25 Ranges library [ranges]

25.7 Range adaptors [range.adaptors]

25.7.12 Drop view [range.drop]

25.7.12.1 Overview [range.drop.overview]

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drop_view produces a view excluding the first N elements from another view, or an empty range if the adapted view contains fewer than N elements.
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The name views​::​drop denotes a range adaptor object ([range.adaptor.object]).
Let E and F be expressions, let T be remove_cvref_t<decltype((E))>, and let D be range_difference_t<decltype((E))>.
If decltype((F)) does not model convertible_to<D>, views​::​drop(E, F) is ill-formed.
Otherwise, the expression views​::​drop(E, F) is expression-equivalent to:
  • (2.1)
    If T is a specialization of empty_view ([range.empty.view]), then ((void)F, decay-copy(E)), except that the evaluations of E and F are indeterminately sequenced.
  • (2.2)
    Otherwise, if T models random_access_range and sized_range and is then U(ranges​::​begin(E) + std​::​min<D>(ranges​::​distance(E), F), ranges​::​end(E)), except that E is evaluated only once, where U is span<typename T​::​element_type> if T is a specialization of span and T otherwise.
  • (2.3)
    Otherwise, if T is a specialization of subrange that models random_access_range and sized_range, then T(ranges​::​begin(E) + std​::​min<D>(ranges​::​distance(E), F), ranges​::​end(E), to-unsigned-like(ranges​::​distance(E) - std​::​min<D>(ranges​::​distance(E), F))), except that E and F are each evaluated only once.
  • (2.4)
    Otherwise, if T is a specialization of repeat_view ([range.repeat.view]):
    • (2.4.1)
      if T models sized_range, then views::repeat(*E.value_, ranges::distance(E) - std::min<D>(ranges::distance(E), F)) except that E is evaluated only once;
    • (2.4.2)
      otherwise, ((void)F, decay-copy(E)), except that the evaluations of E and F are indeterminately sequenced.
  • (2.5)
    Otherwise, drop_view(E, F).
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[Example 1: auto ints = views::iota(0) | views::take(10); for (auto i : ints | views::drop(5)) { cout << i << ' '; // prints 5 6 7 8 9 } — end example]

25.7.12.2 Class template drop_view [range.drop.view]

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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>>; }
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constexpr explicit drop_view(V base, range_difference_t<V> count);
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Preconditions: count >= 0 is true.
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Effects: Initializes base_ with std​::​move(base) and count_ with count.
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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>;
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Returns: ranges​::​next(ranges​::​begin(base_), count_, ranges​::​end(base_)).
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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]