25 Ranges library [ranges]

25.7 Range adaptors [range.adaptors]

25.7.32 Stride view [range.stride]

25.7.32.1 Overview [range.stride.overview]

stride_view presents a view of an underlying sequence, advancing over n elements at a time, as opposed to the usual single-step succession.

The name views::stride denotes a range adaptor object ([range.adaptor.object]).

Given subexpressions E and N, the expression views::stride(E, N) is expression-equivalent to stride_view(E, N).

[Example 1: auto input = views::iota(0, 12) | views::stride(3); ranges::copy(input, ostream_iterator<int>(cout, " ")); // prints 0 3 6 9 ranges::copy(input | views::reverse, ostream_iterator<int>(cout, " ")); // prints 9 6 3 0 — end example]

25.7.32.2 Class template stride_view [range.stride.view]

namespace std::ranges { template<input_range V> requires view<V> class stride_view : public view_interface<stride_view<V>> { V base_; // exposition only range_difference_t<V> stride_; // exposition only // [range.stride.iterator], class template stride_view::iterator template<bool> class iterator; // exposition only public: constexpr explicit stride_view(V base, range_difference_t<V> stride); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr range_difference_t<V> stride() const noexcept; constexpr auto begin() requires (!simple-view<V>) { return iterator<false>(this, ranges::begin(base_)); } constexpr auto begin() const requires 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> && forward_range<V>) { auto missing = (stride_ - ranges::distance(base_) % stride_) % stride_; return iterator<false>(this, ranges::end(base_), missing); } 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 range<const V> { if constexpr (common_range<const V> && sized_range<const V> && forward_range<const V>) { auto missing = (stride_ - ranges::distance(base_) % stride_) % stride_; return iterator<true>(this, ranges::end(base_), missing); } 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>; }; template<class R> stride_view(R&&, range_difference_t<R>) -> stride_view<views::all_t<R>>; }

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constexpr stride_view(V base, range_difference_t<V> stride);

Preconditions: stride > 0 is true.

Effects: Initializes base_ with std::move(base) and stride_ with stride.

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constexpr range_difference_t<V> stride() const noexcept;

Returns: stride_.

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

Effects: Equivalent to: return to-unsigned-like(div-ceil(ranges::distance(base_), stride_));

25.7.32.3 Class template stride_view::iterator [range.stride.iterator]

namespace std::ranges { template<input_range V> requires view<V> template<bool Const> class stride_view<V>::iterator { using Parent = maybe-const<Const, stride_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> stride_ = 0; // exposition only range_difference_t<Base> missing_ = 0; // exposition only constexpr iterator(Parent* parent, iterator_t<Base> current, // exposition only range_difference_t<Base> missing = 0); public: using difference_type = range_difference_t<Base>; using value_type = range_value_t<Base>; using iterator_concept = see below; using iterator_category = see below; // not always present iterator() requires default_initializable<iterator_t<Base>> = default; constexpr iterator(iterator<!Const> other) requires Const && convertible_to<iterator_t<V>, iterator_t<Base>> && convertible_to<sentinel_t<V>, sentinel_t<Base>>; constexpr iterator_t<Base> base() &&; constexpr const iterator_t<Base>& base() const & noexcept; constexpr decltype(auto) operator*() const { return *current_; } constexpr iterator& operator++(); constexpr void operator++(int); constexpr iterator operator++(int) requires forward_range<Base>; constexpr iterator& operator--() requires bidirectional_range<Base>; constexpr iterator operator--(int) requires bidirectional_range<Base>; constexpr iterator& operator+=(difference_type n) requires random_access_range<Base>; constexpr iterator& operator-=(difference_type n) requires random_access_range<Base>; constexpr decltype(auto) operator[](difference_type n) const requires random_access_range<Base> { return *(*this + n); } friend constexpr bool operator==(const iterator& x, default_sentinel_t); friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<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 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& x, difference_type n) requires random_access_range<Base>; friend constexpr iterator operator+(difference_type n, const iterator& x) requires random_access_range<Base>; friend constexpr iterator operator-(const iterator& x, 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>>; friend constexpr range_rvalue_reference_t<Base> iter_move(const iterator& i) noexcept(noexcept(ranges::iter_move(i.current_))); friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.current_, y.current_))) requires indirectly_swappable<iterator_t<Base>>; }; }

iterator::iterator_concept is defined as follows:

(1.1)
If Base models random_access_range, then iterator_concept denotes random_access_iterator_tag.
(1.2)
Otherwise, if Base models bidirectional_range, then iterator_concept denotes bidirectional_iterator_tag.
(1.3)
Otherwise, if Base models forward_range, then iterator_concept denotes forward_iterator_tag.
(1.4)
Otherwise, iterator_concept denotes input_iterator_tag.

The member typedef-name iterator_category is defined if and only if Base models forward_range .

In that case, iterator::iterator_category is defined as follows:

(2.1)
Let C denote the type iterator_traits<iterator_t<Base>>::iterator_category.
(2.2)
If C models derived_from<random_access_iterator_tag>, then iterator_category denotes random_access_iterator_tag.
(2.3)
Otherwise, iterator_category denotes C.

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

Effects: Initializes current_ with std::move(current), end_ with ranges::end(parent->base_), stride_ with parent->stride_, and missing_ with missing.

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

Effects: Initializes current_ with std::move(i.current_), end_ with std::move(i.end_), stride_ with i.stride_, and missing_ with i.missing_.

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constexpr iterator_t<Base> base() &&;

Returns: std::move(current_).

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constexpr const iterator_t<Base>& base() const & noexcept;

Returns: current_.

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constexpr iterator& operator++();

Preconditions: current_ != end_ is true.

Effects: Equivalent to: missing_ = ranges::advance(current_, stride_, end_); return *this;

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constexpr void operator++(int);

Effects: Equivalent to: ++*this;

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constexpr iterator operator++(int) requires forward_range<Base>;

Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;

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

Effects: Equivalent to: ranges::advance(current_, missing_ - stride_); missing_ = 0; return *this;

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

Effects: Equivalent to: auto tmp = *this; --*this; return tmp;

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

Preconditions: If n is positive, ranges::distance(current_, end_) > stride_ * (n - 1) is true.

[Note 1:

If n is negative, the Effects paragraph implies a precondition.

— end note]

Effects: Equivalent to: if (n > 0) { ranges::advance(current_, stride_ * (n - 1)); missing_ = ranges::advance(current_, stride_, end_); } else if (n < 0) { ranges::advance(current_, stride_ * n + missing_); missing_ = 0; } return *this;

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

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

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

Returns: x.current_ == x.end_.

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

Returns: x.current_ == y.current_.

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

Returns: x.current_ < y.current_.

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

Effects: Equivalent to: return y < x;

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

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

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

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

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

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

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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>;

Effects: Equivalent to: auto r = i; r += n; return r;

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

Effects: Equivalent to: auto r = i; r -= n; return r;

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

Returns: Let N be (x.current_ - y.current_).

(25.1)
If Base models forward_range, (N + x.missing_ - y.missing_) / x.stride_.
(25.2)
Otherwise, if N is negative, -div-ceil(-N, x.stride_).
(25.3)
Otherwise, div-ceil(N, x.stride_).

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

Returns: div-ceil(x.end_ - x.current_, x.stride_).

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

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

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friend constexpr range_rvalue_reference_t<Base> iter_move(const iterator& i)
  noexcept(noexcept(ranges::iter_move(i.current_)));

Effects: Equivalent to: return ranges::iter_move(i.current_);

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friend constexpr void iter_swap(const iterator& x, const iterator& y)
  noexcept(noexcept(ranges::iter_swap(x.current_, y.current_)))
  requires indirectly_swappable<iterator_t<Base>>;