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) |
view of an underlying sequence without the elements that fail to satisfy a predicate.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 e is expression-equivalent to expression f, if.
e and f have the same effects, and noexcept(e) == noexcept(f)). Typical implementations of filter_view hold two or three non-static data members:
view of type V (shown here as base_ for exposition only), 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, 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. | (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 |
|
|
(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>) |
|
(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>) |
|
(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>) |
|
(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>) |
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) |
base_ via its default member initializer (= V()) and default-initializes pred_ (which value-initializes the contained Pred).base_ with std::move(base) and initializes pred_ with std::move(pred).| base | - | range to filter |
| pred | - | predicate to filter out elements |
constexpr V base() const& requires std::copy_constructible<V>; | (1) | (since C++20) |
constexpr V base() &&; | (2) | (since C++20) |
return base_;.return std::move(base_);. 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.
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.
constexpr auto end() {
if constexpr (ranges::common_range<V>)
return /*iterator*/{*this, ranges::end(base_)};
else
return /*sentinel*/{*this};
}
| (since C++20) |
template< class R, class Pred > filter_view( R&&, Pred ) -> filter_view<views::all_t<R>, Pred>; | (since C++20) |
|
(C++20) | the iterator type of filter_view (exposition-only member class) |
|
(C++20) | the sentinel type of filter_view when the underlying view is not a common_range (exposition-only member class) |
#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
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 constructorconstructs a filter_view which does not contain an Pred | the filter_view is alsonot default_initializable |
|
(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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