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

 

C++

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Feature test macros (C++20)

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Metaprogramming library (C++11)

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Containers library

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Ranges library (C++20)

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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
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

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) 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) (C++20)	constructs a reverse_view (public member function)
base (C++20)	returns the underlying view V (public member function)
begin (C++20)	returns the beginning iterator of the reverse_view (public member function)
end (C++20)	returns the end iterator of the reverse_view (public member function)
size (C++20)	returns the size of the view if it is bounded (public member function)
Inherited from std::ranges::view_interface
empty (C++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>)
operator bool (C++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 (C++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 (C++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[] (C++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 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> {     return ranges::size(base_); }	(1)	(since C++20)
constexpr auto size() const requires ranges::sized_range<const V> {     return ranges::size(base_); }	(2)	(since C++20)

Returns the size of the view if the view is bounded.

Deduction guides

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

Helper templates

template<class T> inline constexpr bool enable_borrowed_range<std::ranges::reverse_view<T>> =     std::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 <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 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)