149 lines
9.3 KiB
Markdown
149 lines
9.3 KiB
Markdown
[range.drop]
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# 25 Ranges library [[ranges]](./#ranges)
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## 25.7 Range adaptors [[range.adaptors]](range.adaptors#range.drop)
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### 25.7.12 Drop view [range.drop]
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#### [25.7.12.1](#overview) Overview [[range.drop.overview]](range.drop.overview)
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[1](#overview-1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6345)
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drop_view produces a view
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excluding the first N elements from another view, or
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an empty range if the adapted view contains fewer than N elements[.](#overview-1.sentence-1)
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[2](#overview-2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6350)
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The name views::drop denotes
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a range adaptor object ([[range.adaptor.object]](range.adaptor.object "25.7.2 Range adaptor objects"))[.](#overview-2.sentence-1)
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Let E and F be expressions,
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let T be remove_cvref_t<decltype((E))>, and
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let D be range_difference_t<decltype((E))>[.](#overview-2.sentence-2)
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If decltype((F)) does not model[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_to [concept.convertible]")<D>,views::drop(E, F) is ill-formed[.](#overview-2.sentence-3)
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Otherwise, the expression views::drop(E, F) is expression-equivalent to:
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- [(2.1)](#overview-2.1)
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If T is a specialization ofempty_view ([[range.empty.view]](range.empty.view "25.6.2.2 Class template empty_view")),
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then ((void)F, *decay-copy*(E)),
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except that the evaluations of E and F are indeterminately sequenced[.](#overview-2.1.sentence-1)
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- [(2.2)](#overview-2.2)
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Otherwise, if T models[random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]") and [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]") and is
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* [(2.2.1)](#overview-2.2.1)
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a specialization of span ([[views.span]](views.span "23.7.2.2 Class template span")),
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* [(2.2.2)](#overview-2.2.2)
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a specialization of basic_string_view ([[string.view]](string.view "27.3 String view classes")),
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* [(2.2.3)](#overview-2.2.3)
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a specialization of iota_view ([[range.iota.view]](range.iota.view "25.6.4.2 Class template iota_view")), or
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* [(2.2.4)](#overview-2.2.4)
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a specialization of subrange ([[range.subrange]](range.subrange "25.5.4 Sub-ranges"))
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where T::*StoreSize* is false,
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then U(ranges::begin(E) + std::min<D>(ranges::distance(E), F), ranges::end(E)),
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except that E is evaluated only once,
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where U is span<typename T::element_type> if T is a specialization of span and T otherwise[.](#overview-2.2.sentence-1)
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- [(2.3)](#overview-2.3)
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Otherwise,
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if T is
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a specialization of subrange that models [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]") and [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]"),
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thenT(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))),
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except that E and F are each evaluated only once[.](#overview-2.3.sentence-1)
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- [(2.4)](#overview-2.4)
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Otherwise, if T is
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a specialization of repeat_view ([[range.repeat.view]](range.repeat.view "25.6.5.2 Class template repeat_view")):
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* [(2.4.1)](#overview-2.4.1)
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if T models [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]"),
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thenviews::repeat(*E.*value_*, ranges::distance(E) - std::min<D>(ranges::distance(E), F)) except that E is evaluated only once;
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* [(2.4.2)](#overview-2.4.2)
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otherwise, ((void)F, *decay-copy*(E)),
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except that the evaluations of E and F are indeterminately sequenced[.](#overview-2.4.sentence-1)
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- [(2.5)](#overview-2.5)
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Otherwise, drop_view(E, F)[.](#overview-2.5.sentence-1)
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[3](#overview-3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6417)
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[*Example [1](#overview-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} â *end example*]
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#### [25.7.12.2](#view) Class template drop_view [[range.drop.view]](range.drop.view)
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[ð](#lib:drop_view)
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namespace std::ranges {template<[view](range.view#concept:view "25.4.5 Views [range.view]") V>class drop_view : public view_interface<drop_view<V>> {public: drop_view() requires [default_initializable](concept.default.init#concept:default_initializable "18.4.12 Concept default_initializable [concept.default.init]")<V> = default; constexpr explicit drop_view(V base, range_difference_t<V> count); constexpr V base() const & requires [copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_constructible [concept.copyconstructible]")<V> { return *base_*; }constexpr V base() && { return std::move(*base_*); }constexpr auto begin()requires (!([*simple-view*](range.utility.helpers#concept:simple-view "25.5.2 Helper concepts [range.utility.helpers]")<V> &&[random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<const V> && [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V>)); constexpr auto begin() constrequires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<const V> && [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V>; constexpr auto end() requires (<V>){ return ranges::end(*base_*); }constexpr auto end() const requires [range](range.range#concept:range "25.4.2 Ranges [range.range]")<const V>{ return ranges::end(*base_*); }constexpr auto size() requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<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](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V> {const auto s = ranges::size(*base_*); const auto c = static_cast<decltype(s)>(*count_*); return s < c ? 0 : s - c; }constexpr auto reserve_hint() requires [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")<V> {const auto s = static_cast<range_difference_t<V>>(ranges::reserve_hint(*base_*)); return *to-unsigned-like*(s < *count_* ? 0 : s - *count_*); }constexpr auto reserve_hint() const requires [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")<const V> {const auto s = static_cast<range_difference_t<const V>>(ranges::reserve_hint(*base_*)); return *to-unsigned-like*(s < *count_* ? 0 : s - *count_*); }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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[ð](#lib:drop_view,constructor)
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`constexpr explicit drop_view(V base, range_difference_t<V> count);
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`
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[1](#view-1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6494)
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*Preconditions*: count >= 0 is true[.](#view-1.sentence-1)
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[2](#view-2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6498)
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*Effects*: Initializes *base_* with std::move(base) and*count_* with count[.](#view-2.sentence-1)
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[ð](#lib:begin,drop_view)
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`constexpr auto begin()
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requires (!([simple-view](range.utility.helpers#concept:simple-view "25.5.2 Helper concepts [range.utility.helpers]")<V> &&
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[random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<const V> && [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V>));
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constexpr auto begin() const
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requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<const V> && [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V>;
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`
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[3](#view-3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6514)
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*Returns*: ranges::next(ranges::begin(*base_*), *count_*, ranges::end(*base_*))[.](#view-3.sentence-1)
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[4](#view-4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6518)
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*Remarks*: In order to provide the amortized constant-time complexity required
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by the [range](range.range#concept:range "25.4.2 Ranges [range.range]") concept
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when drop_view models [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]"),
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the first overload caches the result within the drop_view for use on subsequent calls[.](#view-4.sentence-1)
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[*Note [1](#view-note-1)*:
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Without this,
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applying a reverse_view over a drop_view would have quadratic iteration complexity[.](#view-4.sentence-2)
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â *end note*]
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