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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

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_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (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
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

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) 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) (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>)
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 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_)};   else     return /*sentinel*/{*this}; }		(since C++20)

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
P2325R3	C++20	if Pred is not default_initializable, the default constructor constructs a filter_view which does not contain an 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)