
[section:filter 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.

[h2 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::bind2nd(std::greater<int>(), -2)
            , numbers, numbers + N)

        , boost::make_filter_iterator(
              std::bind2nd(std::greater<int>(), -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 [@../example/filter_iterator_example.cpp here].

[h2 Reference]


[h3 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`.


[h3 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`.


[h3 Concepts]

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

[table 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 Access
       [[If `Iterator` models             ][then `filter_iterator` models   ]]
       [[Readable Iterator][Readable Iterator]]
       [[Writable Iterator][Writable Iterator]]
       [[Lvalue Iterator  ][Lvalue Iterator  ]]
]

[table 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`.


[h3 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.[br]
[*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).[br]
[*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`.[br]
[*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`.[br]
[*Returns: ] `*this`  


[endsect]