22 Iterators library [iterators]

22.5 Iterator adaptors [predef.iterators]

22.5.1 Reverse iterators [reverse.iterators]

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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.
The fundamental relation between a reverse iterator and its corresponding iterator i is established by the identity: &*(reverse_­iterator(i)) == &*(i - 1).

22.5.1.1 Class template reverse_­iterator [reverse.iterator]

namespace std {
  template<class Iterator>
  class reverse_iterator {
  public:
    using iterator_type     = Iterator;
    using iterator_category = typename iterator_traits<Iterator>::iterator_category;
    using value_type        = typename iterator_traits<Iterator>::value_type;
    using difference_type   = typename iterator_traits<Iterator>::difference_type;
    using pointer           = typename iterator_traits<Iterator>::pointer;
    using reference         = typename iterator_traits<Iterator>::reference;

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

    constexpr Iterator base() const;      // explicit
    constexpr reference operator*() const;
    constexpr pointer   operator->() const;

    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;

  protected:
    Iterator current;
  };

  template<class Iterator1, class Iterator2>
    constexpr bool operator==(
      const reverse_iterator<Iterator1>& x,
      const reverse_iterator<Iterator2>& y);
  template<class Iterator1, class Iterator2>
    constexpr bool operator!=(
      const reverse_iterator<Iterator1>& x,
      const reverse_iterator<Iterator2>& y);
  template<class Iterator1, class Iterator2>
    constexpr bool operator<(
      const reverse_iterator<Iterator1>& x,
      const reverse_iterator<Iterator2>& y);
  template<class Iterator1, class Iterator2>
    constexpr bool operator>(
      const reverse_iterator<Iterator1>& x,
      const reverse_iterator<Iterator2>& y);
  template<class Iterator1, class Iterator2>
    constexpr bool operator<=(
      const reverse_iterator<Iterator1>& x,
      const reverse_iterator<Iterator2>& y);
  template<class Iterator1, class Iterator2>
    constexpr bool operator>=(
      const reverse_iterator<Iterator1>& x,
      const reverse_iterator<Iterator2>& y);
  template<class Iterator1, class Iterator2>
    constexpr auto operator-(
      const reverse_iterator<Iterator1>& x,
      const reverse_iterator<Iterator2>& y) -> decltype(y.base() - x.base());
  template<class Iterator>
    constexpr reverse_iterator<Iterator> operator+(
      typename reverse_iterator<Iterator>::difference_type n,
      const reverse_iterator<Iterator>& x);

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

22.5.1.2 reverse_­iterator requirements [reverse.iter.requirements]

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The template parameter Iterator shall satisfy all the requirements of a Cpp17BidirectionalIterator ([bidirectional.iterators]).
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Additionally, Iterator shall satisfy the requirements of a Cpp17RandomAccessIterator ([random.access.iterators]) if any of the members operator+, operator-, operator+=, operator-= ([reverse.iter.nav]), operator[] ([reverse.iter.elem]), or the non-member operators ([reverse.iter.cmp]) operator<, operator>, operator<=, operator>=, operator-, or operator+ ([reverse.iter.nonmember]) are referenced in a way that requires instantiation ([temp.inst]).

22.5.1.3 reverse_­iterator construction and assignment [reverse.iter.cons]

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constexpr reverse_iterator();
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Effects: Value-initializes current.
Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value-initialized iterator of type Iterator.
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constexpr explicit reverse_iterator(Iterator x);
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Effects: Initializes current with x.
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template<class U> constexpr reverse_iterator(const reverse_iterator<U>& u);
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Effects: Initializes current with u.current.
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template<class U> constexpr reverse_iterator& operator=(const reverse_iterator<U>& u);
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Effects: Assigns u.base() to current.
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Returns: *this.

22.5.1.4 Conversion [reverse.iter.conv]

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constexpr Iterator base() const; // explicit
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Returns: current.

22.5.1.5 reverse_­iterator 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;
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Returns: addressof(operator*()).
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constexpr unspecified operator[](difference_type n) const;
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Returns: current[-n-1].

22.5.1.6 reverse_­iterator 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.

22.5.1.7 reverse_­iterator 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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Returns: x.current == y.current.
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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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Returns: x.current != y.current.
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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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Returns: x.current > y.current.
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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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Returns: x.current < y.current.
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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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Returns: x.current >= y.current.
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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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Returns: x.current <= y.current.

22.5.1.8 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.current - x.current.
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template<class Iterator> constexpr reverse_iterator<Iterator> operator+( typename reverse_iterator<Iterator>::difference_type n, const reverse_iterator<Iterator>& x);
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Returns: reverse_­iterator<Iterator> (x.current - n).
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template<class Iterator> constexpr reverse_iterator<Iterator> make_reverse_iterator(Iterator i);
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Returns: reverse_­iterator<Iterator>(i).