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

From cppreference.com

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 `n` ^th 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

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

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External libraries

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