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

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< cpp‎ | ranges
C++
Ranges library
Range adaptors
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

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 copyable or movable(since C++23).
(exposition-only member object*
begin_ (private)
(conditionally present) 		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

 constructs a filter_view
(public member function)
returns the underlying view V
(public member function)
returns a reference to the predicate stored within filter_view
(public member function)
returns the beginning iterator of the filter_view
(public member function)
returns the sentinel of the filter_view
(public member function)
Inherited from std::ranges::view_interface
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>)
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>)
returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
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 

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

 the iterator type of filter_view
(exposition-only member class*
the sentinel type of filter_view when the underlying view is not a 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 << ' ';
    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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