25 Ranges library [ranges]

25.7 Range adaptors [range.adaptors]

25.7.8 Filter view [range.filter]

25.7.8.2 Class template filter_view [range.filter.view]

namespace std::ranges { template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class filter_view : public view_interface<filter_view<V, Pred>> { private: V base_ = V(); // exposition only movable-box<Pred> pred_; // exposition only // [range.filter.iterator], class filter_view::iterator template<bool> class iterator; // exposition only // [range.filter.sentinel], class filter_view::sentinel template<bool> class sentinel; // exposition only public: filter_view() requires default_initializable<V> && default_initializable<Pred> = default; constexpr explicit filter_view(V base, Pred pred); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr const Pred& pred() const; constexpr iterator<false> begin(); constexpr iterator<true> begin() const requires (input_range<const V> && !forward_range<const V> && indirect_unary_predicate<const Pred, iterator_t<const V>>); constexpr auto end() { if constexpr (common_range<V>) return iterator<false>{*this, ranges::end(base_)}; else return sentinel<false>{*this}; } constexpr sentinel<true> end() const requires (input_range<const V> && !forward_range<const V> && indirect_unary_predicate<const Pred, iterator_t<const V>>) { return sentinel<true>{*this}; } }; template<class R, class Pred> filter_view(R&&, Pred) -> filter_view<views::all_t<R>, Pred>; }

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constexpr explicit filter_view(V base, Pred pred);

Effects: Initializes base_ with std::move(base) and initializes pred_ with std::move(pred).

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constexpr const Pred& pred() const;

Effects: Equivalent to: return *pred_;

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constexpr iterator<false> begin();

Preconditions: pred_.has_value() is true.

Returns: {*this, ranges::find_if(base_, ref(*pred_))}.

Remarks: In order to provide the amortized constant time complexity required by the range concept when filter_view models forward_range, this function caches the result within the filter_view for use on subsequent calls.

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constexpr iterator<true> begin() const
  requires (input_range<const V> && !forward_range<const V> &&
            indirect_unary_predicate<const Pred, iterator_t<const V>>);

Preconditions: pred_.has_value() is true.

Returns: {*this, ranges::find_if(base_, ref(*pred_))}.

[Note 1:

This function does not cache the result within the filter_view.

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