25 Ranges library [ranges]

25.7 Range adaptors [range.adaptors]

25.7.12 Drop view [range.drop]

25.7.12.2 Class template drop_view [range.drop.view]

namespace std::ranges { template<view V> class drop_view : public view_interface<drop_view<V>> { public: drop_view() requires default_initializable<V> = default; constexpr explicit drop_view(V base, range_difference_t<V> count); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr auto begin() requires (!(simple-view<V> && random_access_range<const V> && sized_range<const V>)); constexpr auto begin() const requires random_access_range<const V> && sized_range<const V>; constexpr auto end() requires (!simple-view<V>) { return ranges::end(base_); } constexpr auto end() const requires range<const V> { return ranges::end(base_); } constexpr auto size() requires sized_range<V> { const auto s = ranges::size(base_); const auto c = static_cast<decltype(s)>(count_); return s < c ? 0 : s - c; } constexpr auto size() const requires sized_range<const V> { const auto s = ranges::size(base_); const auto c = static_cast<decltype(s)>(count_); return s < c ? 0 : s - c; } private: V base_ = V(); // exposition only range_difference_t<V> count_ = 0; // exposition only }; template<class R> drop_view(R&&, range_difference_t<R>) -> drop_view<views::all_t<R>>; }

🔗

constexpr explicit drop_view(V base, range_difference_t<V> count);

Preconditions: count >= 0 is true.

Effects: Initializes base_ with std::move(base) and count_ with count.

🔗

constexpr auto begin()
  requires (!(simple-view<V> &&
              random_access_range<const V> && sized_range<const V>));
constexpr auto begin() const
  requires random_access_range<const V> && sized_range<const V>;

Returns: ranges::next(ranges::begin(base_), count_, ranges::end(base_)).

Remarks: In order to provide the amortized constant-time complexity required by the range concept when drop_view models forward_range, the first overload caches the result within the drop_view for use on subsequent calls.

[Note 1:

Without this, applying a reverse_view over a drop_view would have quadratic iteration complexity.

— end note]