std::ranges::views::filter,std::ranges::filter_view(3)

NAME
std::ranges::views::filter,std::ranges::filter_view - std::ranges::views::filter,std::ranges::filter_view

Synopsis
   Defined in header <ranges>
   template< ranges::input_range V,
   std::indirect_unary_predicate<ranges::iterator_t<V>> Pred > (since
   requires ranges::view<V> && std::is_object_v<Pred> (1) C++20)
   class filter_view : public ranges::view_interface<filter_view<V,
   Pred>>
   namespace views {
                        (since
   inline constexpr /* unspecified */ filter = /* unspecified */; (2) C++20)
   }
   Call signature
   template< ranges::viewable_range R, class Pred >
                        (since
   requires /* see below */ C++20)
   constexpr ranges::view auto filter( R&& r, Pred&& pred );
   template< class Pred > (since
   constexpr /*range adaptor closure*/ filter( Pred&& pred ); C++20)
   1) A range adaptor that represents view of an underlying sequence without the
   elements that fail to satisfy a predicate.
   2) Ranges adaptor object. The expression views::filter(e, p) is
   expression-equivalent to filter_view(e, p) for any suitable subexpressions e and p.
   filter_view models the concepts bidirectional_range, forward_range, input_range, and
   common_range when the underlying view V models respective concepts.
  Expression-equivalent
   Expression e is expression-equivalent to expression f, if
     * e and f have the same effects, and
     * either both are constant subexpressions or else neither is a constant
       subexpression, and
     * either both are potentially-throwing or else neither is potentially-throwing
       (i.e. noexcept(e) == noexcept(f)).
  Data members
   Typical implementations of filter_view hold two or three non-static data members:
     * the underlying view of type V (shown here as base_ for exposition only),
     * a wrapper that wraps the predicate used to filter out elements of base_ of type
       /*copyable-box*/<Pred> (shown here as pred_ for exposition only), where
       copyable-box is a wrapper class template that always satisfies copyable,
     * an object of std::optional-like type (shown here as begin_ for exposition only)
       that caches an iterator to the first element of base_ that satisfies the pred_.
       The begin_ may be present only if filter_view models forward_range.

Member functions
   constructor constructs a filter_view
   (C++20) (public member function)
   base returns the underlying view V
   (C++20) (public member function)
   pred returns a reference to the predicate stored within filter_view
   (C++20) (public member function)
   begin returns the beginning iterator of the filter_view
   (C++20) (public member function)
   end returns the sentinel of the filter_view
   (C++20) (public member function)
         Inherited from std::ranges::view_interface
   empty Returns whether the derived view is empty. Provided if it satisfies
   (C++20) sized_range or forward_range.
                 (public member function of std::ranges::view_interface<D>)
   operator bool Returns whether the derived view is not empty. Provided if
   (C++20) ranges::empty is applicable to it.
                 (public member function of std::ranges::view_interface<D>)
   front Returns the first element in the derived view. Provided if it
   (C++20) satisfies forward_range.
                 (public member function of std::ranges::view_interface<D>)
   back Returns the last element in the derived view. Provided if it satisfies
   (C++20) bidirectional_range and common_range.
                 (public member function of std::ranges::view_interface<D>) std::ranges::filter_view::filter_view
   filter_view() requires std::default_initializable<V> && (1) (since C++20)
   std::default_initializable<Pred> = default;
   constexpr filter_view( V base, Pred pred ); (2) (since C++20)
   1) Value-initializes base_ via its default member initializer (= V()) and
   default-initializes pred_ (which value-initializes the contained Pred).
   2) Initializes base_ with std::move(base) and initializes pred_ with
   std::move(pred).

Parameters
   base - range to filter
   pred - predicate to filter out elements std::ranges::filter_view::base
   constexpr V base() const& requires std::copy_constructible<V>; (1) (since C++20)
   constexpr V base() &&; (2) (since C++20)
   1) Equivalent to return base_;.
   2) Equivalent to return std::move(base_);. std::ranges::filter_view::pred
   constexpr const Pred& pred() const; (since C++20)
   Returns a reference to the contained Pred object. The behavior is undefined if pred_
   does not contain a value. std::ranges::filter_view::begin
   constexpr /*iterator*/ begin(); (since C++20)
   In order to provide the amortized constant time complexity required by the range
   concept, this function caches the result within the filter_view object for use on
   subsequent calls. Equivalent to
 if constexpr (!ranges::forward_range<V>) {
     return /*iterator*/{*this, ranges::find_if(base_, std::ref(*pred_))};
 } else {
     if (!begin_.has_value())
         begin_ = ranges::find_if(base_, std::ref(*pred_)); // caching
     return /*iterator*/{*this, begin_.value())};
 }
   The behavior is undefined if pred_ does not contain a value. std::ranges::filter_view::end
   constexpr auto end() {
   if constexpr (ranges::common_range<V>)
   return /*iterator*/{*this, ranges::end(base_)}; (since C++20)
   else
   return /*sentinel*/{*this};
   }
  Deduction guides
   template< class R, class Pred > (since C++20)
   filter_view( R&&, Pred ) -> filter_view<views::all_t<R>, Pred>;
  Nested classes
   iterator the iterator type of filter_view
   (C++20) (exposition-only member class)
   sentinel the sentinel type of filter_view when the underlying view is not a
   (C++20) common_range
            (exposition-only member class)

Example
// Run this code
 #include <iostream>
 #include <ranges>
 int main() {
     auto even = [](int i) { return 0 == i % 2; };
     auto square = [](int i) { return i * i; };
     for (int i : std::views::iota(0, 6)
                | std::views::filter(even)
                | std::views::transform(square)) {
         std::cout << i << ' ';
     }
 }

Output:
 0 4 16
  Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
                      if Pred is not default_initializable,
   P2325R3 C++20 the default constructor the filter_view is also
                      constructs a filter_view which does not not default_initializable
                      contain an Pred

See also
   ranges::take_while_view a view consisting of the initial elements of another view,
   views::take_while until the first element on which a predicate returns false
   (C++20) (class template) (range adaptor object)