Windowing range adaptors: views::chunk and views::slide

Document #: P2442R0
Date: 2021-09-14
Project: Programming Language C++
Audience: LEWG
Reply-to: Tim Song
<>

1 Abstract

This paper proposes two range adaptors, views::chunk and views::slide, as described in section 3.5 of [P2214R0].

2 Example

std::vector v = {1, 2, 3, 4, 5};
fmt::print("{}\n", v | std::views::chunk(2));   // [[1, 2], [3, 4], [5]]
fmt::print("{}\n", v | std::views::slide(2));   // [[1, 2], [2, 3], [3, 4], [4, 5]]

3 Design

Both of these range adaptors are well-known quantities that have been shipped in range-v3 for years and are further discussed in section 3.5 of [P2214R0]. The discussion below assumes familiarity with that paper.

3.1 chunk

The basic idea of chunk is simple: views::chunk(R, N) divides R into non-overlapping N-sized chunks, except that the last chunk can be smaller than N in size. It is a precondition that N is positive.

3.1.1 Input range support

Matching the range-v3 implementation, the proposed chunk supports input ranges, even though the algorithm necessary for such support is significantly different.

In particular, for input ranges, the underlying iterator as well as related iteration state is tracked in the chunk_view object itself. This means that this chunk_view can only support non-const iteration. As a point of departure from range-v3, both outer and inner iterators are move-only input iterators.

Because the inner iterator and outer iterator share state, and moving from the stored underlying iterators can invalidate both iterators, only the non-mutating base() const& overload is provided for the inner iterator to avoid this sort of spooky-action-at-a-distance invalidation:

auto v = some_input_view | views::chunk(2);
auto outer = v.begin();
auto range = *outer;
range.begin().base(); // if this moved the underlying iterator, outer would be invalidated

Providing base() for the outer iterator would be misleading as the stored iterator mutates when the inner range is being iterated over.

3.1.2 Value type

For input ranges, chunk has a bespoke value type that is itself an input view. For forward and stronger ranges, chunk defers to views::take for its value type.

3.1.3 Conditionally common

In range-v3, chunk is never a common range. chunk as proposed here is a common range if the underlying range is forward, common, and either sized or non-bidirectional.

For bidirectional and stronger ranges, the need to size the last chunk correctly from the end iterator requires the underlying range to be sized.

3.1.4 Conditionally borrowed

As with range-v3, this paper proposes that chunk is borrowed if the underlying view is forward and borrowed.

3.1.5 Implementation notes

For input ranges, chunk_view stores

Incrementing the inner iterator increments current_ and decrements remainder_, setting it to zero if the end of the chunk is reached.

Incrementing the outer iterator increments current_ remainder_ times so that we start at the next N-element chunk even if the inner view isn’t iterated to end, and then sets remainder_ to the chunk size.

For forward and stronger ranges, chunk_view’s iterator stores an exposition-only data member offset_. This data member can only be nonzero when the iterator is the past-the-end iterator, in which case it represents the difference between N and the size of the last chunk.

3.2 slide

views::slide(R, N) produces a range of ranges such that the M-th range is a view into the M-th through (M+N-1)-th elements of R. It is similar to views::adjacent ([P2321R2]), except that the size of the window N is provided at runtime. It is a precondition that N is positive.

3.2.1 Forward ranges only

Unlike chunk, and similar to adjacent, slide does not support input ranges. It might be possible to support a window size of 1 - but then users can just use chunk instead. Caching elements is not considered a viable approach, essentially for the reasons discussed in section 4.3.4 of [P2321R2].

3.2.2 Value type

slide defers to views::counted for its value type.

3.2.3 Conditionally common and borrowed

slide is a common range if the underlying range is (or can be trivially made one), and is a borrowed range if the underlying range is.

3.2.4 Implementation notes

There are basically two strategies for slide. For simplicity the discussion below ignores the case where the number of elements in the source view is less than N.

3.3 Implementation experience

Both chunk and slide (under the name sliding) have been implemented in range-v3, and much of the subtler aspects of the implementation logic in the wording below are taken from there (any errors are mine, of course, though hopefully there isn’t any).

I also have implemented a version that directly follows the proposed wording below without issue.

4 Wording

4.1 Addition to <ranges>

Add the following to 24.2 [ranges.syn], header <ranges> synopsis:

// [...]
namespace std::ranges {
  // [...]

  // [range.chunk], chunk view
  template<view V>
    requires input_range<V>
  class chunk_view;

  template<view V>
    requires forward_range<V>
  class chunk_view<V>;

  template<class V>
    inline constexpr bool enable_borrowed_range<chunk_view<V>> =
      forward_range<V> && enable_borrowed_range<V>;

  namespace views {
    inline constexpr unspecified chunk = unspecified;
  }

  // [range.slide], slide view
  template<view V>
    requires forward_range<V>
  class slide_view;

  template<class V>
    inline constexpr bool enable_borrowed_range<slide_view<V>> =
      enable_borrowed_range<V>;

  namespace views {
    inline constexpr unspecified slide = unspecified;
  }
}

4.2 chunk

Add the following subclause to 24.7 [range.adaptors].

24.7.? Chunk view [range.chunk]

24.7.?.1 Overview [range.chunk.overview]

1 chunk_view takes a view and a number N and produces a range of views that are N-sized non-overlapping contiguous chunks of the elements of the original view, in order. The last view in the range can have fewer than N elements.

2 The name views::chunk denotes a range adaptor object (24.7.2 [range.adaptor.object]). Given subexpressions E and N, the expression views::chunk(E, N) is expression-equivalent to chunk_view(E, N).

[ Example 1: 
vector v = {1, 2, 3, 4, 5};

for (auto r : v | views::chunk(2)) {
  cout << '[';
  auto sep = "";
  for(auto i : r) {
    cout << sep << i;
    sep = ", ";
  }
  cout << "] ";
}
// prints: [1, 2] [3, 4] [5]
— end example ]

24.7.?.2 chunk_view for input ranges [range.chunk.view.input]

namespace std::ranges {

template<view V>
  requires input_range<V>
class chunk_view : public view_interface<chunk_view<V>>{
  V base_ = V();                        // exposition only
  range_difference_t<V> n_ = 0;         // exposition only
  range_difference_t<V> remainder_ = 0; // exposition only

  non-propagating-cache<iterator_t<V>> current_; // exposition only

  // [range.chunk.outer.iter], class chunk_view::outer-iterator
  class outer-iterator;                 // exposition only
  // [range.chunk.inner.iter], class chunk_view::inner-iterator
  class inner-iterator;                 // exposition only

public:
  chunk_view() requires default_initializable<V> = default;
  constexpr explicit chunk_view(V base, range_difference_t<V> n);

  constexpr V base() const & requires copy_constructible<V> { return base_; }
  constexpr V base() && { return std::move(base_); }

  constexpr outer-iterator begin();
  constexpr default_sentinel_t end();

  constexpr auto size() requires sized_range<V>;
  constexpr auto size() const requires sized_range<const V>;
};

template<class R>
  chunk_view(R&& r, range_difference_t<R>) -> chunk_view<views::all_t<R>>;

}
constexpr explicit chunk_view(V base, range_difference_t<V> n);

1 Preconditions: n > 0 is true.

2 Effects: Initializes base_ with std::move(base) and n_ with n.

constexpr outer-iterator begin();

3 Effects: Equivalent to:

current_ = ranges::begin(base_);
remainder_ = n_;
return outer-iterator(*this);
constexpr default_sentinel_t end();

4 Returns: default_sentinel.

constexpr auto size() requires sized_range<V>;
constexpr auto size() const requires sized_range<const V>;

5 Effects: Equivalent to:

auto sz = (ranges::distance(base_) + n_ - 1) / n_;
return to-unsigned-like(sz);

24.7.?.3 Class chunk_view::outer-iterator [range.chunk.outer.iter]

namespace std::ranges {
  template<view V>
    requires input_range<V>
  class chunk_view<V>::outer-iterator {
    chunk_view* parent_;                                    // exposition only

    constexpr explicit outer-iterator(chunk_view& parent);  // exposition only
  public:
    using iterator_concept = input_iterator_tag;
    using difference_type  = range_difference_t<V>;

    // [range.chunk.outer.value], class chunk_view::outer-iterator::value_type
    struct value_type;

    outer-iterator(outer-iterator&&) = default;
    outer-iterator& operator=(outer-iterator&&) = default;

    constexpr value_type operator*() const;
    constexpr outer-iterator& operator++();
    constexpr void operator++(int);

    friend constexpr bool operator==(const outer-iterator& x, default_sentinel_t);

    friend constexpr difference_type operator-(default_sentinel_t y, const outer-iterator& x)
      requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
    friend constexpr difference_type operator-(const outer-iterator& x, default_sentinel_t y)
      requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
  };
}
constexpr explicit outer-iterator(chunk_view& parent);

1 Effects: Initializes parent_ with addressof(parent).

constexpr value_type operator*() const;

2 Preconditions: *this == default_sentinel is false.

3 Returns: value_type(*parent_).

constexpr outer-iterator& operator++();

4 Preconditions: *this == default_sentinel is false.

5 Effects: Equivalent to:

   ranges::advance(*parent_->current_, parent_->remainder_, ranges::end(parent_->base_));
   parent_->remainder_ = parent_->n_;
   return *this;
constexpr void operator++(int);

6 Effects: Equivalent to ++*this.

friend constexpr bool operator==(const outer-iterator& x, default_sentinel_t);

7 Returns: *x.parent_->current_ == ranges::end(x.parent_->base_) && x.parent_->remainder_ != 0.

friend constexpr difference_type operator-(default_sentinel_t y, const outer-iterator& x)
  requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

8 Effects: Equivalent to:

const auto dist = ranges::end(x.parent_->base_) - *x.parent_->current_;

if (dist < x.parent_->remainder_) {
  return dist == 0 ? 0 : 1;
}

return (dist - x.parent_->remainder_ + x.parent_->n_ - 1) / x.parent_->n_ + 1;
friend constexpr difference_type operator-(const outer-iterator& x, default_sentinel_t y)
  requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

9 Effects: Equivalent to: return -(y - x);

24.7.?.4 Class chunk_view::outer-iterator::value_type [range.chunk.outer.value]

namespace std::ranges {
  template<view V>
    requires input_range<V>
  struct chunk_view<V>::outer-iterator::value_type : view_interface<value_type> {
    chunk_view* parent_;                                // exposition only

    constexpr explicit value_type(chunk_view& parent);    // exposition only
  public:
    constexpr inner-iterator begin() const;
    constexpr default_sentinel_t end() const;

    constexpr auto size() const
      requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
  };
}
constexpr explicit value_type(chunk_view& parent);

1 Effects: Initializes parent_ with addressof(parent).

constexpr inner-iterator begin() const;

2 Returns: inner-iterator(*parent_).

constexpr default_sentinel_t end() const;

3 Returns: default_sentinel.

constexpr auto size() const
  requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

4 Returns: ranges::min(parent_->remainder_, ranges::end(parent_->base_) - *parent_->current_).

24.7.?.5 Class chunk_view::inner-iterator [range.chunk.inner.iter]

namespace std::ranges {
  template<view V>
    requires input_range<V>
  class chunk_view<V>::inner-iterator {
    chunk_view* parent_;                                // exposition only

    constexpr explicit inner-iterator(chunk_view& parent);    // exposition only
  public:
    using iterator_concept  = input_iterator_tag;
    using difference_type = range_difference_t<V>;
    using value_type = range_value_t<V>;

    inner-iterator(inner-iterator&&) = default;
    inner-iterator& operator=(inner-iterator&&) = default;

    constexpr const iterator_t<V>& base() const &;

    constexpr range_reference_t<V> operator*() const;
    constexpr inner-iterator& operator++();
    constexpr void operator++(int);

    friend constexpr bool operator==(const inner-iterator& x, default_sentinel_t);

    friend constexpr difference_type operator-(default_sentinel_t y, const inner-iterator& x)
      requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
    friend constexpr difference_type operator-(const inner-iterator& x, default_sentinel_t y)
      requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
  };
}
constexpr explicit inner-iterator(chunk_view& parent);

1 Effects: Initializes parent_ with addressof(parent).

const iterator_t<V>& base() const &;

2 Returns: *parent_->current_.

constexpr range_reference_t<V> operator*() const;

3 Preconditions: *this == default_sentinel is false.

4 Effects: Equivalent to: return **parent_->current_;

constexpr iterator& operator++();

5 Preconditions: *this == default_sentinel is false.

6 Effects: Equivalent to:

  ++*parent_->current_;
  if (*parent_->current_ == ranges::end(parent_->base_)) {
    parent_->remainder_ = 0;
  }
  else {
    --parent_->remainder_;
  }
  return *this;
constexpr void operator++(int);

7 Effects: Equivalent to ++*this.

friend constexpr bool operator==(const inner-iterator& x, default_sentinel_t);

8 Returns: x.parent_->remainder_ == 0.

friend constexpr difference_type operator-(default_sentinel_t y, const inner-iterator& x)
  requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

9 Returns: ranges::min(x.parent_->remainder_, ranges::end(x.parent_->base_) - *x.parent_->current_).

friend constexpr difference_type operator-(const inner-iterator& x, default_sentinel_t y)
  requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

10 Effects: Equivalent to: return -(y - x);

24.7.?.6 chunk_view for forward ranges [range.chunk.view.fwd]

namespace std::ranges {

template<view V>
  requires forward_range<V>
class chunk_view<V> : public view_interface<chunk_view<V>>{
  V base_ = V();                     // exposition only
  range_difference_t<V> n_ = 0;      // exposition only
  template<bool> class iterator;     // exposition only

public:
  chunk_view() requires default_initializable<V> = default;
  constexpr explicit chunk_view(V base, range_difference_t<V> n);

  constexpr V base() const & requires copy_constructible<V> { return base_; }
  constexpr V base() && { return std::move(base_); }

  constexpr auto begin() requires (!simple-view<V>) {
    return iterator<false>(this, ranges::begin(base_));
  }

  constexpr auto begin() const requires forward_range<const V> {
    return iterator<true>(this, ranges::begin(base_));
  }

  constexpr auto end() requires (!simple-view<V>) {
    if constexpr (common_range<V> && sized_range<V>) {
      auto offset = (n_ - ranges::distance(base_) % n_) % n_;
      return iterator<false>(this, ranges::end(base_), offset);
    }
    else if constexpr (common_range<V> && !bidirectional_range<V>) {
      return iterator<false>(this, ranges::end(base_));
    }
    else {
      return default_sentinel;
    }
  }

  constexpr auto end() const requires forward_range<const V> {
    if constexpr (common_range<const V> && sized_range<const V>) {
      auto offset = (n_ - ranges::distance(base_) % n_) % n_;
      return iterator<true>(this, ranges::end(base_), offset);
    }
    else if constexpr (common_range<const V> && !bidirectional_range<const V>) {
      return iterator<true>(this, ranges::end(base_));
    }
    else {
      return default_sentinel;
    }
  }

  constexpr auto size() requires sized_range<V>;
  constexpr auto size() const requires sized_range<const V>;
};

}
constexpr explicit chunk_view(V base, range_difference_t<V> n);

1 Preconditions: n > 0 is true.

2 Effects: Initializes base_ with std::move(base) and n_ with n.

constexpr auto size() requires sized_range<V>;
constexpr auto size() const requires sized_range<const V>;

3 Effects: Equivalent to:

auto sz = (ranges::distance(base_) + n_ - 1) / n_;
return to-unsigned-like(sz);

24.7.?.7 Class template chunk_view<V>::iterator for forward ranges [range.chunk.fwd.iter]

namespace std::ranges {
  template<view V>
    requires forward_range<V>
  template<bool Const>
  class chunk_view<V>::iterator {
    using Parent = maybe-const<Const, chunk_view>;              // exposition only
    using Base = maybe-const<Const, V>;                         // exposition only

    iterator_t<Base> current_ = iterator_t<Base>();             // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();                 // exposition only
    range_difference_t<Base> n_ = 0;                            // exposition only
    range_difference_t<Base> offset_ = 0;                       // exposition only

    constexpr iterator(Parent* parent, iterator_t<Base> current,  // exposition only
                       range_difference_t<Base> offset = 0);
  public:
    using iterator_category = input_iterator_tag;
    using iterator_concept  = see below;
    using value_type = decltype(views::take(subrange(current_, end_), n_));
    using difference_type = range_difference_t<Base>;

    iterator() = default;
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>
                     && convertible_to<sentinel_t<V>, sentinel_t<Base>>;

    constexpr iterator_t<Base> base() const;

    constexpr value_type operator*() const;
    constexpr iterator& operator++();
    constexpr iterator operator++(int);

    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;

    constexpr iterator& operator+=(difference_type x)
      requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type x)
      requires random_access_range<Base>;

    constexpr value_type operator[](difference_type n) const
      requires random_access_range<Base>;

    friend constexpr bool operator==(const iterator& x, const iterator& y);
    friend constexpr bool operator==(const iterator& x, default_sentinel_t);

    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> &&
               three_way_comparable<iterator_t<Base>>;

    friend constexpr iterator operator+(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type n, const iterator& i)
      requires random_access_range<Base>;
    friend constexpr iterator operator-(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;

    friend constexpr difference_type operator-(default_sentinel_t y, const iterator& x)
      requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
    friend constexpr difference_type operator-(const iterator& x, default_sentinel_t y)
      requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
  };
}

1 iterator::iterator_concept is defined as follows:

constexpr iterator(Parent* parent, iterator_t<Base> current,
                     range_difference_t<Base> offset = 0);

2 Effects: Initializes current_ with current, end_ with ranges::end(parent->base_), n_ with parent->n_, and offset_ with offset.

constexpr iterator(iterator<!Const> i)
  requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>
                 && convertible_to<sentinel_t<V>, sentinel_t<Base>>;

3 Effects: Initializes current_ with std::move(i.current_), end_ with std::move(i.end_), n_ with i.n_, and offset_ with i.offset_.

constexpr iterator_t<Base> base() const;

4 Returns: current_.

constexpr value_type operator*() const;

5 Preconditions: current_ != end_ is true.

6 Returns: views::take(subrange(current_, end_), n_)

constexpr iterator& operator++();

7 Preconditions: current_ != end_ is true.

8 Effects: Equivalent to:

   offset_ = ranges::advance(current_, n_, end_);
   return *this;
constexpr iterator operator++(int);

9 Effects: Equivalent to:

  auto tmp = *this;
  ++*this;
  return tmp;
constexpr iterator& operator--() requires bidirectional_range<Base>;

10 Effects: Equivalent to:

   ranges::advance(current_, offset_ - n_);
   offset_ = 0;
   return *this;
constexpr iterator operator--(int) requires bidirectional_range<Base>;

11 Effects: Equivalent to:

  auto tmp = *this;
  --*this;
  return tmp;
constexpr iterator& operator+=(difference_type x)
  requires random_access_range<Base>;

12 Preconditions: If x is positive, ranges::distance(current_, end_) > n_ * (x - 1) is true.

13 Effects: Equivalent to:

if (x > 0) {
  offset_ = ranges::advance(current_, n_ * x, end_);
}
else if (x < 0) {
  ranges::advance(current_, n_ * x + offset_);
  offset_ = 0;
}
return *this;
constexpr iterator& operator-=(difference_type x)
  requires random_access_range<Base>;

14 Effects: Equivalent to: return *this += -x;

constexpr value_type operator[](difference_type n) const
  requires random_access_range<Base>;

15 Returns: *(*this + n).

friend constexpr bool operator==(const iterator& x, const iterator& y);

16 Returns: x.current_ == y.current_.

friend constexpr bool operator==(const iterator& x, default_sentinel_t);

17 Returns: x.current_ == x.end_;

friend constexpr bool operator<(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

18 Returns: x.current_ < y.current_.

friend constexpr bool operator>(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

19 Effects: Equivalent to: return y < x;

friend constexpr bool operator<=(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

20 Effects: Equivalent to: return !(y < x);

friend constexpr bool operator>=(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

21 Effects: Equivalent to: return !(x < y);

friend constexpr auto operator<=>(const iterator& x, const iterator& y)
  requires random_access_range<Base> &&
           three_way_comparable<iterator_t<Base>>;

22 Returns: x.current_ <=> y.current_.

friend constexpr iterator operator+(const iterator& i, difference_type n)
  requires random_access_range<Base>;
friend constexpr iterator operator+(difference_type n, const iterator& i)
  requires random_access_range<Base>;

23 Effects: Equivalent to:

  auto r = i;
  r += n;
  return r;
friend constexpr iterator operator-(const iterator& i, difference_type n)
  requires random_access_range<Base>;

24 Effects: Equivalent to:

  auto r = i;
  r -= n;
  return r;
friend constexpr difference_type operator-(const iterator& x, const iterator& y)
  requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;

25 Returns: (x.current_ - y.current_ + x.offset_ - y.offset_) / x.n_.

friend constexpr difference_type operator-(default_sentinel_t y, const iterator& x)
  requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

26 Returns: (x.end_ - x.current_ + x.n_ - 1) / x.n_.

friend constexpr difference_type operator-(const iterator& x, default_sentinel_t y)
  requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

27 Effects: Equivalent to: return -(y - x);

4.3 slide

Add the following subclause to 24.7 [range.adaptors].

24.7.? Slide view [range.slide]

24.7.?.1 Overview [range.slide.overview]

1 slide_view takes a view and a number N and produces a view whose M th element is a view over the M th through (M + N - 1)th elements of the original view. If the original view has fewer than N elements, the resulting view is empty.

2 The name views::slide denotes a range adaptor object (24.7.2 [range.adaptor.object]). Given subexpressions E and N, the expression views::slide(E, N) is expression-equivalent to slide_view(E, N).

[ Example 1: 
vector v = {1, 2, 3, 4};

for (auto i : v | views::slide(2)) {
  cout << '[' << i[0] << ', ' << i[1] << "] "; // prints: [1, 2] [2, 3] [3, 4]
}
— end example ]

24.7.?.2 Class template slide_view [range.slide.view]

namespace std::ranges {

template<class V>
  concept slide-caches-nothing = random_access_range<V> && sized_range<V>; // exposition only

template<class V>
  concept slide-caches-last = !slide-caches-nothing<V> && bidirectional_range<V> && common_range<V>; // exposition only

template<class V>
  concept slide-caches-first = !slide-caches-nothing<V> && !slide-caches-last<V>; // exposition only

template<forward_range V>
  requires view<V>
class slide_view : public view_interface<slide_view<V>>{
  V base_ = V();                     // exposition only
  range_difference_t<V> n_ = 0;      // exposition only

  template<bool> class iterator;     // exposition only
  class sentinel;                    // exposition only

public:
  slide_view() requires default_initializable<V> = default;
  constexpr explicit slide_view(V base, range_difference_t<V> n);

  constexpr auto begin()
    requires (!(simple-view<V> && slide-caches-nothing<const V>));
  constexpr auto begin() const requires slide-caches-nothing<const V>;

  constexpr auto end()
    requires (!(simple-view<V> && slide-caches-nothing<const V>));
  constexpr auto end() const requires slide-caches-nothing<const V>;

  constexpr auto size() requires sized_range<V>;
  constexpr auto size() const requires sized_range<const V>;
};

template<class R>
  slide_view(R&& r, range_difference_t<R>) -> slide_view<views::all_t<R>>;

}
constexpr explicit slide_view(V base, range_difference_t<V> n);

1 Effects: Initializes base_ with std::move(base) and n_ with n.

constexpr auto begin()
  requires (!(simple-view<V> && slide-caches-nothing<const V>));

2 Returns:

  • (2.1) If V models slide-caches-first, iterator<false>(ranges::begin(base_), ranges::next(ranges::begin(base_), n_ - 1, ranges::end(base_)), n_).
  • (2.2) Otherwise, iterator<false>(ranges::begin(base_), n_).

3 Remarks: In order to provide the amortized constant-time complexity required by the range concept, this function caches the result within the slide_view for use on subsequent calls when V models slide-caches-first.

constexpr auto begin() const requires slide-caches-nothing<const V>;

4 Returns: iterator<true>(ranges::begin(base_), n_).

constexpr auto end()
  requires (!(simple-view<V> && slide-caches-nothing<const V>));

5 Returns:

  • (5.1) If V models slide-caches-nothing, iterator<false>(ranges::begin(base_) + range_difference_t<V>(size()), n_);
  • (5.2) Otherwise, if V models slide-caches-last, iterator<false>(ranges::prev(ranges::end(base_), n_ - 1, ranges::begin(base_)), n_);
  • (5.3) Otherwise, if V models common_range, iterator<false>(ranges::end(base_), ranges::end(base_), n_);
  • (5.4) Otherwise, sentinel(ranges::end(base_)).

6 Remarks: In order to provide the amortized constant-time complexity required by the range concept, this function caches the result within the slide_view for use on subsequent calls when V models slide-caches-last.

constexpr auto end() const requires slide-caches-nothing<const V>;

7 Returns: begin() + range_difference_t<const V>(size()).

constexpr auto size() requires sized_range<V>;
constexpr auto size() const requires sized_range<const V>;

8 Effects: Equivalent to:

auto sz = ranges::distance(base_) - n_ + 1;
if (sz < 0) sz = 0;
return to-unsigned-like(sz);

24.7.?.3 Class template slide_view::iterator [range.slide.iterator]

namespace std::ranges {
  template<forward_range V>
    requires view<V>
  template<bool Const>
  class slide_view<V>::iterator {
    using Base = maybe-const<Const, V>;                     // exposition only
    iterator_t<Base> current_   = iterator_t<Base>();       // exposition only
    iterator_t<Base> last_ele_  = iterator_t<Base>();       // exposition only, present only if Base models slide-caches-first
    range_difference_t<Base> n_ = 0;                        // exposition only

    constexpr iterator(iterator_t<Base> current, range_difference_t<Base> n) // exposition only
      requires (!slide-caches-first<Base>);

    constexpr iterator(iterator_t<Base> current, iterator_t<Base> last_ele, range_difference_t<Base> n) // exposition only
      requires slide-caches-first<Base>;
  public:
    using iterator_category = input_iterator_tag;
    using iterator_concept  = see below;
    using value_type = decltype(views::counted(current_, n_));
    using difference_type = range_difference_t<Base>;

    iterator() = default;
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;

    constexpr auto operator*() const;
    constexpr iterator& operator++();
    constexpr iterator operator++(int);

    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;

    constexpr iterator& operator+=(difference_type x)
      requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type x)
      requires random_access_range<Base>;

    constexpr auto operator[](difference_type n) const
      requires random_access_range<Base>;

    friend constexpr bool operator==(const iterator& x, const iterator& y);

    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> &&
               three_way_comparable<iterator_t<Base>>;

    friend constexpr iterator operator+(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type n, const iterator& i)
      requires random_access_range<Base>;
    friend constexpr iterator operator-(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
  };
}

1 iterator::iterator_concept is defined as follows:

2 If the invocation of any non-const member function of iterator exits via an exception, the iterator acquires a singular value.

constexpr iterator(iterator_t<Base> current, range_difference_t<Base> n)
  requires (!slide-caches-first<Base>);

3 Effects: Initializes current_ with current and n_ with n.

constexpr iterator(iterator_t<Base> current, iterator_t<Base> last_ele, range_difference_t<Base> n); // exposition only
  requires slide-caches-first<Base>;

4 Effects: Initializes current_ with current, last_ele_ with last_ele, and n_ with n.

constexpr iterator(iterator<!Const> i)
  requires Const && (convertible_to<iterator_t<V>, iterator_t<Base>>;

5 Effects: Initializes current_ with std::move(i.current_) and n_ with i.n_. [ Note 1: iterator<true> can only be formed when Base models slide-caches-nothing, in which case last_ele_ is not present. — end note ]

constexpr auto operator*() const;

6 Returns: views::counted(current_, n_).

constexpr iterator& operator++();

7 Preconditions: current_ and last_ele_ (if present) are incrementable.

8 Postconditions: current_ and last_ele_ (if present) are each equal to ranges::next(i), where i is the value of that data member before the call.

9 Returns: *this.

constexpr iterator operator++(int);

10 Effects: Equivalent to:

  auto tmp = *this;
  ++*this;
  return tmp;
constexpr iterator& operator--() requires bidirectional_range<Base>;

11 Preconditions: current_ and last_ele_ (if present) are decrementable.

12 Postconditions: current_ and last_ele_ (if present) are each equal to ranges::prev(i), where i is the value of that data member before the call.

13 Returns: *this.

constexpr iterator operator--(int) requires bidirectional_range<Base>;

14 Effects: Equivalent to:

  auto tmp = *this;
  --*this;
  return tmp;
constexpr iterator& operator+=(difference_type x)
  requires random_access_range<Base>;

15 Preconditions: current_ + x and last_ele_ + x (if present) has well-defined behavior.

16 Postconditions: current_ and last_ele_ (if present) are each equal to i + x, where i is the value of that data member before the call.

17 Returns: *this.

  constexpr iterator& operator-=(difference_type x)
    requires random_access_range<Base>;

18 Preconditions: current_ - x and last_ele_ - x (if present) has well-defined behavior.

19 Postconditions: current_ and last_ele_ (if present) are each equal to i - x, where i is the value of that data member before the call.

20 Returns: *this.

constexpr auto operator[](difference_type n) const
  requires random_access_range<Base>;

21 Effects: Equivalent to: return views::counted(current_ + n, n_);

friend constexpr bool operator==(const iterator& x, const iterator& y);

22 Returns: If last_ele_ is present, x.last_ele_ == y.last_ele_; otherwise, x.current_ == y.current_.

friend constexpr bool operator<(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

23 Returns: x.current_ < y.current_.

friend constexpr bool operator>(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

24 Effects: Equivalent to: return y < x;

friend constexpr bool operator<=(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

25 Effects: Equivalent to: return !(y < x);

friend constexpr bool operator>=(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

26 Effects: Equivalent to: return !(x < y);

friend constexpr auto operator<=>(const iterator& x, const iterator& y)
  requires random_access_range<Base> &&
           three_way_comparable<iterator_t<Base>>;

27 Returns: x.current_ <=> y.current_.

friend constexpr iterator operator+(const iterator& i, difference_type n)
  requires random_access_range<Base>;
friend constexpr iterator operator+(difference_type n, const iterator& i)
  requires random_access_range<Base>;

28 Effects: Equivalent to:

  auto r = i;
  r += n;
  return r;
friend constexpr iterator operator-(const iterator& i, difference_type n)
  requires random_access_range<Base>;

29 Effects: Equivalent to:

  auto r = i;
  r -= n;
  return r;
friend constexpr difference_type operator-(const iterator& x, const iterator& y)
  requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;

30 Returns: If last_ele_ is present, x.last_ele_ - y.last_ele_; otherwise, x.current_ - y.current_.

24.7.?.4 Class slide_view::sentinel [range.slide.sentinel]

namespace std::ranges {
  template<forward_range V>
    requires view<V>
  class slide_view<V>::sentinel {
    sentinel_t<V> end_ = sentinel_t<V>();             // exposition only
    constexpr explicit sentinel(sentinel_t<V> end);   // exposition only
  public:
    sentinel() = default;

    friend constexpr bool operator==(const iterator<false>& x, const sentinel& y);

    friend constexpr range_difference_t<V>
      operator-(const iterator<false>& x, const sentinel& y)
        requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

    friend constexpr range_difference_t<V>
      operator-(const sentinel& y, const iterator<false>& x)
        requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
  };
}

1 [ Note 1: sentinel is only used when slide-caches-first<V> is true. — end note ]

constexpr explicit sentinel(sentinel_t<V> end);

2 Effects: Initializes end_ with end.

friend constexpr bool operator==(const iterator<false>& x, const sentinel& y);

3 Returns: x.last_ele_ == y.end_.

friend constexpr range_difference_t<V>
  operator-(const iterator<false>& x, const sentinel& y)
    requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

4 Returns: x.last_ele_ - y.end_.

friend constexpr range_difference_t<V>
  operator-(const sentinel& y, const iterator<false>& x)
    requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

5 Returns: y.end_ - x.last_ele_.

4.4 Feature-test macro

Add the following macro definition to 17.3.2 [version.syn], header <version> synopsis, with the value selected by the editor to reflect the date of adoption of this paper:

#define __cpp_lib_ranges_chunk_slide 20XXXXL // also in <ranges>

5 References

[P2214R0] Barry Revzin, Conor Hoekstra, Tim Song. 2020-10-15. A Plan for C++23 Ranges.
https://wg21.link/p2214r0

[P2321R2] Tim Song. 2021-06-11. zip.
https://wg21.link/p2321r2