std::ranges::views::reverse, std::ranges::reverse_view

template< ranges::view V > requires ranges::bidirectional_range<V> class reverse_view : public ranges::view_interface<reverse_view<V>>	(1)	(since C++20)
namespace views { inline constexpr /unspecified/ reverse = /unspecified/; }	(2)	(since C++20)

1) A range adaptor that represents a view of underlying view with reversed order.

2) The expression views::reverse(E) is expression-equivalent one of the following expressions, except that E is evaluated only once:

E.base(), if the type of E is a (possibly cv-qualified) specialization of reverse_view;
otherwise, if the type of E is (possibly cv-qualified) ranges::subrange<std::reverse_iterator<I>, std::reverse_iterator<I>, K> for some iterator type I and value K of type ranges::subrange_kind:

ranges::subrange<I, I, K>(E.end().base(), E.begin().base(), E.size()), if K is ranges::subrange_kind::sized;
otherwise ranges::subrange<I, I, K>(E.end().base(), E.begin().base());

otherwise ranges::reverse_view{E}.

In other words, views::reverse unwraps reversed views if possible.

Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

Data members

std::ranges::reverse_view::base_

V base_ = V(); /* exposition-only */

the underlying view

Member functions

std::ranges::reverse_view::reverse_view

reverse_view() = default;	(1)
constexpr reverse_view(V base);	(2)
template<ranges::viewable_range R> requires ranges::bidirectional_range<R> && std::constructible_from<V, ranges::all_view<R>> constexpr reverse_view(R&& r);	(3)

1) Value-initializes base_

2) Initializes base_ with std::move(base).

3) Initializes base_ with views::all(std::forward<R>(r)).

Parameters

r

-

range to reverse

std::ranges::reverse_view::base

constexpr V base() const;

Equivalent to return base_;

std::ranges::reverse_view::begin

constexpr std::reverse_iterator<ranges::iterator_t<V>> begin();	(1)
constexpr std::reverse_iterator<ranges::iterator_t<V>> begin() requires ranges::common_range<V>;	(2)
constexpr std::reverse_iterator<ranges::iterator_t<const V>> begin() const requires ranges::common_range<const V>;	(2)

1) Returns std::make_reverse_iterator(ranges::next(ranges::begin(base_), ranges::end(base_))). In order to provide the amortized constant time complexity required by the range concept, this function caches the result within the reverse_view object for use on subsequent calls.

2) Equivalent to return std::make_reverse_iterator(ranges::end(base_));.

std::ranges::reverse_view::end

constexpr std::reverse_iterator<ranges::iterator_t<V>> end() requires ranges::common_range<V>;
constexpr std::reverse_iterator<ranges::iterator_t<const V>> end() const requires ranges::common_range<const V>;

Equivalent to return std::make_reverse_iterator(ranges::begin(base_));.

std::ranges::reverse_view::size

constexpr auto size() requires ranges::sized_range<V> { return ranges::size(base_); }
constexpr auto size() const requires ranges::sized_range<const V> { return ranges::size(base_); }

Returns the size of the view if the view is bounded.

Deduction guides

template<class R> reverse_view(R&&) -> reverse_view<ranges::all_view<R>>;

Example

Run this code

#include <array>
#include <ranges>
#include <iostream>
 
int main()
{
    std::array<int, 6> il {3, 1, 4, 1, 5, 9};
    std::ranges::reverse_view rv {il};
    for (int i : rv)
        std::cout << i << ' ';
}

Output:

9 5 1 4 1 3

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