9 Iterators library [iterators]

9.7.4.1 Class template common_iterator [common.iterator]

namespace std { namespace experimental { namespace ranges { inline namespace v1 {
  template <Iterator I, Sentinel<I> S>
    requires !Same<I, S>
  class common_iterator {
  public:
    using difference_type = difference_type_t<I>;

    constexpr common_iterator();
    constexpr common_iterator(I i);
    constexpr common_iterator(S s);
    constexpr common_iterator(const common_iterator<ConvertibleTo<I>, ConvertibleTo<S>>& u);
    common_iterator& operator=(const common_iterator<ConvertibleTo<I>, ConvertibleTo<S>>& u);

    decltype(auto) operator*();
    decltype(auto) operator*() const
      requires dereferenceable<const I>;
    decltype(auto) operator->() const
      requires see below;

    common_iterator& operator++();
    decltype(auto) operator++(int);
    common_iterator operator++(int)
      requires ForwardIterator<I>;

    friend rvalue_reference_t<I> iter_move(const common_iterator& i)
      noexcept(see below)
        requires InputIterator<I>;
    template <IndirectlySwappable<I> I2, class S2>
      friend void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y)
        noexcept(see below);

  private:
    bool is_sentinel; // exposition only
    I iter;           // exposition only
    S sentinel;       // exposition only
  };

  template <Readable I, class S>
  struct value_type<common_iterator<I, S>> {
    using type = value_type_t<I>;
  };

  template <InputIterator I, class S>
  struct iterator_category<common_iterator<I, S>> {
    using type = input_iterator_tag;
  };

  template <ForwardIterator I, class S>
  struct iterator_category<common_iterator<I, S>> {
    using type = forward_iterator_tag;
  };

  template <class I1, class I2, Sentinel<I2> S1, Sentinel<I1> S2>
  bool operator==(
    const common_iterator<I1, S1>& x, const common_iterator<I2, S2>& y);
  template <class I1, class I2, Sentinel<I2> S1, Sentinel<I1> S2>
    requires EqualityComparableWith<I1, I2>
  bool operator==(
    const common_iterator<I1, S1>& x, const common_iterator<I2, S2>& y);
  template <class I1, class I2, Sentinel<I2> S1, Sentinel<I1> S2>
  bool operator!=(
    const common_iterator<I1, S1>& x, const common_iterator<I2, S2>& y);

  template <class I2, SizedSentinel<I2> I1, SizedSentinel<I2> S1, SizedSentinel<I1> S2>
  difference_type_t<I2> operator-(
    const common_iterator<I1, S1>& x, const common_iterator<I2, S2>& y);
}}}}