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

From cppreference.com
< cpp‎ | ranges
 
 
 
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)
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:
  • 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.

Contents

[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] 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.

[edit] 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
(C++20)
Returns whether the derived view is empty. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
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>) [edit]
(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>) [edit]
(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>) [edit]
Returns the nth element in the derived view. Provided if it satisfies random_access_range.
(public member function of std::ranges::view_interface<D>) [edit]

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 form the underlying view. Equivalent to return base_;.
2) Move-constructs the result form the underlying view. Equivalent to return std::move(base_);.

std::ranges::reverse_view::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

(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.

[edit] Deduction guides

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

[edit] 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.

[edit] Example

#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

[edit] 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

[edit] See also

iterator adaptor for reverse-order traversal
(class template) [edit]
reverses the order of elements in a range
(niebloid) [edit]
creates a copy of a range that is reversed
(niebloid) [edit]