25 Ranges library [ranges]

25.7 Range adaptors [range.adaptors]

25.7.21 Reverse view [range.reverse]

25.7.21.1 Overview [range.reverse.overview]

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reverse_view takes a bidirectional view and produces another view that iterates the same elements in reverse order.
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The name views​::​reverse denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E, the expression views​::​reverse(E) is expression-equivalent to:
  • (2.1)
    If the type of E is a (possibly cv-qualified) specialization of reverse_view, then E.base().
  • (2.2)
    Otherwise, if the type of E is cv subrange<reverse_iterator<I>, reverse_iterator<I>, K> for some iterator type I and value K of type subrange_kind,
    • (2.2.1)
      if K is subrange_kind​::​sized, then subrange<I, I, K>(E.end().base(), E.begin().base(), E.size());
    • (2.2.2)
      otherwise, subrange<I, I, K>(E.end().base(), E.begin().base()).
    However, in either case E is evaluated only once.
  • (2.3)
    Otherwise, reverse_view{E}.
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[Example 1: vector<int> is {0,1,2,3,4}; for (int i : is | views::reverse) cout << i << ' '; // prints 4 3 2 1 0 — end example]

25.7.21.2 Class template reverse_view [range.reverse.view]

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namespace std::ranges { template<view V> requires bidirectional_range<V> class reverse_view : public view_interface<reverse_view<V>> { private: V base_ = V(); // exposition only public: reverse_view() requires default_initializable<V> = default; constexpr explicit reverse_view(V r); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr reverse_iterator<iterator_t<V>> begin(); constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>; constexpr auto begin() const requires common_range<const V>; constexpr reverse_iterator<iterator_t<V>> end(); constexpr auto end() const requires common_range<const V>; constexpr auto size() requires sized_range<V> { return ranges::size(base_); } constexpr auto size() const requires sized_range<const V> { return ranges::size(base_); } }; template<class R> reverse_view(R&&) -> reverse_view<views::all_t<R>>; }
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constexpr explicit reverse_view(V base);
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Effects: Initializes base_ with std​::​move(base).
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constexpr reverse_iterator<iterator_t<V>> begin();
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Returns: make_reverse_iterator(ranges::next(ranges::begin(base_), ranges::end(base_)))
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Remarks: In order to provide the amortized constant time complexity required by the range concept, this function caches the result within the reverse_view for use on subsequent calls.
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constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>; constexpr auto begin() const requires common_range<const V>;
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Effects: Equivalent to: return make_reverse_iterator(ranges​::​end(base_));
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constexpr reverse_iterator<iterator_t<V>> end(); constexpr auto end() const requires common_range<const V>;
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Effects: Equivalent to: return make_reverse_iterator(ranges​::​begin(base_));