std::ranges::views::reverse,std::ranges::reverse_view(3)

NAME
std::ranges::views::reverse,std::ranges::reverse_view - std::ranges::views::reverse,std::ranges::reverse_view

Synopsis
   Defined in header <ranges>
   template< ranges::view V >
       requires ranges::bidirectional_range<V> (1) (since C++20)
   class reverse_view
       : public ranges::view_interface<reverse_view<V>>
   namespace views {
       inline constexpr /* unspecified */ reverse = /* unspecified (2) (since C++20)
   */;
   }
   Call signature
   template< ranges::viewable_range R >
       requires /* see below */ (since C++20)
   constexpr ranges::view auto reverse( R&& r );
   1) A range adaptor that represents a view of underlying view with reversed order.
   2) RangeAdaptorObject. The expression views::reverse(e) is expression-equivalent to
   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.
   A reverse_view always models bidirectional_range and common_range, and it models
   borrowed_range, sized_range, or random_access_range if the underlying view type V
   models the corresponding concept.

Member functions
   constructor constructs a reverse_view
   (C++20) (public member function)
   base returns the underlying view V
   (C++20) (public member function)
   begin returns the beginning iterator of the reverse_view
   (C++20) (public member function)
   end returns the end iterator of the reverse_view
   (C++20) (public member function)
   size returns the size of the view if it is bounded
   (C++20) (public member function)
         Inherited from std::ranges::view_interface
   empty returns whether the derived view is empty. Provided if it satisfies
   (C++20) sized_range or forward_range.
                 (public member function of std::ranges::view_interface<D>)
   cbegin returns a constant iterator to the beginning of the range.
   (C++23) (public member function of std::ranges::view_interface<D>)
   cend returns a sentinel for the constant iterator of the range.
   (C++23) (public member function of std::ranges::view_interface<D>)
   operator bool returns whether the derived view is not empty. Provided if
   (C++20) ranges::empty is applicable to it.
                 (public member function of std::ranges::view_interface<D>)
   front returns the first element in the derived view. Provided if it
   (C++20) satisfies forward_range.
                 (public member function of std::ranges::view_interface<D>)
   back returns the last element in the derived view. Provided if it satisfies
   (C++20) bidirectional_range and common_range.
                 (public member function of std::ranges::view_interface<D>)
   operator[] returns the n^th element in the derived view. Provided if it satisfies
   (C++20) random_access_range.
                 (public member function of std::ranges::view_interface<D>) std::ranges::reverse_view::reverse_view
   reverse_view() requires std::default_initializable<V> = default; (1) (since C++20)
   constexpr reverse_view( V r ); (2) (since C++20)
   1) Value-initializes base_ via its default member initializer (= V()).
   2) Initializes base_ with std::move(r).

Parameters
   r - range to reverse std::ranges::reverse_view::base
   constexpr V base() const& requires std::copy_constructible<V>; (1) (since C++20)
   constexpr V base() &&; (2) (since C++20)
   Returns the underlying view.
   1) Copy-constructs the result from the underlying view. Equivalent to return base_;.
   2) Move-constructs the result from the underlying view. Equivalent to return
   std::move(base_);. std::ranges::reverse_view::begin
   constexpr std::reverse_iterator<ranges::iterator_t<V>> begin(); (1) (since C++20)
   constexpr std::reverse_iterator<ranges::iterator_t<V>> begin() (2) (since C++20)
       requires ranges::common_range<V>;
   constexpr auto begin() const requires ranges::common_range<const (3) (since C++20)
   V>;
   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 cache
   object for use on subsequent calls.
   2,3) Equivalent to return std::make_reverse_iterator(ranges::end(base_));. std::ranges::reverse_view::end
   constexpr std::reverse_iterator<ranges::iterator_t<V>> end(); (1) (since C++20)
   constexpr auto end() const requires ranges::common_range<const V>; (2) (since C++20)
   Equivalent to return std::make_reverse_iterator(ranges::begin(base_));. std::ranges::reverse_view::size
   constexpr auto size() requires ranges::sized_range<V>; (1) (since C++20)
   constexpr auto size() const requires ranges::sized_range<const V>; (2) (since C++20)
   Returns the size of the view if the view is bounded.
   1,2) Equivalent to return ranges::size(base_);.
   Deduction guides
   template< class R > (since C++20)
   reverse_view( R&& ) -> reverse_view<views::all_t<R>>;
   Helper templates
   template< class T >
   inline constexpr bool (since C++20)
   enable_borrowed_range<std::ranges::reverse_view<T>> =
       std::ranges::enable_borrowed_range<T>;
   This specialization of std::ranges::enable_borrowed_range makes reverse_view satisfy
   borrowed_range when the underlying view satisfies it.

Example
// Run this code
 #include <iostream>
 #include <ranges>
 int main()
 {
     static constexpr auto il = {3, 1, 4, 1, 5, 9};
     std::ranges::reverse_view rv{il};
     for (int i : rv)
         std::cout << i << ' ';
     std::cout << '\n';
     for (int i : il | std::views::reverse)
         std::cout << i << ' ';
     std::cout << '\n';
     // operator[] is inherited from std::view_interface
     for (auto i{0U}; i != rv.size(); ++i)
         std::cout << rv[i] << ' ';
     std::cout << '\n';
 }

Output:
 9 5 1 4 1 3
 9 5 1 4 1 3
 9 5 1 4 1 3
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3494 C++20 reverse_view was never a it is a borrowed_range if its
                       borrowed_range underlying view is

See also
   reverse_iterator iterator adaptor for reverse-order traversal
                        (class template)
   ranges::reverse reverses the order of elements in a range
   (C++20) (niebloid)
   ranges::reverse_copy creates a copy of a range that is reversed
   (C++20) (niebloid)