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

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 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

constructs a filter_view
(public member function)
returns the underlying view V
(public member function)
returns a reference to the predicate stored within filter_view
(public member function)
returns the beginning iterator of the filter_view
(public member function)
returns the sentinel of the filter_view
(public member function)
Inherited from std::ranges::view_interface
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>)
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>)
Returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
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)
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).


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)
1) Equivalent to return base_;.
2) Equivalent to 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_)};
    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

the iterator type of filter_view
(exposition-only member class)
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 << ' ';


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

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)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.