24 Ranges library [ranges]

24.7 Range adaptors [range.adaptors]

24.7.14 Common view [range.common]

24.7.14.1 Overview [range.common.overview]

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common_­view takes a view which has different types for its iterator and sentinel and turns it into a view of the same elements with an iterator and sentinel of the same type.
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[Note 1:
common_­view is useful for calling legacy algorithms that expect a range's iterator and sentinel types to be the same.
— end note]
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The name views​::​common denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E, the expression views​::​common(E) is expression-equivalent to:
  • (3.1)
    views​::​all(E), if decltype((E)) models common_­range and views​::​all(E) is a well-formed expression.
  • (3.2)
    Otherwise, common_­view{E}.
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[Example 1: // Legacy algorithm: template<class ForwardIterator> size_t count(ForwardIterator first, ForwardIterator last); template<forward_­range R> void my_algo(R&& r) { auto&& common = common_view{r}; auto cnt = count(common.begin(), common.end()); // ... } — end example]

24.7.14.2 Class template common_­view [range.common.view]

namespace std::ranges { template<view V> requires (!common_range<V> && copyable<iterator_t<V>>) class common_view : public view_interface<common_view<V>> { private: V base_ = V(); // exposition only public: common_view() = default; constexpr explicit common_view(V r); template<viewable_­range R> requires (!common_range<R> && constructible_­from<V, views::all_t<R>>) constexpr explicit common_view(R&& r); constexpr V base() const& requires copy_­constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr auto begin() { if constexpr (random_­access_­range<V> && sized_­range<V>) return ranges::begin(base_); else return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_)); } constexpr auto begin() const requires range<const V> { if constexpr (random_­access_­range<const V> && sized_­range<const V>) return ranges::begin(base_); else return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::begin(base_)); } constexpr auto end() { if constexpr (random_­access_­range<V> && sized_­range<V>) return ranges::begin(base_) + ranges::size(base_); else return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_)); } constexpr auto end() const requires range<const V> { if constexpr (random_­access_­range<const V> && sized_­range<const V>) return ranges::begin(base_) + ranges::size(base_); else return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::end(base_)); } 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> common_view(R&&) -> common_view<views::all_t<R>>; }
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constexpr explicit common_view(V base);
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Effects: Initializes base_­ with std​::​move(base).
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template<viewable_range R> requires (!common_range<R> && constructible_­from<V, views::all_t<R>>) constexpr explicit common_view(R&& r);
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Effects: Initializes base_­ with views​::​all(std​::​forward<R>(r)).