[range.filter]

The name views::filter denotes a range adaptor object ([range.adaptor.object]).

Given subexpressions E and P, the expression views::filter(E, P) is expression-equivalent to filter_view(E, P).

[Example 1: vector<int> is{ 0, 1, 2, 3, 4, 5, 6 }; auto evens = views::filter(is, [](int i) { return 0 == i % 2; }); for (int i : evens) cout << i << ' '; / prints 0 2 4 6 — end example]

25.7.8.2 Class template filter_view [range.filter.view]

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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 class iterator; / exposition only / [range.filter.sentinel], class filter_view::sentinel 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 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>; }

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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 begin();

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.

25.7.8.3 Class filter_view::iterator [range.filter.iterator]

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namespace std::ranges { template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class filter_view<V, Pred>::iterator { private: iterator_t<V> current_ = iterator_t<V>(); / exposition only filter_view* parent_ = nullptr; / exposition only public: using iterator_concept = see below; using iterator_category = see below; / not always present using value_type = range_value_t<V>; using difference_type = range_difference_t<V>; iterator() requires default_initializable<iterator_t<V>> = default; constexpr iterator(filter_view& parent, iterator_t<V> current); constexpr const iterator_t<V>& base() const & noexcept; constexpr iterator_t<V> base() &&; constexpr range_reference_t<V> operator*() const; constexpr iterator_t<V> operator->() const requires has-arrow<iterator_t<V>> && copyable<iterator_t<V>>; constexpr iterator& operator+(); constexpr void operator+(int); constexpr iterator operator+(int) requires forward_range<V>; constexpr iterator& operator-() requires bidirectional_range<V>; constexpr iterator operator-(int) requires bidirectional_range<V>; friend constexpr bool operator=(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<V>>; friend constexpr range_rvalue_reference_t<V> iter_move(const iterator& i) noexcept(noexcept(ranges::iter_move(i.current_)); friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.current_, y.current_)) requires indirectly_swappable<iterator_t<V>>; }; }

Modification of the element a filter_view::iterator denotes is permitted, but results in undefined behavior if the resulting value does not satisfy the filter predicate.

iterator::iterator_concept is defined as follows:

(2.1)
If V models bidirectional_range, then iterator_concept denotes bidirectional_iterator_tag.
(2.2)
Otherwise, if V models forward_range, then iterator_concept denotes forward_iterator_tag.
(2.3)
Otherwise, iterator_concept denotes input_iterator_tag.

The member typedef-name iterator_category is defined if and only if V models forward_range .

In that case, iterator::iterator_category is defined as follows:

(3.1)
Let C denote the type iterator_traits<iterator_t<V>>::iterator_category.
(3.2)
If C models derived_from<bidirectional_iterator_tag>, then iterator_category denotes bidirectional_iterator_tag.
(3.3)
Otherwise, if C models derived_from<forward_iterator_tag>, then iterator_category denotes forward_iterator_tag.
(3.4)
Otherwise, iterator_category denotes C.

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constexpr iterator(filter_view& parent, iterator_t<V> current);

Effects: Initializes current_ with std::move(current) and parent_ with addressof(parent).

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constexpr const iterator_t<V>& base() const & noexcept;

Effects: Equivalent to: return current_;

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constexpr iterator_t<V> base() &&;

Effects: Equivalent to: return std::move(current_);

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constexpr range_reference_t<V> operator*() const;

Effects: Equivalent to: return *current_;

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constexpr iterator_t<V> operator->() const
  requires has-arrow<iterator_t<V>> && copyable<iterator_t<V>>;

Effects: Equivalent to: return current_;

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constexpr iterator& operator+();

Effects: Equivalent to: current_ = ranges::find_if(std::move(+current_), ranges::end(parent_->base_), ref(*parent_->pred_)); return *this;

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constexpr void operator+(int);

Effects: Equivalent to ++*this.

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constexpr iterator operator+(int) requires forward_range<V>;

Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;

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constexpr iterator& operator-() requires bidirectional_range<V>;

Effects: Equivalent to: do --current_; while (!invoke(*parent_->pred_, *current_)); return *this;

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constexpr iterator operator-(int) requires bidirectional_range<V>;

Effects: Equivalent to: auto tmp = *this; --*this; return tmp;

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friend constexpr bool operator=(const iterator& x, const iterator& y)
  requires equality_comparable<iterator_t<V>>;

Effects: Equivalent to: return x.current_ == y.current_;

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friend constexpr range_rvalue_reference_t<V> iter_move(const iterator& i)
  noexcept(noexcept(ranges::iter_move(i.current_);

Effects: Equivalent to: return ranges::iter_move(i.current_);

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friend constexpr void iter_swap(const iterator& x, const iterator& y)
  noexcept(noexcept(ranges::iter_swap(x.current_, y.current_)
  requires indirectly_swappable<iterator_t<V>>;

Effects: Equivalent to ranges::iter_swap(x.current_, y.current_).

25.7.8.4 Class filter_view::sentinel [range.filter.sentinel]

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namespace std::ranges { template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class filter_view<V, Pred>::sentinel { private: sentinel_t<V> end_ = sentinel_t<V>(); / exposition only public: sentinel() = default; constexpr explicit sentinel(filter_view& parent); constexpr sentinel_t<V> base() const; friend constexpr bool operator=(const iterator& x, const sentinel& y); }; }

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constexpr explicit sentinel(filter_view& parent);

Effects: Initializes end_ with ranges::end(parent.base_).

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constexpr sentinel_t<V> base() const;

Effects: Equivalent to: return end_;

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friend constexpr bool operator=(const iterator& x, const sentinel& y);

Effects: Equivalent to: return x.current_ == y.end_;

25.7.8.2 Class template filter_view [range.filter.view]

25.7.8.3 Class filter_view​::​iterator [range.filter.iterator]

25.7.8.4 Class filter_view​::​sentinel [range.filter.sentinel]

25.7.8.3 Class filter_view::iterator [range.filter.iterator]

25.7.8.4 Class filter_view::sentinel [range.filter.sentinel]