[range.reverse] # 25 Ranges library [[ranges]](./#ranges) ## 25.7 Range adaptors [[range.adaptors]](range.adaptors#range.reverse) ### 25.7.21 Reverse view [range.reverse] #### [25.7.21.1](#overview) Overview [[range.reverse.overview]](range.reverse.overview) [1](#overview-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L9881) reverse_view takes a bidirectional view and produces another view that iterates the same elements in reverse order[.](#overview-1.sentence-1) [2](#overview-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L9885) The name views​::​reverse denotes a range adaptor object ([[range.adaptor.object]](range.adaptor.object "25.7.2 Range adaptor objects"))[.](#overview-2.sentence-1) Given a subexpression E, the expressionviews​::​reverse(E) is expression-equivalent to: - [(2.1)](#overview-2.1) If the type of E is a (possibly cv-qualified) specialization of reverse_view, then E.base()[.](#overview-2.1.sentence-1) - [(2.2)](#overview-2.2) Otherwise, if the type of E is cv subrange, reverse_iterator, K> for some iterator type I and value K of type subrange_kind, * [(2.2.1)](#overview-2.2.1) if K is subrange_kind​::​sized, thensubrange(E.end().base(), E.begin().base(), E.size()); * [(2.2.2)](#overview-2.2.2) otherwise, subrange(E.end().base(), E.begin().base())[.](#overview-2.2.sentence-1) However, in either case E is evaluated only once[.](#overview-2.2.sentence-2) - [(2.3)](#overview-2.3) Otherwise, reverse_view{E}[.](#overview-2.3.sentence-1) [3](#overview-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L9912) [*Example [1](#overview-example-1)*: vector is {0,1,2,3,4};for (int i : is | views::reverse) cout << i << ' '; // prints 4 3 2 1 0 — *end example*] #### [25.7.21.2](#view) Class template reverse_view [[range.reverse.view]](range.reverse.view) [🔗](#lib:reverse_view) namespace std::ranges {template<[view](range.view#concept:view "25.4.5 Views [range.view]") V>requires [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")class reverse_view : public view_interface> {private: V *base_* = V(); // *exposition only*public: reverse_view() requires [default_initializable](concept.default.init#concept:default_initializable "18.4.12 Concept default_­initializable [concept.default.init]") = default; constexpr explicit reverse_view(V r); 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 reverse_iterator> begin(); constexpr reverse_iterator> begin() requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]"); constexpr auto begin() const requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]"); constexpr reverse_iterator> end(); constexpr auto end() const requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]"); constexpr auto size() requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]") {return ranges::size(*base_*); }constexpr auto size() const requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]") {return ranges::size(*base_*); }constexpr auto reserve_hint() requires [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]") {return ranges::reserve_hint(*base_*); }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]") {return ranges::reserve_hint(*base_*); }}; template reverse_view(R&&) -> reverse_view>;} [🔗](#lib:reverse_view,constructor) `constexpr explicit reverse_view(V base); ` [1](#view-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L9976) *Effects*: Initializes *base_* with std​::​move(base)[.](#view-1.sentence-1) [🔗](#lib:begin,reverse_view) `constexpr reverse_iterator> begin(); ` [2](#view-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L9987) *Returns*: make_reverse_iterator(ranges::next(ranges::begin(*base_*), ranges::end(*base_*))) [3](#view-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L9993) *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, this function caches the result within thereverse_view for use on subsequent calls[.](#view-3.sentence-1) [🔗](#lib:begin,reverse_view_) `constexpr reverse_iterator> begin() requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]"); constexpr auto begin() const requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]"); ` [4](#view-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L10007) *Effects*: Equivalent to: return make_reverse_iterator(ranges​::​end(*base_*)); [🔗](#lib:end,reverse_view) `constexpr reverse_iterator> end(); constexpr auto end() const requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]"); ` [5](#view-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L10019) *Effects*: Equivalent to: return make_reverse_iterator(ranges​::​begin(*base_*));