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std::ranges::views::filter,std::ranges::filter_view(3) | C++ Standard Libary | std::ranges::views::filter,std::ranges::filter_view(3) |
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)
2022.07.31 | http://cppreference.com |