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)