[range.take] # 25 Ranges library [[ranges]](./#ranges) ## 25.7 Range adaptors [[range.adaptors]](range.adaptors#range.take) ### 25.7.10 Take view [range.take] #### [25.7.10.1](#overview) Overview [[range.take.overview]](range.take.overview) [1](#overview-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5864) take_view produces a view of the first N elements from another view, or all the elements if the adapted view contains fewer than N[.](#overview-1.sentence-1) [2](#overview-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5869) The name views​::​take 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​::​take(E, F) is ill-formed[.](#overview-2.sentence-3) Otherwise, the expression views​::​take(E, F) is expression-equivalent to: - [(2.1)](#overview-2.1) If T is a specialization of empty_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 a specialization ofspan ([[views.span]](views.span "23.7.2.2 Class template span")),basic_string_view ([[string.view]](string.view "27.3 String view classes")), orsubrange ([[range.subrange]](range.subrange "25.5.4 Sub-ranges")), thenU(ranges​::​begin(E), ranges​::​begin(E) + std​::​min(ranges​::​distance(E), F)), except that E is evaluated only once, where U is a type determined as follows: * [(2.2.1)](#overview-2.2.1) if T is a specialization of span, then U is span; * [(2.2.2)](#overview-2.2.2) otherwise, if T is a specialization of basic_string_view, then U is T; * [(2.2.3)](#overview-2.2.3) otherwise, T is a specialization of subrange, andU is subrange>; - [(2.3)](#overview-2.3) otherwise, if T is a specialization of iota_view ([[range.iota.view]](range.iota.view "25.6.4.2 Class template iota_­view")) 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]"), theniota_view(*ranges​::​begin(E),*(ranges​::​begin(E) + std​::​min(ranges​::​distance(E), F))), except that E is 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_*, std::min(ranges::distance(E), F)) except that E is evaluated only once; * [(2.4.2)](#overview-2.4.2) otherwise, views​::​repeat(*E.*value_*, static_cast(F))[.](#overview-2.4.sentence-1) - [(2.5)](#overview-2.5) Otherwise, take_view(E, F)[.](#overview-2.5.sentence-1) [3](#overview-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5938) [*Example [1](#overview-example-1)*: vector is{0,1,2,3,4,5,6,7,8,9};for (int i : is | views::take(5)) cout << i << ' '; // prints 0 1 2 3 4 — *end example*] #### [25.7.10.2](#view) Class template take_view [[range.take.view]](range.take.view) [🔗](#lib:take_view) namespace std::ranges {template<[view](range.view#concept:view "25.4.5 Views [range.view]") V>class take_view : public view_interface> {private: V *base_* = V(); // *exposition only* range_difference_t *count_* = 0; // *exposition only*// [[range.take.sentinel]](#sentinel "25.7.10.3 Class template take_­view​::​sentinel"), class template take_view​::​*sentinel*template class *sentinel*; // *exposition only*public: take_view() requires [default_initializable](concept.default.init#concept:default_initializable "18.4.12 Concept default_­initializable [concept.default.init]") = default; constexpr explicit take_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]")) {if constexpr ([sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")) {if constexpr ([random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")) {return ranges::begin(*base_*); } else {auto sz = range_difference_t(size()); return counted_iterator(ranges::begin(*base_*), sz); }} else if constexpr ([sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]"), iterator_t>) {auto it = ranges::begin(*base_*); auto sz = std::min(*count_*, ranges::end(*base_*) - it); return counted_iterator(std::move(it), sz); } else {return counted_iterator(ranges::begin(*base_*), *count_*); }}constexpr auto begin() const requires [range](range.range#concept:range "25.4.2 Ranges [range.range]") {if constexpr ([sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")) {if constexpr ([random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")) {return ranges::begin(*base_*); } else {auto sz = range_difference_t(size()); return counted_iterator(ranges::begin(*base_*), sz); }} else if constexpr ([sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]"), iterator_t>) {auto it = ranges::begin(*base_*); auto sz = std::min(*count_*, ranges::end(*base_*) - it); return counted_iterator(std::move(it), sz); } else {return counted_iterator(ranges::begin(*base_*), *count_*); }}constexpr auto end() requires (![*simple-view*](range.utility.helpers#concept:simple-view "25.5.2 Helper concepts [range.utility.helpers]")) {if constexpr ([sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")) {if constexpr ([random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]"))return ranges::begin(*base_*) + range_difference_t(size()); elsereturn default_sentinel; } else if constexpr ([sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]"), iterator_t>) {return default_sentinel; } else {return *sentinel*{ranges::end(*base_*)}; }}constexpr auto end() const requires [range](range.range#concept:range "25.4.2 Ranges [range.range]") {if constexpr ([sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")) {if constexpr ([random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]"))return ranges::begin(*base_*) + range_difference_t(size()); elsereturn default_sentinel; } else if constexpr ([sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]"), iterator_t>) {return default_sentinel; } else {return *sentinel*{ranges::end(*base_*)}; }}constexpr auto size() requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]") {auto n = ranges::size(*base_*); return ranges::min(n, static_cast(*count_*)); }constexpr auto size() const requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]") {auto n = ranges::size(*base_*); return ranges::min(n, static_cast(*count_*)); }constexpr auto reserve_hint() {if constexpr ([approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")) {auto n = static_cast>(ranges::reserve_hint(*base_*)); return *to-unsigned-like*(ranges::min(n, *count_*)); }return *to-unsigned-like*(*count_*); }constexpr auto reserve_hint() const {if constexpr ([approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")) {auto n = static_cast>(ranges::reserve_hint(*base_*)); return *to-unsigned-like*(ranges::min(n, *count_*)); }return *to-unsigned-like*(*count_*); }}; template take_view(R&&, range_difference_t)-> take_view>;} [🔗](#lib:take_view,constructor) `constexpr explicit take_view(V base, range_difference_t count); ` [1](#view-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6071) *Preconditions*: count >= 0 is true[.](#view-1.sentence-1) [2](#view-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6075) *Effects*: Initializes *base_* with std​::​move(base) and*count_* with count[.](#view-2.sentence-1) #### [25.7.10.3](#sentinel) Class template take_view​::​*sentinel* [[range.take.sentinel]](range.take.sentinel) [🔗](#lib:take_view::sentinel) namespace std::ranges {template<[view](range.view#concept:view "25.4.5 Views [range.view]") V>templateclass take_view::*sentinel* {private:using *Base* = *maybe-const*; // *exposition only*templateusing *CI* = counted_iterator>>; // *exposition only* sentinel_t<*Base*> *end_* = sentinel_t<*Base*>(); // *exposition only*public:*sentinel*() = default; constexpr explicit *sentinel*(sentinel_t<*Base*> end); constexpr *sentinel*(*sentinel* s)requires Const && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]"), sentinel_t<*Base*>>; constexpr sentinel_t<*Base*> base() const; friend constexpr bool operator==(const *CI*& y, const *sentinel*& x); templaterequires [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]"), iterator_t<*maybe-const*>>friend constexpr bool operator==(const *CI*& y, const *sentinel*& x); };} [🔗](#lib:take_view::sentinel,constructor) `constexpr explicit sentinel(sentinel_t end); ` [1](#sentinel-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6118) *Effects*: Initializes *end_* with end[.](#sentinel-1.sentence-1) [🔗](#lib:take_view::sentinel,constructor_) `constexpr sentinel(sentinel s) requires Const && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]"), sentinel_t>; ` [2](#sentinel-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6130) *Effects*: Initializes *end_* with std​::​move(s.*end_*)[.](#sentinel-2.sentence-1) [🔗](#lib:base,take_view::sentinel) `constexpr sentinel_t base() const; ` [3](#sentinel-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6141) *Effects*: Equivalent to: return *end_*; [🔗](#lib:operator==,take_view::sentinel) `friend constexpr bool operator==(const CI& y, const sentinel& x); template requires [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]"), iterator_t>> friend constexpr bool operator==(const CI& y, const sentinel& x); ` [4](#sentinel-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6156) *Effects*: Equivalent to:return y.count() == 0 || y.base() == x.*end_*;