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) Range adaptor object. 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.
  Expression-equivalent
   Expression e is expression-equivalent to expression f, if
     * e and f have the same effects, and
     * either both are constant subexpressions or else neither is a constant
       subexpression, and
     * either both are potentially-throwing or else neither is potentially-throwing
       (i.e. noexcept(e) == noexcept(f)).
  Data members
   Typical implementations of reverse_view hold only one or two member objects:
     * the underlying view of type V (shown here as base_, the name is exposition
       only), and
     * a std::optional-like cache object that holds either no value or the end
       iterator/position of the underlying view, which exists only if the underlying
       view type V does not model common_range.

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>)
   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 nth 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> {
   return ranges::size(base_); (1) (since C++20)
   }
   constexpr auto size() const requires ranges::sized_range<const V>
   {
                        (2) (since C++20)
   return ranges::size(base_);
   }
   Returns the size of the view if the view is bounded.
  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 <ranges>
 #include <iostream>
 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 `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)