24 Iterators library [iterators]

24.5 Iterator adaptors [predef.iterators]

24.5.4 Move iterators and sentinels [move.iterators]

24.5.4.1 General [move.iterators.general]

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.

[Example 1: list<string> s; // populate the list s vector<string> v1(s.begin(), s.end()); // copies strings into v1 vector<string> 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]

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namespace std { template<class Iterator> class move_iterator { public: using iterator_type = Iterator; using iterator_concept = see below; using iterator_category = see below; // not always present using value_type = iter_value_t<Iterator>; using difference_type = iter_difference_t<Iterator>; using pointer = Iterator; using reference = iter_rvalue_reference_t<Iterator>; constexpr move_iterator(); constexpr explicit move_iterator(Iterator i); template<class U> constexpr move_iterator(const move_iterator& u); template<class U> 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<Iterator> S> friend constexpr bool operator==(const move_iterator& x, const move_sentinel<S>& y); template<sized_sentinel_for<Iterator> S> friend constexpr iter_difference_t<Iterator> operator-(const move_sentinel<S>& x, const move_iterator& y); template<sized_sentinel_for<Iterator> S> friend constexpr iter_difference_t<Iterator> operator-(const move_iterator& x, const move_sentinel<S>& y); friend constexpr iter_rvalue_reference_t<Iterator> iter_move(const move_iterator& i) noexcept(noexcept(ranges::iter_move(i.current))); template<indirectly_swappable<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))); private: Iterator current; // exposition only }; }

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.

The member typedef-name iterator_category is defined if and only if the qualified-id iterator_traits<Iterator>::iterator_category is valid and denotes a type.

In that case, iterator_category denotes

(2.1)
random_access_iterator_tag if the type iterator_traits<Iterator>::iterator_category models derived_from<random_access_iterator_tag>, and
(2.2)
iterator_traits<Iterator>::iterator_category otherwise.

24.5.4.3 Requirements [move.iter.requirements]

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 model random_access_iterator ([iterator.concept.random.access]).

24.5.4.4 Construction and assignment [move.iter.cons]

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constexpr move_iterator();

Effects: Value-initializes current.

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constexpr explicit move_iterator(Iterator i);

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

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

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

Effects: Initializes current with u.current.

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

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

Effects: Assigns u.current to current.

Returns: *this.

24.5.4.5 Conversion [move.iter.op.conv]

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constexpr const Iterator& base() const & noexcept;

Returns: current.

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constexpr Iterator base() &&;

Returns: std::move(current).

24.5.4.6 Element access [move.iter.elem]

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constexpr reference operator*() const;

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

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constexpr reference operator[](difference_type n) const;

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

24.5.4.7 Navigation [move.iter.nav]

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constexpr move_iterator& operator++();

Effects: As if by ++current.

Returns: *this.

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constexpr auto operator++(int);

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

Otherwise, equivalent to ++current.

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constexpr move_iterator& operator--();

Effects: As if by --current.

Returns: *this.

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constexpr move_iterator operator--(int);

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

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constexpr move_iterator operator+(difference_type n) const;

Returns: move_iterator(current + n).

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constexpr move_iterator& operator+=(difference_type n);

Effects: As if by: current += n;

Returns: *this.

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constexpr move_iterator operator-(difference_type n) const;

Returns: move_iterator(current - n).

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constexpr move_iterator& operator-=(difference_type n);

Effects: As if by: current -= n;

Returns: *this.

24.5.4.8 Comparisons [move.iter.op.comp]

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template<class Iterator1, class Iterator2>
  constexpr bool operator==(const move_iterator<Iterator1>& x,
                            const move_iterator<Iterator2>& y);
template<sentinel_for<Iterator> S>
  friend constexpr bool operator==(const move_iterator& x,
                                   const move_sentinel<S>& y);

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

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

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

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

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

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

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

Returns: y < x.

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

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

Returns: !(y < x).

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

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

Returns: !(x < y).

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

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

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

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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> S>
  friend constexpr iter_difference_t<Iterator>
    operator-(const move_sentinel<S>& x, const move_iterator& y);
template<sized_sentinel_for<Iterator> S>
  friend constexpr iter_difference_t<Iterator>
    operator-(const move_iterator& x, const move_sentinel<S>& y);

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

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

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

Returns: x + n.

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

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

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template<indirectly_swappable<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)));

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

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

Returns: move_iterator<Iterator>(std::move(i)).

24.5.4.10 Class template move_sentinel [move.sentinel]

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

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

[Example 1:

A move_if algorithm is easily implemented with copy_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<S>{last}, std::move(out), pred); }

— end example]

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

24.5.4.11 Operations [move.sent.ops]

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constexpr move_sentinel();

Effects: Value-initializes last.

If is_trivially_default_constructible_v<S> is true, then this constructor is a constexpr constructor.

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constexpr explicit move_sentinel(S s);

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

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template<class S2>
  requires convertible_to<const S2&, S>
    constexpr move_sentinel(const move_sentinel<S2>& s);

Effects: Initializes last with s.last.

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template<class S2>
  requires assignable_from<S&, const S2&>
    constexpr move_sentinel& operator=(const move_sentinel<S2>& s);

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

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constexpr S base() const;

Returns: last.