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<input_or_output_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 convertible_to<const I2&, I>
        constexpr counted_iterator(const counted_iterator<I2>& x);

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

    constexpr I base() const & requires copy_constructible<I>;
    constexpr I base() &&;
    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 forward_iterator<I>;
    constexpr counted_iterator& operator--()
      requires bidirectional_iterator<I>;
    constexpr counted_iterator operator--(int)
      requires bidirectional_iterator<I>;

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

    constexpr counted_iterator operator-(iter_difference_t<I> n) const
      requires random_access_iterator<I>;
    template<common_with<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 random_access_iterator<I>;

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

    template<common_with<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);

    template<common_with<I> I2>
      friend constexpr strong_ordering 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 input_iterator<I>;
    template<indirectly_swappable<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<input_iterator I>
  struct iterator_traits<counted_iterator<I>> : iterator_traits<I> {
    using pointer = void;
  };
}

23.5.6.2 Constructors and conversions [counted.iter.const]

constexpr counted_iterator(I i, iter_difference_t<I> n);
Preconditions: n >= 0.
Effects: Initializes current with i and length with n.
template<class I2> requires convertible_to<const I2&, I> constexpr counted_iterator(const counted_iterator<I2>& x);
Effects: Initializes current with x.current and length with x.length.
template<class I2> requires assignable_from<I&, const I2&> constexpr counted_iterator& operator=(const counted_iterator<I2>& x);
Effects: Assigns x.current to current and x.length to length.
Returns: *this.

23.5.6.3 Accessors [counted.iter.access]

constexpr I base() const & requires copy_constructible<I>;
Effects: Equivalent to: return current;
constexpr I base() &&;
Returns: std​::​move(current).
constexpr iter_difference_t<I> count() const noexcept;
Effects: Equivalent to: return length;

23.5.6.4 Element access [counted.iter.elem]

constexpr decltype(auto) operator*(); constexpr decltype(auto) operator*() const requires dereferenceable<const I>;
Effects: Equivalent to: return *current;
constexpr decltype(auto) operator[](iter_difference_t<I> n) const requires random_access_iterator<I>;
Preconditions: n < length.
Effects: Equivalent to: return current[n];

23.5.6.5 Navigation [counted.iter.nav]

constexpr counted_iterator& operator++();
Preconditions: length > 0.
Effects: Equivalent to:
++current;
--length;
return *this;
decltype(auto) operator++(int);
Preconditions: length > 0.
Effects: Equivalent to:
--length;
try { return current++; }
catch(...) { ++length; throw; }
constexpr counted_iterator operator++(int) requires forward_iterator<I>;
Effects: Equivalent to:
counted_iterator tmp = *this;
++*this;
return tmp;
constexpr counted_iterator& operator--(); requires bidirectional_iterator<I>
Effects: Equivalent to:
--current;
++length;
return *this;
constexpr counted_iterator operator--(int) requires bidirectional_iterator<I>;
Effects: Equivalent to:
counted_iterator tmp = *this;
--*this;
return tmp;
constexpr counted_iterator operator+(iter_difference_t<I> n) const requires random_access_iterator<I>;
Effects: Equivalent to: return counted_­iterator(current + n, length - n);
friend constexpr counted_iterator operator+( iter_difference_t<I> n, const counted_iterator& x) requires random_access_iterator<I>;
Effects: Equivalent to: return x + n;
constexpr counted_iterator& operator+=(iter_difference_t<I> n) requires random_access_iterator<I>;
Preconditions: n <= length.
Effects: Equivalent to:
current += n;
length -= n;
return *this;
constexpr counted_iterator operator-(iter_difference_t<I> n) const requires random_access_iterator<I>;
Effects: Equivalent to: return counted_­iterator(current - n, length + n);
template<common_with<I> I2> friend constexpr iter_difference_t<I2> operator-( const counted_iterator& x, const counted_iterator<I2>& y);
Preconditions: x and y refer to elements of the same sequence ([counted.iterator]).
Effects: Equivalent to: return y.length - x.length;
friend constexpr iter_difference_t<I> operator-( const counted_iterator& x, default_sentinel_t);
Effects: Equivalent to: return -x.length;
friend constexpr iter_difference_t<I> operator-( default_sentinel_t, const counted_iterator& y);
Effects: Equivalent to: return y.length;
constexpr counted_iterator& operator-=(iter_difference_t<I> n) requires random_access_iterator<I>;
Preconditions: -n <= length.
Effects: Equivalent to:
current -= n;
length += n;
return *this;

23.5.6.6 Comparisons [counted.iter.cmp]

template<common_with<I> I2> friend constexpr bool operator==( const counted_iterator& x, const counted_iterator<I2>& y);
Preconditions: x and y refer to elements of the same sequence ([counted.iterator]).
Effects: Equivalent to: return x.length == y.length;
friend constexpr bool operator==( const counted_iterator& x, default_sentinel_t);
Effects: Equivalent to: return x.length == 0;
template<common_with<I> I2> friend constexpr strong_ordering operator<=>( const counted_iterator& x, const counted_iterator<I2>& y);
Preconditions: x and y refer to elements of the same sequence ([counted.iterator]).
Effects: Equivalent to: return y.length <=> x.length;
Note
:
The argument order in the Effects: element is reversed because length counts down, not up.
— end note
 ]

23.5.6.7 Customizations [counted.iter.cust]

friend constexpr iter_rvalue_reference_t<I> iter_move(const counted_iterator& i) noexcept(noexcept(ranges::iter_move(i.current))) requires input_iterator<I>;
Effects: Equivalent to: return ranges​::​iter_­move(i.current);
template<indirectly_swappable<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)));
Effects: Equivalent to ranges​::​iter_­swap(x.current, y.current).