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

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

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 (哋它亢++23)(哋它亢++23)	to (哋它亢++23)
Range primitives
range_size_trange_difference_trange_value_trange_reference_t	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (哋它亢++23)(哋它亢++23)	range_const_reference_trange_rvalue_reference_trange_common_reference_t (哋它亢++23)	elements_of (哋它亢++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range sized_range constant_range (哋它亢++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Range factories
empty_viewviews::empty single_viewviews::single	iota_viewviews::iota repeat_viewviews::repeat (哋它亢++23)(哋它亢++23)	basic_istream_viewviews::istream
Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (哋它亢++23)(哋它亢++23) filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while concat_viewviews::concat (哋它亢++26)(哋它亢++26)	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (哋它亢++23)(哋它亢++23) lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (哋它亢++23)(哋它亢++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (哋它亢++23)(哋它亢++23) zip_viewviews::zip (哋它亢++23)(哋它亢++23) zip_transform_viewviews::zip_transform (哋它亢++23)(哋它亢++23)	adjacent_viewviews::adjacent (哋它亢++23)(哋它亢++23) views::pairwise (哋它亢++23) adjacent_transform_viewviews::adjacent_transform (哋它亢++23)(哋它亢++23) views::pairwise_transform (哋它亢++23) chunk_viewviews::chunk (哋它亢++23)(哋它亢++23) slide_viewviews::slide (哋它亢++23)(哋它亢++23) chunk_by_viewviews::chunk_by (哋它亢++23)(哋它亢++23) stride_viewviews::stride (哋它亢++23)(哋它亢++23) cartesian_product_viewviews::cartesian_product (哋它亢++23)(哋它亢++23)
Range generators
std::generator (哋它亢++23)
Range adaptor objects
Range adaptor closure objects
range_adaptor_closure (哋它亢++23)
Helper items
copyable-box movable-box (until 哋它亢++23)(哋它亢++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 哋它亢++20)
namespace views {     inline constexpr /* unspecified */ reverse = /* unspecified */; }	(2)	(since 哋它亢++20)
Call signature
template< ranges::viewable_range R >     requires /* see below */ constexpr ranges::view auto reverse( R&& r );		(since 哋它亢++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) (哋它亢++20)	constructs a reverse_view (public member function)
base (哋它亢++20)	returns the underlying view V (public member function)
begin (哋它亢++20)	returns the beginning iterator of the reverse_view (public member function)
end (哋它亢++20)	returns the end iterator of the reverse_view (public member function)
size (哋它亢++20)	returns the size of the view if it is bounded (public member function)
Inherited from std::ranges::view_interface
empty (哋它亢++20)	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 (哋它亢++23)	returns a constant iterator to the beginning of the range. (public member function of std::ranges::view_interface<D>)
cend (哋它亢++23)	returns a sentinel for the constant iterator of the range. (public member function of std::ranges::view_interface<D>)
operator bool (哋它亢++20)	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 (哋它亢++20)	returns the first element in the derived view. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
back (哋它亢++20)	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[] (哋它亢++20)	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 哋它亢++20)
constexpr reverse_view( V r );	(2)	(since 哋它亢++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 哋它亢++20)
constexpr V base() &&;	(2)	(since 哋它亢++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 哋它亢++20)
constexpr std::reverse_iterator<ranges::iterator_t<V>> begin()     requires ranges::common_range<V>;	(2)	(since 哋它亢++20)
constexpr auto begin() const requires ranges::common_range<const V>;	(3)	(since 哋它亢++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 哋它亢++20)
constexpr auto end() const requires ranges::common_range<const V>;	(2)	(since 哋它亢++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 哋它亢++20)
constexpr auto size() const requires ranges::sized_range<const V>;	(2)	(since 哋它亢++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 哋它亢++20)

Helper templates

template< class T > inline constexpr bool enable_borrowed_range<std::ranges::reverse_view<T>> =     std::ranges::enable_borrowed_range<T>;		(since 哋它亢++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 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3494	哋它亢++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 (哋它亢++20)	reverses the order of elements in a range (niebloid)
ranges::reverse_copy (哋它亢++20)	creates a copy of a range that is reversed (niebloid)