From Jason Turner

[range.reverse]

C++17 → C++20 3.5 KB 118 lines
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tmp/tmp3q1mhzye/{from.md → to.md} RENAMED
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+ ### Reverse view <a id="range.reverse">[[range.reverse]]</a>
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+
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+ #### Overview <a id="range.reverse.overview">[[range.reverse.overview]]</a>
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+
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+ `reverse_view` takes a bidirectional `view` and produces another `view`
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+ that iterates the same elements in reverse order.
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+
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+ The name `views::reverse` denotes a range adaptor object
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+ [[range.adaptor.object]]. Given a subexpression `E`, the expression
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+ `views::reverse(E)` is expression-equivalent to:
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+
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+ - If the type of `E` is a (possibly cv-qualified) specialization of
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+ `reverse_view`, equivalent to `E.base()`.
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+ - Otherwise, if the type of `E` is cv-qualified
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+ ``` cpp
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+ subrange<reverse_iterator<I>, reverse_iterator<I>, K>
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+ ```
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+
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+ for some iterator type `I` and value `K` of type `subrange_kind`,
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+ - if `K` is `subrange_kind::sized`, equivalent to:
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+ ``` cpp
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+ subrange<I, I, K>(E.end().base(), E.begin().base(), E.size())
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+ ```
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+ - otherwise, equivalent to:
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+ ``` cpp
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+ subrange<I, I, K>(E.end().base(), E.begin().base())
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+ ```
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+
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+ However, in either case `E` is evaluated only once.
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+ - Otherwise, equivalent to `reverse_view{E}`.
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+
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+ [*Example 1*:
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+
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+ ``` cpp
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+ vector<int> is {0,1,2,3,4};
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+ reverse_view rv {is};
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+ for (int i : rv)
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+ cout << i << ' '; // prints: 4 3 2 1 0
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+ ```
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+
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+ — *end example*]
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+
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+ #### Class template `reverse_view` <a id="range.reverse.view">[[range.reverse.view]]</a>
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+
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+ ``` cpp
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+ namespace std::ranges {
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+ template<view V>
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+ requires bidirectional_range<V>
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+ class reverse_view : public view_interface<reverse_view<V>> {
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+ private:
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+ V base_ = V(); // exposition only
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+ public:
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+ reverse_view() = default;
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+
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+ constexpr explicit reverse_view(V r);
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+
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+ constexpr V base() const& requires copy_constructible<V> { return base_; }
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+ constexpr V base() && { return std::move(base_); }
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+
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+ constexpr reverse_iterator<iterator_t<V>> begin();
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+ constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>;
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+ constexpr auto begin() const requires common_range<const V>;
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+
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+ constexpr reverse_iterator<iterator_t<V>> end();
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+ constexpr auto end() const requires common_range<const V>;
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+
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+ constexpr auto size() requires sized_range<V> {
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+ return ranges::size(base_);
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+ }
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+ constexpr auto size() const requires sized_range<const V> {
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+ return ranges::size(base_);
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+ }
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+ };
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+
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+ template<class R>
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+ reverse_view(R&&) -> reverse_view<views::all_t<R>>;
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+ }
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+ ```
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+
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+ ``` cpp
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+ constexpr explicit reverse_view(V base);
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+ ```
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+
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+ *Effects:* Initializes *base\_* with `std::move(base)`.
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+
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+ ``` cpp
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+ constexpr reverse_iterator<iterator_t<V>> begin();
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+ ```
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+
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+ *Returns:*
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+
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+ ``` cpp
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+ make_reverse_iterator(ranges::next(ranges::begin(base_), ranges::end(base_)))
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+ ```
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+
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+ *Remarks:* In order to provide the amortized constant time complexity
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+ required by the `range` concept, this function caches the result within
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+ the `reverse_view` for use on subsequent calls.
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+
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+ ``` cpp
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+ constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>;
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+ constexpr auto begin() const requires common_range<const V>;
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+ ```
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+
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+ *Effects:* Equivalent to:
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+ `return make_reverse_iterator(ranges::end(base_));`
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+
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+ ``` cpp
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+ constexpr reverse_iterator<iterator_t<V>> end();
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+ constexpr auto end() const requires common_range<const V>;
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+ ```
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+
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+ *Effects:* Equivalent to:
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+ `return make_reverse_iterator(ranges::begin(base_));`
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+