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### Filter Iterator

The filter iterator adaptor creates a view of an iterator range in which some elements of the range are skipped. A predicate function object controls which elements are skipped. When the predicate is applied to an element, if it returns `true` then the element is retained and if it returns `false` then the element is skipped over. When skipping over elements, it is necessary for the filter adaptor to know when to stop so as to avoid going past the end of the underlying range. A filter iterator is therefore constructed with pair of iterators indicating the range of elements in the unfiltered sequence to be traversed.

### Example

This example uses `filter_iterator` and then `make_filter_iterator` to output only the positive integers from an array of integers. Then `make_filter_iterator` is is used to output the integers greater than `-2`.

    struct is_positive_number {
      bool operator()(int x) { return 0 < x; }
    };

    int main()
    {
      int numbers_[] = { 0, -1, 4, -3, 5, 8, -2 };
      const int N = sizeof(numbers_)/sizeof(int);

      typedef int* base_iterator;
      base_iterator numbers(numbers_);

      // Example using filter_iterator
      typedef boost::filter_iterator<is_positive_number, base_iterator>
        FilterIter;

      is_positive_number predicate;
      FilterIter filter_iter_first(predicate, numbers, numbers + N);
      FilterIter filter_iter_last(predicate, numbers + N, numbers + N);

      std::copy(filter_iter_first, filter_iter_last, std::ostream_iterator<int>(std::cout, " "));
      std::cout << std::endl;

      // Example using make_filter_iterator()
      std::copy(boost::make_filter_iterator<is_positive_number>(numbers, numbers + N),
                boost::make_filter_iterator<is_positive_number>(numbers + N, numbers + N),
                std::ostream_iterator<int>(std::cout, " "));
      std::cout << std::endl;

      // Another example using make_filter_iterator()
      std::copy(
          boost::make_filter_iterator(
              std::bind(std::greater<int>(), std::placeholders::_1, -2)
            , numbers, numbers + N)

        , boost::make_filter_iterator(
              std::bind(std::greater<int>(), std::placeholders::_1, -2)
            , numbers + N, numbers + N)

        , std::ostream_iterator<int>(std::cout, " ")
      );

      std::cout << std::endl;

      return boost::exit_success;
    }

The output is:

    4 5 8
    4 5 8
    0 -1 4 5 8

The source code for this example can be found here.

### Reference

#### Synopsis

    template <class Predicate, class Iterator>
    class filter_iterator
    {
     public:
        typedef iterator_traits<Iterator>::value_type value_type;
        typedef iterator_traits<Iterator>::reference reference;
        typedef iterator_traits<Iterator>::pointer pointer;
        typedef iterator_traits<Iterator>::difference_type difference_type;
        typedef /* see below */ iterator_category;

        filter_iterator();
        filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());
        filter_iterator(Iterator x, Iterator end = Iterator());
        template<class OtherIterator>
        filter_iterator(
            filter_iterator<Predicate, OtherIterator> const& t
            , typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
            );
        Predicate predicate() const;
        Iterator end() const;
        Iterator const& base() const;
        reference operator*() const;
        filter_iterator& operator++();
    private:
        Predicate m_pred; // exposition only
        Iterator m_iter;  // exposition only
        Iterator m_end;   // exposition only
    };

If `Iterator` models Readable Lvalue Iterator and Bidirectional Traversal Iterator then `iterator_category` is convertible to `std::bidirectional_iterator_tag`. Otherwise, if `Iterator` models Readable Lvalue Iterator and Forward Traversal Iterator then `iterator_category` is convertible to `std::forward_iterator_tag`. Otherwise `iterator_category` is convertible to `std::input_iterator_tag`.

#### Requirements

The `Iterator` argument shall meet the requirements of Readable Iterator and Single Pass Iterator or it shall meet the requirements of Input Iterator.

The `Predicate` argument must be Assignable, Copy Constructible, and the expression `p(x)` must be valid where `p` is an object of type `Predicate`, `x` is an object of type `iterator_traits<Iterator>::value_type`, and where the type of `p(x)` must be convertible to `bool`.

#### Concepts

The concepts that `filter_iterator` models are dependent on which concepts the `Iterator` argument models, as specified in the following tables.

Table 1.12. Traversal

If `Iterator` models | then `filter_iterator` models
--- | ---
Single Pass Iterator | Single Pass Iterator
Forward Traversal Iterator | Forward Traversal Iterator
Bidirectional Traversal Iterator | Bidirectional Traversal Iterator

Table 1.13. Access

If `Iterator` models | then `filter_iterator` models
--- | ---
Readable Iterator | Readable Iterator
Writable Iterator | Writable Iterator
Lvalue Iterator | Lvalue Iterator

Table 1.14. C++03

If `Iterator` models | then `filter_iterator` models
--- | ---
Readable Iterator, Single Pass Iterator | Input Iterator
Readable Lvalue Iterator, Forward Traversal Iterator | Forward Iterator
Writable Lvalue Iterator, Forward Traversal Iterator | Mutable Forward Iterator
Writable Lvalue Iterator, Bidirectional Iterator | Mutable Bidirectional Iterator

`filter_iterator<P1, X>` is interoperable with `filter_iterator<P2, Y>` if and only if `X` is interoperable with `Y`.

#### Operations

In addition to those operations required by the concepts that `filter_iterator` models, `filter_iterator` provides the following operations.

    filter_iterator();

Requires: `Predicate` and `Iterator` must be Default Constructible.
Effects: Constructs a `filter_iterator` whose`m_pred`, `m_iter`, and `m_end` members are a default constructed.

    filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());

Effects: Constructs a `filter_iterator` where `m_iter` is either the first position in the range `[x,end)` such that `f(*m_iter) == true` or else`m_iter == end`. The member `m_pred` is constructed from `f` and `m_end` from `end`.

    filter_iterator(Iterator x, Iterator end = Iterator());

Requires: `Predicate` must be Default Constructible and `Predicate` is a class type (not a function pointer).
Effects: Constructs a `filter_iterator` where `m_iter` is either the first position in the range `[x,end)` such that `m_pred(*m_iter) == true` or else`m_iter == end`. The member `m_pred` is default constructed.

    template <class OtherIterator>
    filter_iterator(
        filter_iterator<Predicate, OtherIterator> const& t
        , typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
        );

Requires: `OtherIterator` is implicitly convertible to `Iterator`.
Effects: Constructs a filter iterator whose members are copied from `t`.

    Predicate predicate() const;

Returns: `m_pred`

    Ierator end() const;

Returns: `m_end`

    Iterator const& base() const;

Returns: `m_iterator`

    reference operator*() const;

Returns: `*m_iter`

    filter_iterator& operator++();

Effects: Increments `m_iter` and then continues to increment `m_iter` until either `m_iter == m_end` or `m_pred(*m_iter) == true`.
Returns: `*this`