27 Iterators library [iterators]

27.5 Iterator adaptors [predef.iterators]

27.5.1 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).

27.5.1.1 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);
}

27.5.1.2 reverse_­iterator requirements [reverse.iter.requirements]

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The template parameter Iterator shall meet all the requirements of a Bidirectional Iterator ([bidirectional.iterators]).
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Additionally, Iterator shall meet the requirements of a random access iterator ([random.access.iterators]) if any of the members operator+ ([reverse.iter.op+]), operator- ([reverse.iter.op-]), operator+= ([reverse.iter.op+=]), operator-= ([reverse.iter.op-=]), operator[] ([reverse.iter.opindex]), or the non-member operators operator< ([reverse.iter.op<]), operator> ([reverse.iter.op>]),
operator<= ([reverse.iter.op<=]), operator>= ([reverse.iter.op>=]), operator- ([reverse.iter.opdiff]) or operator+ ([reverse.iter.opsum]) are referenced in a way that requires instantiation ([temp.inst]).

27.5.1.3 reverse_­iterator operations [reverse.iter.ops]

27.5.1.3.1 reverse_­iterator constructor [reverse.iter.cons]

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.
constexpr explicit reverse_iterator(Iterator x);
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Effects: Initializes current with x.
template <class U> constexpr reverse_iterator(const reverse_iterator<U>& u);
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Effects: Initializes current with u.current.

27.5.1.3.2 reverse_­iterator​::​operator= [reverse.iter.op=]

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.

27.5.1.3.3 Conversion [reverse.iter.conv]

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

27.5.1.3.4 operator* [reverse.iter.op.star]

constexpr reference operator*() const;
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Effects: As if by: 
Iterator tmp = current;
return *--tmp;

27.5.1.3.5 operator-> [reverse.iter.opref]

constexpr pointer operator->() const;
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Returns: addressof(operator*()).

27.5.1.3.6 operator++ [reverse.iter.op++]

constexpr reverse_iterator& operator++();
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Effects: As if by: --current;
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Returns: *this.
constexpr reverse_iterator operator++(int);
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Effects: As if by: 
reverse_iterator tmp = *this;
--current;
return tmp;

27.5.1.3.7 operator-- [reverse.iter.op--]

constexpr reverse_iterator& operator--();
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Effects: As if by ++current.
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Returns: *this.
constexpr reverse_iterator operator--(int);
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Effects: As if by: 
reverse_iterator tmp = *this;
++current;
return tmp;

27.5.1.3.8 operator+ [reverse.iter.op+]

constexpr reverse_iterator operator+(difference_type n) const;
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Returns: reverse_­iterator(current-n).

27.5.1.3.9 operator+= [reverse.iter.op+=]

constexpr reverse_iterator& operator+=(difference_type n);
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Effects: As if by: current -= n;
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Returns: *this.

27.5.1.3.10 operator- [reverse.iter.op-]

constexpr reverse_iterator operator-(difference_type n) const;
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Returns: reverse_­iterator(current+n).

27.5.1.3.11 operator-= [reverse.iter.op-=]

constexpr reverse_iterator& operator-=(difference_type n);
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Effects: As if by: current += n;
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Returns: *this.

27.5.1.3.12 operator[] [reverse.iter.opindex]

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

27.5.1.3.13 operator== [reverse.iter.op==]

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.

27.5.1.3.14 operator< [reverse.iter.op<]

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.

27.5.1.3.15 operator!= [reverse.iter.op!=]

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.

27.5.1.3.16 operator> [reverse.iter.op>]

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.

27.5.1.3.17 operator>= [reverse.iter.op>=]

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.

27.5.1.3.18 operator<= [reverse.iter.op<=]

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.

27.5.1.3.19 operator- [reverse.iter.opdiff]

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.

27.5.1.3.20 operator+ [reverse.iter.opsum]

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).

27.5.1.3.21 Non-member function make_­reverse_­iterator() [reverse.iter.make]

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