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std::ranges::view::filter, std::ranges::filter_view

template< InputRange V, IndirectUnaryPredicate<iterator_t<V>> Pred >
    requires View<V> && std::is_object_v<Pred>
class filter_view : public ranges::view_interface<filter_view<V, Pred>>
 (1) (since C++20) 
namespace view {
    inline constexpr /*unspecified*/ filter = /*unspecified*/;
}
 (2) (since C++20)
1) A range adaptor that represents View of an underlying sequence without the elements that fail to satisfy a predicate.
2) The expression view::filter(E, P) is expression-equivalent to (has the same effect as) filter_view{E, P} for any suitable subexpressions E and P.

filter_view models the concepts BidirectionalRange, ForwardRange, InputRange, and CommonRange when the underlying view V models respective concepts.

Data members

std::ranges::filter_view::base_ 

V base_ = V(); /* exposition-only */

the underlying view.

std::ranges::filter_view::pred_ 

semiregular<Pred> pred_; /* exposition-only */

the predicate used to filter out elements of base_

semiregular is a wrapper class template which behaves exactly like std::optional except for some small differences. The name semiregular is for exposition purposes only and not normative.

Member functions

std::ranges::filter_view::filter_view 

filter_view() = default;
 (1) 
constexpr filter_view(V base, Pred pred);
 (2) 
template<InputRange R>
requires ViewableRange<R> && Constructible<V, all_view<R>>
constexpr filter_view(R&& r, Pred pred);
 (3)
1) Default-initializes base_ and pred_
2) Initializes base_ with std::move(base) and initializes pred_ with std::move(pred).
3) Initializes base_ with view::all(std::forward<R>(r)) and initializes pred_ with std::​move(pred).

Parameters

r - range to filter
pred - predicate to filter out elements

std::ranges::filter_view::base

constexpr V base() const;

Equivalent to return base_;

std::ranges::filter_view::begin

constexpr iterator begin();

Returns the iterator initialized with {*this, ranges::find_if(base_, std::ref(*pred_))}. 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.

The behavior is undefined unless pred_.has_value().

std::ranges::filter_view::end

constexpr auto end() {
  if constexpr (CommonRange<V>)
    return iterator{*this, ranges::end(base_)};
  else
    return sentinel{*this};
}

Deduction guides

template<class R, class Pred>
filter_view(R&&, Pred) -> filter_view<all_view<R>, Pred>;

Nested classes

std::ranges::filter_view::iterator

template<class V, class Pred>
class filter_view<V, Pred>::iterator /* exposition-only */

The return type of filter_view::begin.

This is a BidirectionalIterator if V models BidirectionalRange, a ForwardIterator if V models ForwardRange, and InputIterator otherwise.

Modification of the element denoted by this iterator is permitted, but results in undefined behavior if the resulting value does not satisfy the filter's predicate.

std::ranges::filter_view::iterator::iterator

iterator() = default;
constexpr iterator(filter_view& parent, iterator_t<V> current);

Initializes exposition-only data member current_ with current and exposition-only data members parent_ with addressof(parent).

std::ranges::filter_view::iterator::operator++

constexpr iterator& operator++()

Equivalent to.

current_ = ranges::find_if(++current_, ranges::end(parent_->base_), ref(*parent_->pred_));
return *this;

std::ranges::filter_view::iterator::operator--

constexpr iterator& operator--() requires BidirectionalRange<V>;

Equivalent to.

do
  --current_;
while (!invoke(*parent_->pred_, *current_));
return *this;

Other members as expected of an iterator.

std::ranges::filter_view::sentinel

template<class V, class Pred>
class filter_view<V, Pred>::sentinel /* exposition-only */

The return type of filter_view::end.

std::ranges::filter_view::sentinel::end_

sentinel_t<V> end_ = sentinel_t<V>(); /* exposition only */

Exposition-only data member holding the sentinel of the underlying View.

std::ranges::filter_view::sentinel::sentinel

sentinel() = default;
constexpr explicit sentinel(filter_view& parent);

Initializes exposition-only data member end_ with parent.

std::ranges::filter_view::sentinel::base

constexpr sentinel_t<V> base() const;

Equivalent to: return end_;

std::ranges::filter_view::sentinel::operator==

friend constexpr bool operator==(const iterator& x, const sentinel& y);
friend constexpr bool operator==(const sentinel& x, const iterator& y);

Equivalent to: return x.current_ == y.end_; and return y == x; respectively.

std::ranges::filter_view::sentinel::operator!=

friend constexpr bool operator!=(const iterator& x, const sentinel& y);
friend constexpr bool operator!=(const sentinel& x, const iterator& y);

Equivalent to return !(x == y); and return !(y == x); respectively.

Example

#include <vector>
#include <ranges>
#include <iostream>
 
int main()
{
  std::vector<int> ints{0,1,2,3,4,5};
  auto even = [](int i){ return 0 == i % 2; };
  auto square = [](int i) { return i * i; };
 
  for (int i : ints | std::view::filter(even) | std::view::transform(square)) {
    std::cout << i << ' ';
  }
}

Output:

0 4 16

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