std::ranges...reverse_view(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

💖 NAME 💖

okay so welcome to the C++ Standard Libary manual page for std::ranges...reverse_view(3), bestie. This is the page for:

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

It’s giving “reverse range adaptor with main character energy.” 💅

🌸 Synopsis 🌸

Defined in header <ranges>

template< ranges::view V >
    requires ranges::bidirectional_range<V>
class reverse_view
    : public ranges::view_interface<reverse_view<V>>

That’s (1) and it’s (since C++20).

namespace views {
    inline constexpr /* unspecified */ reverse = /* unspecified */;
}

That’s (2) and it’s (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. iykyk ✨

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. concept vibe check: passed 🫶

🧸 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>)
  • 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 (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 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)
constexpr reverse_view( V r );                                   (2)

Both are (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)
constexpr V base() &&;                                         (2)

Both are (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)
constexpr std::reverse_iterator<ranges::iterator_t<V>> begin() (2)
    requires ranges::common_range<V>;
constexpr auto begin() const requires ranges::common_range<const V>; (3)

All three are (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_));.

the cache object is doing unpaid labor so your iterator stays fast, periodt 💅

🌈 std::ranges::reverse_view::end 🌈

constexpr std::reverse_iterator<ranges::iterator_t<V>> end(); (1)
constexpr auto end() const requires ranges::common_range<const V>; (2)

Both are (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)
constexpr auto size() const requires ranges::sized_range<const V>; (2)

Both are (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 >
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. borrowed-range behavior: inherited, no cap 🫶

🍓 Example 🍓

The example literally says // 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 LWG 3494 — Applied to C++20 — Behavior as published: reverse_view was never a borrowed_range — Correct behavior: 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)

made with so much love, receipts from http://cppreference.com, version 2024.06.10, and the page std::ranges...reverse_view(3) — mwah 💋✨🎀

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