23 Iterators library [iterators]

23.5 Iterator adaptors [predef.iterators]

23.5.6 Counted iterators [iterators.counted]

23.5.6.1 Class template counted_­iterator [counted.iterator]

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.
[Example
:
list<string> s;
// populate the list s with at least 10 strings
vector<string> v;
// copies 10 strings into v:
ranges::copy(counted_iterator(s.begin(), 10), default_sentinel, back_inserter(v));
end example
]
Two values i1 and i2 of types counted_­iterator<I1> and counted_­iterator<I2> refer to elements of the same sequence if and only if next(i1.base(), i1.count()) and next(i2.base(), i2.count()) refer to the same (possibly past-the-end) element.
namespace std {
  template<Iterator I>
  class counted_iterator {
  public:
    using iterator_type = I;

    constexpr counted_iterator() = default;
    constexpr counted_iterator(I x, iter_difference_t<I> n);
    template<class I2>
      requires ConvertibleTo<const I2&, I>
        constexpr counted_iterator(const counted_iterator<I2>& x);

    template<class I2>
      requires Assignable<I&, const I2&>
        constexpr counted_iterator& operator=(const counted_iterator<I2>& x);

    constexpr I base() const;
    constexpr iter_difference_t<I> count() const noexcept;
    constexpr decltype(auto) operator*();
    constexpr decltype(auto) operator*() const
      requires dereferenceable<const I>;

    constexpr counted_iterator& operator++();
    decltype(auto) operator++(int);
    constexpr counted_iterator operator++(int)
      requires ForwardIterator<I>;
    constexpr counted_iterator& operator--()
      requires BidirectionalIterator<I>;
    constexpr counted_iterator operator--(int)
      requires BidirectionalIterator<I>;

    constexpr counted_iterator operator+(iter_difference_t<I> n) const
      requires RandomAccessIterator<I>;
    friend constexpr counted_iterator operator+(
      iter_difference_t<I> n, const counted_iterator& x)
        requires RandomAccessIterator<I>;
    constexpr counted_iterator& operator+=(iter_difference_t<I> n)
      requires RandomAccessIterator<I>;

    constexpr counted_iterator operator-(iter_difference_t<I> n) const
      requires RandomAccessIterator<I>;
    template<Common<I> I2>
      friend constexpr iter_difference_t<I2> operator-(
        const counted_iterator& x, const counted_iterator<I2>& y);
    friend constexpr iter_difference_t<I> operator-(
      const counted_iterator& x, default_sentinel_t);
    friend constexpr iter_difference_t<I> operator-(
      default_sentinel_t, const counted_iterator& y);
    constexpr counted_iterator& operator-=(iter_difference_t<I> n)
      requires RandomAccessIterator<I>;

    constexpr decltype(auto) operator[](iter_difference_t<I> n) const
      requires RandomAccessIterator<I>;

    template<Common<I> I2>
      friend constexpr bool operator==(
        const counted_iterator& x, const counted_iterator<I2>& y);
    friend constexpr bool operator==(
      const counted_iterator& x, default_sentinel_t);
    friend constexpr bool operator==(
      default_sentinel_t, const counted_iterator& x);

    template<Common<I> I2>
      friend constexpr bool operator!=(
        const counted_iterator& x, const counted_iterator<I2>& y);
    friend constexpr bool operator!=(
      const counted_iterator& x, default_sentinel_t y);
    friend constexpr bool operator!=(
      default_sentinel_t x, const counted_iterator& y);

    template<Common<I> I2>
      friend constexpr bool operator<(
        const counted_iterator& x, const counted_iterator<I2>& y);
    template<Common<I> I2>
      friend constexpr bool operator>(
        const counted_iterator& x, const counted_iterator<I2>& y);
    template<Common<I> I2>
      friend constexpr bool operator<=(
        const counted_iterator& x, const counted_iterator<I2>& y);
    template<Common<I> I2>
      friend constexpr bool operator>=(
        const counted_iterator& x, const counted_iterator<I2>& y);

    friend constexpr iter_rvalue_reference_t<I> iter_move(const counted_iterator& i)
      noexcept(noexcept(ranges::iter_move(i.current)))
        requires InputIterator<I>;
    template<IndirectlySwappable<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)));

  private:
    I current = I();                    // exposition only
    iter_difference_t<I> length = 0;    // exposition only
  };

  template<class I>
  struct incrementable_traits<counted_iterator<I>> {
    using difference_type = iter_difference_t<I>;
  };

  template<InputIterator I>
  struct iterator_traits<counted_iterator<I>> : iterator_traits<I> {
    using pointer = void;
  };
}