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)