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std::ranges::views::filter, std::ranges::filter_view

Defined in header <ranges> 
template< ranges::input_range V,
          std::indirect_unary_predicate<ranges::iterator_t<V>> Pred >
    requires ranges::view<V> && std::is_object_v<Pred>
class filter_view
    : public ranges::view_interface<filter_view<V, Pred>>
 (1) (since C++20) 
namespace views {
    inline constexpr /* unspecified */ filter = /* unspecified */;
}
 (2) (since C++20)
Call signature 
template< ranges::viewable_range R, class Pred >
    requires /* see below */
constexpr ranges::view auto filter( R&& r, Pred&& pred );
 (since C++20) 
template< class Pred >
constexpr /* range adaptor closure */ filter( Pred&& pred );
 (since C++20)
1) A range adaptor that represents view of an underlying sequence without the elements that fail to satisfy a predicate.
2) RangeAdaptorObject. 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.

Data members

Member name Definition
base_ (private) The underlying view of type V.
(exposition-only member object*)
pred_ (private) Wraps the predicate used to filter out elements of base_ of type copyable-box<Pred>(until C++23)movable-box<Pred>(since C++23) augmenting Pred with assignability when needed and hence always satisfies copyableor movable(since C++23).
(exposition-only member object*)
begin_ (private)
(optional)		 An object of optional-like type that caches an iterator to the first element of base_ that satisfies the pred_. Present only if filter_view models forward_range.
(exposition-only member object*)

Member functions

(C++20)
constructs a filter_view
(public member function)
(C++20)
returns the underlying view V
(public member function)
(C++20)
returns a reference to the predicate stored within filter_view
(public member function)
(C++20)
returns the beginning iterator of the filter_view
(public member function)
(C++20)
returns the sentinel of the filter_view
(public member function)
Inherited from std::ranges::view_interface
(C++20)
returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
(C++23)
returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>)
(C++23)
returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>)
(C++20)
returns whether the derived view is not empty. Provided if ranges::empty is applicable to it.
(public member function of std::ranges::view_interface<D>)
(C++20)
returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
(C++20)
returns the last element in the derived view. Provided if it satisfies 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> &&
                       std::default_initializable<Pred> = default;
 (1) (since C++20) 
constexpr explicit 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();
 (exposition only*)

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();
 (since C++20)

Returns an iterator to the end. Equivalent to

if constexpr (ranges::common_range<V>)
    return /* iterator */{*this, ranges::end(base_)};
else
    return /* sentinel */{*this};

Deduction guides

template< class R, class Pred >
filter_view( R&&, Pred ) -> filter_view<views::all_t<R>, Pred>;
 (since C++20)

Nested classes

(C++20)
the iterator type of filter_view
(exposition-only member class*)
(C++20)
the sentinel type of filter_view when the underlying view is not a common_range
(exposition-only member class*)

Example

#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 << ' ';
    std::cout << '\n';
}

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
LWG 3714
(P2711R1)		 C++20 the multi-parameter constructor was not explicit made explicit
P2325R3 C++20 if Pred is not default_initializable, the default constructor
constructs a filter_view which does not contain a Pred		 the filter_view is also
not default_initializable

See also

(C++20)
a view consisting of the initial elements of another view, until the first element on which a predicate returns false
(class template) (range adaptor object)

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