24 Ranges library [ranges]

24.7 Range adaptors [range.adaptors]

24.7.5 Filter view [range.filter] 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 semiregular-box<Pred> pred_; // exposition only // [range.filter.iterator], class filter_­view​::​iterator class iterator; // exposition only // [range.filter.sentinel], class filter_­view​::​sentinel class sentinel; // exposition only public: filter_view() = default; constexpr 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 begin(); constexpr auto end() { if constexpr (common_range<V>) return iterator{*this, ranges::end(base_)}; else return sentinel{*this}; } }; template<class R, class Pred> filter_view(R&&, Pred) -> filter_view<views::all_t<R>, Pred>; }
constexpr filter_view(V base, Pred pred);
Effects: Initializes base_­ with std​::​move(base) and initializes pred_­ with std​::​move(pred).
constexpr const Pred& pred() const;
Effects: Equivalent to: return *pred_­;
constexpr iterator begin();
Preconditions: pred_­.has_­value().
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.