26 Ranges library [ranges]

26.7 Range adaptors [range.adaptors]

26.7.19 Common view [range.common]

26.7.19.1 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 = views::common(r); auto cnt = count(common.begin(), common.end()); // ... } — end example]

26.7.19.2 Class template common_view [range.common.view]

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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() requires default_initializable<V> = default; constexpr explicit common_view(V 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::distance(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::distance(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).