adaptor with the same behavior as the underlying iterator except that its indirection operator implicitly converts the value returned by the underlying iterator's indirection operator to an rvalue.

Some generic algorithms can be called with move iterators to replace copying with moving.

2

#

[Example 1: list s;// populate the list s vector v1(s.begin(), s.end()); // copies strings into v1 vector v2(make_move_iterator(s.begin()), make_move_iterator(s.end())); // moves strings into v2 — end example]

24.5.4.2 Class template move_iterator [move.iterator]

🔗

namespace std {templateclass move_iterator {public:using iterator_type = Iterator; using iterator_concept = see below; using iterator_category = see below; // not always presentusing value_type = iter_value_t; using difference_type = iter_difference_t; using pointer = Iterator; using reference = iter_rvalue_reference_t; constexpr move_iterator(); constexpr explicit move_iterator(Iterator i); template constexpr move_iterator(const move_iterator& u); template constexpr move_iterator& operator=(const move_iterator& u); constexpr const Iterator& base() const & noexcept; constexpr Iterator base() &&; constexpr reference operator*() const; constexpr move_iterator& operator++(); constexpr auto operator++(int); constexpr move_iterator& operator--(); constexpr move_iterator operator--(int); constexpr move_iterator operator+(difference_type n) const; constexpr move_iterator& operator+=(difference_type n); constexpr move_iterator operator-(difference_type n) const; constexpr move_iterator& operator-=(difference_type n); constexpr reference operator[](difference_type n) const; template<sentinel_for S>friend constexpr booloperator==(const move_iterator& x, const move_sentinel& y); template<sized_sentinel_for S>friend constexpr iter_difference_toperator-(const move_sentinel& x, const move_iterator& y); template<sized_sentinel_for S>friend constexpr iter_difference_toperator-(const move_iterator& x, const move_sentinel& y); friend constexpr iter_rvalue_reference_t iter_move(const move_iterator& i)noexcept(noexcept(ranges::iter_move(i.current))); template<indirectly_swappable Iterator2>friend constexpr void iter_swap(const move_iterator& x, const move_iterator& y)noexcept(noexcept(ranges::iter_swap(x.current, y.current))); private: Iterator current; // exposition only};}

1

#

The member typedef-name iterator_concept is defined as follows:

• (1.1)

  If Iterator models random_access_iterator, then iterator_concept denotes random_access_iterator_tag.

• (1.2)

  Otherwise, if Iterator models bidirectional_iterator, then iterator_concept denotes bidirectional_iterator_tag.

• (1.3)

  Otherwise, if Iterator models forward_iterator, then iterator_concept denotes forward_iterator_tag.

• (1.4)

  Otherwise, iterator_concept denotes input_iterator_tag.

2

#

The member typedef-name iterator_category is defined if and only if the qualified-iditerator_traits​::​iterator_category is valid and denotes a type.

In that case, iterator_category denotes

• (2.1)

random_access_iterator_tag if the typeiterator_traits<​Iterator>​::​iterator_category modelsderived_from<random_access_iterator_tag>, and

• (2.2)

iterator_traits<​Iterator>​::​iterator_category otherwise.

24.5.4.3 Requirements [move.iter.requirements]

1

#

The template parameter Iterator shall either meet the Cpp17InputIterator requirements ([input.iterators]) or model input_iterator ([iterator.concept.input]).

Additionally, if any of the bidirectional traversal functions are instantiated, the template parameter shall either meet the Cpp17BidirectionalIterator requirements ([bidirectional.iterators]) or model bidirectional_iterator ([iterator.concept.bidir]).

If any of the random access traversal functions are instantiated, the template parameter shall either meet the Cpp17RandomAccessIterator requirements ([random.access.iterators]) or modelrandom_access_iterator ([iterator.concept.random.access]).

24.5.4.4 Construction and assignment [move.iter.cons]

🔗

constexpr move_iterator();

1

#

Effects: Value-initializes current.

🔗

constexpr explicit move_iterator(Iterator i);

2

#

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

🔗

template<class U> constexpr move_iterator(const move_iterator<U>& u);

3

#

Constraints: is_same_v<U, Iterator> is false andconst U& models convertible_to.

4

#

Effects: Initializes current with u.current.

🔗

template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);

5

#

Constraints: is_same_v<U, Iterator> is false,const U& models convertible_to, andassignable_from<Iterator&, const U&> is modeled.

6

#

Effects: Assigns u.current tocurrent.

7

#

Returns: *this.

24.5.4.5 Conversion [move.iter.op.conv]

🔗

constexpr const Iterator& base() const & noexcept;

1

#

Returns: current.

🔗

constexpr Iterator base() &&;

2

#

Returns: std​::​move(current).

24.5.4.6 Element access [move.iter.elem]

🔗

constexpr reference operator*() const;

1

#

Effects: Equivalent to: return ranges​::​iter_move(current);

🔗

constexpr reference operator[](difference_type n) const;

2

#

Effects: Equivalent to: return ranges​::​iter_move(current + n);

24.5.4.7 Navigation [move.iter.nav]

🔗

constexpr move_iterator& operator++();

1

#

Effects: As if by ++current.

2

#

Returns: *this.

🔗

constexpr auto operator++(int);

3

#

Effects: If Iterator models forward_iterator, equivalent to:move_iterator tmp = *this;++current;return tmp;

Otherwise, equivalent to ++current.

🔗

constexpr move_iterator& operator--();

4

#

Effects: As if by --current.

5

#

Returns: *this.

🔗

constexpr move_iterator operator--(int);

6

#

Effects: As if by:move_iterator tmp = *this;--current;return tmp;

🔗

constexpr move_iterator operator+(difference_type n) const;

7

#

Returns: move_iterator(current + n).

🔗

constexpr move_iterator& operator+=(difference_type n);

8

#

Effects: As if by: current += n;

9

#

Returns: *this.

🔗

constexpr move_iterator operator-(difference_type n) const;

10

#

Returns: move_iterator(current - n).

🔗

constexpr move_iterator& operator-=(difference_type n);

11

#

Effects: As if by: current -= n;

12

#

Returns: *this.

24.5.4.8 Comparisons [move.iter.op.comp]

🔗

template<class Iterator1, class Iterator2> constexpr bool operator==(const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y); template<[sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<Iterator> S> friend constexpr bool operator==(const move_iterator& x, const move_sentinel<S>& y);

1

#

Constraints: x.base() == y.base() is well-formed and convertible to bool.

2

#

Returns: x.base() == y.base().

🔗

template<class Iterator1, class Iterator2> constexpr bool operator<(const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y);

3

#

Constraints: x.base() < y.base() is well-formed and convertible to bool.

4

#

Returns: x.base() < y.base().

🔗

template<class Iterator1, class Iterator2> constexpr bool operator>(const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y);

5

#

Constraints: y.base() < x.base() is well-formed and convertible to bool.

6

#

Returns: y < x.

🔗

template<class Iterator1, class Iterator2> constexpr bool operator<=(const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y);

7

#

Constraints: y.base() < x.base() is well-formed and convertible to bool.

8

#

Returns: !(y < x).

🔗

template<class Iterator1, class Iterator2> constexpr bool operator>=(const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y);

9

#

Constraints: x.base() < y.base() is well-formed and convertible to bool.

10

#

Returns: !(x < y).

🔗

template<class Iterator1, [three_way_comparable_with](cmp.concept#concept:three_way_comparable_with "17.12.4 Concept three_­way_­comparable [cmp.concept]")<Iterator1> Iterator2> constexpr compare_three_way_result_t<Iterator1, Iterator2> operator<=>(const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y);

11

#

Returns: x.base() <=> y.base().

24.5.4.9 Non-member functions [move.iter.nonmember]

🔗

template<class Iterator1, class Iterator2> constexpr auto operator-( const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base()); template<[sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<Iterator> S> friend constexpr iter_difference_t<Iterator> operator-(const move_sentinel<S>& x, const move_iterator& y); template<[sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<Iterator> S> friend constexpr iter_difference_t<Iterator> operator-(const move_iterator& x, const move_sentinel<S>& y);

1

#

Returns: x.base() - y.base().

🔗

template<class Iterator> constexpr move_iterator<Iterator> operator+(iter_difference_t<Iterator> n, const move_iterator<Iterator>& x);

2

#

Constraints: x.base() + n is well-formed and has type Iterator.

3

#

Returns: x + n.

🔗

friend constexpr iter_rvalue_reference_t<Iterator> iter_move(const move_iterator& i) noexcept(noexcept(ranges::iter_move(i.current)));

4

#

Effects: Equivalent to: return ranges​::​iter_move(i.current);

🔗

template<[indirectly_swappable](alg.req.ind.swap#concept:indirectly_swappable "24.3.7.4 Concept indirectly_­swappable [alg.req.ind.swap]")<Iterator> Iterator2> friend constexpr void iter_swap(const move_iterator& x, const move_iterator<Iterator2>& y) noexcept(noexcept(ranges::iter_swap(x.current, y.current)));

5

#

Effects: Equivalent to: ranges​::​iter_swap(x.current, y.current).

🔗

template<class Iterator> constexpr move_iterator<Iterator> make_move_iterator(Iterator i);

6

#

Returns: move_iterator(std​::​move(i)).

24.5.4.10 Class template move_sentinel [move.sentinel]

1

#

Class template move_sentinel is a sentinel adaptor useful for denoting ranges together with move_iterator.

When an input iterator typeI and sentinel type S model sentinel_for<S, I>,move_sentinel and move_iterator modelsentinel_for<move_sentinel, move_iterator> as well.

2

#

[Example 1:

A move_if algorithm is easily implemented withcopy_if using move_iterator and move_sentinel:template<input_iterator I, sentinel_for S, weakly_incrementable O, indirect_unary_predicate Pred>requires indirectly_movable<I, O>void move_if(I first, S last, O out, Pred pred) { ranges::copy_if(move_iterator{std::move(first)}, move_sentinel{last}, std::move(out), pred);}

— end example]

🔗

namespace std {template<semiregular S>class move_sentinel {public:constexpr move_sentinel(); constexpr explicit move_sentinel(S s); templaterequires convertible_to<const S2&, S>constexpr move_sentinel(const move_sentinel& s); templaterequires assignable_from<S&, const S2&>constexpr move_sentinel& operator=(const move_sentinel& s); constexpr S base() const; private: S last; // exposition only};}

24.5.4.11 Operations [move.sent.ops]

🔗

constexpr move_sentinel();

1

#

Effects: Value-initializes last.

If is_trivially_default_constructible_v is true, then this constructor is a constexpr constructor.

🔗

constexpr explicit move_sentinel(S s);

2

#

Effects: Initializes last with std​::​move(s).

🔗

template<class S2> requires [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<const S2&, S> constexpr move_sentinel(const move_sentinel<S2>& s);

3

#

Effects: Initializes last with s.last.

🔗

template<class S2> requires [assignable_from](concept.assignable#concept:assignable_from "18.4.8 Concept assignable_­from [concept.assignable]")<S&, const S2&> constexpr move_sentinel& operator=(const move_sentinel<S2>& s);

4

#

Effects: Equivalent to: last = s.last; return *this;

🔗

constexpr S base() const;

5

#

Returns: last.

24.5.5 Common iterators [iterators.common]

24.5.5.1 Class template common_iterator [common.iterator]

1

#

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.

2

#

[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]

3

#

[Example 1: templatevoid fun(ForwardIterator begin, ForwardIterator end);

list s;// populate the list susing CI = common_iterator<counted_iterator<list::iterator>, default_sentinel_t>;// call fun on a range of 10 ints fun(CI(counted_iterator(s.begin(), 10)), CI(default_sentinel)); — end example]

🔗

namespace std {template<input_or_output_iterator I, sentinel_for S>requires (<I, S> && copyable)class common_iterator {public:constexpr common_iterator() requires default_initializable = 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*() constrequires dereferenceable; constexpr auto operator->() constrequires see below; constexpr common_iterator& operator++(); constexpr decltype(auto) operator++(int); template<class I2, sentinel_for 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 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 I2, sized_sentinel_for S2>requires sized_sentinel_for<S, I2>friend constexpr iter_difference_t 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; template<indirectly_swappable 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_; // exposition only}; template<class I, class S>struct incrementable_traits<common_iterator<I, S>> {using difference_type = iter_difference_t; }; template<input_iterator I, class S>struct iterator_traits<common_iterator<I, S>> {using iterator_concept = see below; using iterator_category = see below; // not always presentusing value_type = iter_value_t; using difference_type = iter_difference_t; using pointer = see below; using reference = iter_reference_t; };}

24.5.5.2 Associated types [common.iter.types]

1

#

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 is an integral type.

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

2

#

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

• (2.1)

  iterator_concept denotes forward_iterator_tag if I models forward_iterator; otherwise it denotes input_iterator_tag.

• (2.2)

  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]

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constexpr common_iterator(I i);

1

#

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

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constexpr common_iterator(S s);

2

#

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

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

3

#

Hardened preconditions: x.v_.valueless_by_exception() is false.

4

#

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

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template<class I2, class S2> requires [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<const I2&, I> && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<const S2&, S> && [assignable_from](concept.assignable#concept:assignable_from "18.4.8 Concept assignable_­from [concept.assignable]")<I&, const I2&> && [assignable_from](concept.assignable#concept:assignable_from "18.4.8 Concept assignable_­from [concept.assignable]")<S&, const S2&> constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);

5

#

Hardened preconditions: x.v_.valueless_by_exception() is false.

6

#

Effects: Equivalent to:

• (6.1)

  If v_.index() == x.v_.index(), thenget(v_) = get(x.v_).

• (6.2)

  Otherwise, v_.emplace(get(x.v_)).

where i is x.v_.index().

7

#

Returns: *this.

24.5.5.4 Accessors [common.iter.access]

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

1

#

Hardened preconditions: holds_alternative(v_) is true.

2

#

Effects: Equivalent to: return *get(v_);

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constexpr auto operator->() const requires see below;

3

#

The expression in the requires-clause is equivalent to:indirectly_readable &&(requires(const I& i) { i.operator->(); } || is_reference_v<iter_reference_t> ||constructible_from<iter_value_t, iter_reference_t>)

4

#

Hardened preconditions: holds_alternative(v_) is true.

5

#

Effects:

• (5.1)

If I is a pointer type or if the expressionget(v_).operator->() is well-formed, equivalent to: return get(v_);

• (5.2)

Otherwise, if iter_reference_t is a reference type, equivalent to:auto&& tmp = *get(v_);return addressof(tmp);

• (5.3)

Otherwise, equivalent to:return proxy(get(v_)); whereproxy* is the exposition-only class:class proxy { iter_value_t keep_; constexpr proxy(iter_reference_t&& x): keep_(std::move(x)) {}public:constexpr const iter_value_t* operator->() const noexcept {return addressof(keep_); }};

24.5.5.5 Navigation [common.iter.nav]

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constexpr common_iterator& operator++();

1

#

Hardened preconditions: holds_alternative(v_) is true.

2

#

Effects: Equivalent to ++get(v_).

3

#

Returns: *this.

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

4

#

Hardened preconditions: holds_alternative(v_) is true.

5

#

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 && constructible_from<iter_value_t, iter_reference_t> &&move_constructible<iter_value_t> is false, equivalent to:return get(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 keep_; constexpr postfix-proxy(iter_reference_t&& x): keep_(std::forward<iter_reference_t>(x)) {}public:constexpr const iter_value_t& operator() const noexcept {return keep_; }};

24.5.5.6 Comparisons [common.iter.cmp]

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template<class I2, [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<I> S2> requires [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "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);

1

#

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

2

#

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

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template<class I2, [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<I> S2> requires [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<S, I2> && [equality_comparable_with](concept.equalitycomparable#concept:equality_comparable_with "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")<I, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y);

3

#

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

4

#

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

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template<[sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<I> I2, [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<I> S2> requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<S, I2> friend constexpr iter_difference_t<I2> operator-( const common_iterator& x, const common_iterator<I2, S2>& y);

5

#

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

6

#

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

24.5.5.7 Customizations [common.iter.cust]

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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_iterator "24.3.4.9 Concept input_­iterator [iterator.concept.input]")<I>;

1

#

Hardened preconditions: holds_alternative(i.v_) is true.

2

#

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

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template<[indirectly_swappable](alg.req.ind.swap#concept:indirectly_swappable "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&>())));

3

#

Hardened preconditions: holds_alternative(x.v_) and holds_alternative(y.v_) are each true.

4

#

Effects: Equivalent to ranges​::​iter_swap(get(x.v_), get(y.v_)).

24.5.6 Default sentinel [default.sentinel]

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namespace std { struct default_sentinel_t { }; }

1

#

Class default_sentinel_t is an empty type used to denote the end of a range.

It can be used together with iterator types that know the bound of their range (e.g., counted_iterator ([counted.iterator])).

24.5.7 Counted iterators [iterators.counted]

24.5.7.1 Class template counted_iterator [counted.iterator]

1

#

Class template counted_iterator is an iterator adaptor with the same behavior as the underlying iterator except that it keeps track of the distance to the end of its range.

It can be used together with default_sentinel in calls to generic algorithms to operate on a range of N elements starting at a given position without needing to know the end position a priori.

2

#

[Example 1: list s;// populate the list s with at least 10 strings vector v;// copies 10 strings into v: ranges::copy(counted_iterator(s.begin(), 10), default_sentinel, back_inserter(v)); — end example]

3

#

Two values i1 and i2 of typescounted_iterator andcounted_iterator refer to elements of the same sequence if and only if there exists some integer n such thatnext(i1.base(), i1.count() + n) andnext(i2.base(), i2.count() + n) refer to the same (possibly past-the-end) element.

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namespace std {template<input_or_output_iterator I>class counted_iterator {public:using iterator_type = I; using value_type = iter_value_t; // present only// if I models indirectly_readableusing difference_type = iter_difference_t; using iterator_concept = typename I::iterator_concept; // present only// if the qualified-id I​::​iterator_concept is valid and denotes a typeusing iterator_category = typename I::iterator_category; // present only// if the qualified-id I​::​iterator_category is valid and denotes a typeconstexpr counted_iterator() requires default_initializable = default; constexpr counted_iterator(I x, iter_difference_t n); templaterequires convertible_to<const I2&, I>constexpr counted_iterator(const counted_iterator& x); templaterequires assignable_from<I&, const I2&>constexpr counted_iterator& operator=(const counted_iterator& x); constexpr const I& base() const & noexcept; constexpr I base() &&; constexpr iter_difference_t count() const noexcept; constexpr decltype(auto) operator*(); constexpr decltype(auto) operator*() constrequires dereferenceable; constexpr auto operator->() const noexceptrequires contiguous_iterator; constexpr counted_iterator& operator++(); constexpr decltype(auto) operator++(int); constexpr counted_iterator operator++(int)requires forward_iterator; constexpr counted_iterator& operator--()requires bidirectional_iterator; constexpr counted_iterator operator--(int)requires bidirectional_iterator; constexpr counted_iterator operator+(iter_difference_t n) constrequires random_access_iterator; friend constexpr counted_iterator operator+( iter_difference_t n, const counted_iterator& x)requires random_access_iterator; constexpr counted_iterator& operator+=(iter_difference_t n)requires random_access_iterator; constexpr counted_iterator operator-(iter_difference_t n) constrequires random_access_iterator; template<common_with I2>friend constexpr iter_difference_t operator-(const counted_iterator& x, const counted_iterator& y); friend constexpr iter_difference_t operator-(const counted_iterator& x, default_sentinel_t) noexcept; friend constexpr iter_difference_t operator-( default_sentinel_t, const counted_iterator& y) noexcept; constexpr counted_iterator& operator-=(iter_difference_t n)requires random_access_iterator; constexpr decltype(auto) operator[](iter_difference_t n) constrequires random_access_iterator; template<common_with I2>friend constexpr bool operator==(const counted_iterator& x, const counted_iterator& y); friend constexpr bool operator==(const counted_iterator& x, default_sentinel_t) noexcept; template<common_with I2>friend constexpr strong_ordering operator<=>(const counted_iterator& x, const counted_iterator& y); friend constexpr decltype(auto) iter_move(const counted_iterator& i)noexcept(noexcept(ranges::iter_move(i.current)))requires input_iterator; template<indirectly_swappable I2>friend constexpr void iter_swap(const counted_iterator& x, const counted_iterator& y)noexcept(noexcept(ranges::iter_swap(x.current, y.current))); private: I current = I(); // exposition only iter_difference_t length = 0; // exposition only}; template<input_iterator I>requires same_as<ITER_TRAITS(I), iterator_traits> // see [iterator.concepts.general]struct iterator_traits<counted_iterator> : iterator_traits {using pointer = conditional_t<contiguous_iterator, add_pointer_t<iter_reference_t>, void>; };}

24.5.7.2 Constructors and conversions [counted.iter.const]

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constexpr counted_iterator(I i, iter_difference_t<I> n);

1

#

Hardened preconditions: n >= 0 is true.

2

#

Effects: Initializes current with std​::​move(i) andlength with n.

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template<class I2> requires [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<const I2&, I> constexpr counted_iterator(const counted_iterator<I2>& x);

3

#

Effects: Initializes current with x.current andlength with x.length.

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template<class I2> requires [assignable_from](concept.assignable#concept:assignable_from "18.4.8 Concept assignable_­from [concept.assignable]")<I&, const I2&> constexpr counted_iterator& operator=(const counted_iterator<I2>& x);

4

#

Effects: Assigns x.current to current andx.length to length.

5

#

Returns: *this.

24.5.7.3 Accessors [counted.iter.access]

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constexpr const I& base() const & noexcept;

1

#

Effects: Equivalent to: return current;

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constexpr I base() &&;

2

#

Returns: std​::​move(current).

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constexpr iter_difference_t<I> count() const noexcept;

3

#

Effects: Equivalent to: return length;

24.5.7.4 Element access [counted.iter.elem]

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

1

#

Hardened preconditions: length > 0 is true.

2

#

Effects: Equivalent to: return *current;

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constexpr auto operator->() const noexcept requires [contiguous_iterator](iterator.concept.contiguous#concept:contiguous_iterator "24.3.4.14 Concept contiguous_­iterator [iterator.concept.contiguous]")<I>;

3

#

Effects: Equivalent to: return to_address(current);

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constexpr decltype(auto) operator[](iter_difference_t<I> n) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<I>;

4

#

Hardened preconditions: n < length is true.

5

#

Effects: Equivalent to: return current[n];

24.5.7.5 Navigation [counted.iter.nav]

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constexpr counted_iterator& operator++();

1

#

Hardened preconditions: length > 0 is true.

2

#

Effects: Equivalent to:++current;--length;return *this;

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

3

#

Hardened preconditions: length > 0 is true.

4

#

Effects: Equivalent to:--length;try { return current++; }catch(...) { ++length; throw; }

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constexpr counted_iterator operator++(int) requires [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11 Concept forward_­iterator [iterator.concept.forward]")<I>;

5

#

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

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constexpr counted_iterator& operator--() requires [bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]")<I>;

6

#

Effects: Equivalent to:--current;++length;return *this;

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constexpr counted_iterator operator--(int) requires [bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]")<I>;

7

#

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

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constexpr counted_iterator operator+(iter_difference_t<I> n) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<I>;

8

#

Effects: Equivalent to: return counted_iterator(current + n, length - n);

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friend constexpr counted_iterator operator+( iter_difference_t<I> n, const counted_iterator& x) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<I>;

9

#

Effects: Equivalent to: return x + n;

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constexpr counted_iterator& operator+=(iter_difference_t<I> n) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<I>;

10

#

Hardened preconditions: n <= length is true.

11

#

Effects: Equivalent to:current += n; length -= n;return *this;

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constexpr counted_iterator operator-(iter_difference_t<I> n) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<I>;

12

#

Effects: Equivalent to: return counted_iterator(current - n, length + n);

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template<[common_with](concept.common#concept:common_with "18.4.6 Concept common_­with [concept.common]")<I> I2> friend constexpr iter_difference_t<I2> operator-( const counted_iterator& x, const counted_iterator<I2>& y);

13

#

Preconditions: x and y refer to elements of the same sequence ([counted.iterator]).

14

#

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

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friend constexpr iter_difference_t<I> operator-( const counted_iterator& x, default_sentinel_t) noexcept;

15

#

Effects: Equivalent to:return -x.length;

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friend constexpr iter_difference_t<I> operator-( default_sentinel_t, const counted_iterator& y) noexcept;

16

#

Effects: Equivalent to: return y.length;

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constexpr counted_iterator& operator-=(iter_difference_t<I> n) requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<I>;

17

#

Hardened preconditions: -n <= length is true.

18

#

Effects: Equivalent to:current -= n; length += n;return *this;

24.5.7.6 Comparisons [counted.iter.cmp]

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template<[common_with](concept.common#concept:common_with "18.4.6 Concept common_­with [concept.common]")<I> I2> friend constexpr bool operator==( const counted_iterator& x, const counted_iterator<I2>& y);

1

#

Preconditions: x and y refer to elements of the same sequence ([counted.iterator]).

2

#

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

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friend constexpr bool operator==( const counted_iterator& x, default_sentinel_t) noexcept;

3

#

Effects: Equivalent to: return x.length == 0;

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template<[common_with](concept.common#concept:common_with "18.4.6 Concept common_­with [concept.common]")<I> I2> friend constexpr strong_ordering operator<=>( const counted_iterator& x, const counted_iterator<I2>& y);

4

#

Preconditions: x and y refer to elements of the same sequence ([counted.iterator]).

5

#

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

6

#

[Note 1:

The argument order in the Effects: element is reversed because length counts down, not up.

— end note]

24.5.7.7 Customizations [counted.iter.cust]

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

1

#

Hardened preconditions: i.length > 0 is true.

2

#

Effects: Equivalent to: return ranges​::​iter_move(i.current);

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template<[indirectly_swappable](alg.req.ind.swap#concept:indirectly_swappable "24.3.7.4 Concept indirectly_­swappable [alg.req.ind.swap]")<I> I2> friend constexpr void iter_swap(const counted_iterator& x, const counted_iterator<I2>& y) noexcept(noexcept(ranges::iter_swap(x.current, y.current)));

3

#

Hardened preconditions: Both x.length > 0 and y.length > 0 are true.

4

#

Effects: Equivalent to ranges​::​iter_swap(x.current, y.current).

24.5.8 Unreachable sentinel [unreachable.sentinel]

1

#

Class unreachable_sentinel_t can be used with any weakly_incrementable type to denote the “upper bound” of an unbounded interval.

2

#

[Example 1: char* p;// set p to point to a character buffer containing newlineschar* nl = find(p, unreachable_sentinel, '\n');

Provided a newline character really exists in the buffer, the use ofunreachable_sentinel above potentially makes the call to find more efficient since the loop test against the sentinel does not require a conditional branch.

— end example]

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namespace std {struct unreachable_sentinel_t {template<weakly_incrementable I>friend constexpr bool operator==(unreachable_sentinel_t, const I&) noexcept{ return false; }};}