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

 

C++

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Feature test macros (C++20)

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Technical specifications

Symbols index

External libraries

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_trange_value_trange_reference_t	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)	elements_of (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Range factories
empty_viewviews::empty single_viewviews::single	iota_viewviews::iota repeat_viewviews::repeat (C++23)(C++23)	basic_istream_viewviews::istream
Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23) filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23) lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)
Range generators
std::generator (C++23)
Range adaptor objects
Range adaptor closure objects
range_adaptor_closure (C++23)
Helper items
copyable-box movable-box (until C++23)(C++23)	simple-view non-propagating-cache

 

std::ranges::filter_view

Member functions

filter_view::filter_view (C++20)

filter_view::base (C++20)

filter_view::pred (C++20)

filter_view::begin (C++20)

filter_view::end (C++20)

Deduction guides (C++20)

Iterator

filter_view::iterator::iterator (C++20)

filter_view::iterator::base (C++20)

filter_view::iterator::operator*filter_view::iterator::operator-> (C++20)(C++20)

filter_view::iterator::operator++ filter_view::iterator::operator++(int) (C++20)(C++20)

filter_view::iterator::operator-- filter_view::iterator::operator--(int) (C++20)(C++20)

operator==(filter_view::iterator) (C++20)

iter_move(filter_view::iterator) (C++20)

iter_swap(filter_view::iterator) (C++20)

Sentinel

filter_view::sentinel::sentinel (C++20)

filter_view::sentinel::base (C++20)

operator==(filter_view::iterator, filter_view::sentinel) (C++20)

 

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

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> (until C++23)movable-box<Pred> (since C++23) (shown here as pred_ for exposition only), where the wrapper augument Pred with assignability when needed and hence always satisfies copyable or movable (since C++23),
• 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) (C++20)	constructs a filter_view (public member function)
base (C++20)	returns the underlying view V (public member function)
pred (C++20)	returns a reference to the predicate stored within filter_view (public member function)
begin (C++20)	returns the beginning iterator of the filter_view (public member function)
end (C++20)	returns the sentinel of the filter_view (public member function)
Inherited from std::ranges::view_interface
empty (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>)
cbegin (C++23)	returns a constant iterator to the beginning of the range. (public member function of std::ranges::view_interface<D>)
cend (C++23)	returns a sentinel for the constant iterator of the range. (public member function of std::ranges::view_interface<D>)
operator bool (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>)
front (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>)
back (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();		(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();		(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

iterator (C++20)	the iterator type of filter_view (exposition-only member class*)
sentinel (C++20)	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 << ' ';
}

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

ranges::take_while_viewviews::take_while (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)