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