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

 

C++

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Freestanding and hosted

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Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

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Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range    sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while concat_viewviews::concat (C++26)(C++26)	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

Defined in header <ranges>
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)
Call signature
template< ranges::viewable_range R >     requires /* see below */ constexpr ranges::view auto reverse( R&& r );		(since C++20)

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.

Data members

Member name	Definition
base_ (private)	The underlying view of type V. (exposition-only member object*)
cached_end_ (private)	An 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. (exposition-only member object*)

Member functions

(constructor)	constructs a reverse_view (public member function)
base	returns the underlying view V (public member function)
begin	returns the beginning iterator of the reverse_view (public member function)
end	returns the end iterator of the reverse_view (public member function)
size	returns the size of the view if it is bounded (public member function)
Inherited from std::ranges::view_interface
empty	returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range. (public member function of std::ranges::view_interface<D>)
cbegin (C++23)	returns a constant iterator to the beginning of the range. (public member function of std::ranges::view_interface<D>)
cend (C++23)	returns a sentinel for the constant iterator of the range. (public member function of std::ranges::view_interface<D>)
operator bool	returns whether the derived view is not empty. Provided if 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 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 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 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()     requires ranges::common_range<V>;	(2)	(since C++20)
constexpr auto begin() const requires ranges::common_range<const V>;	(3)	(since C++20)

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 > reverse_view( R&& ) -> reverse_view<views::all_t<R>>;		(since C++20)

Helper templates

template< class T > constexpr bool enable_borrowed_range<std::ranges::reverse_view<T>> =     ranges::enable_borrowed_range<T>;		(since C++20)

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 borrowed_range	it is a borrowed_range if its underlying view is

See also

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