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 Forward Traversal Iterator then iterator_category is convertible to std::forward_iterator_tag. Otherwise iterator_category is convertible to std::input_iterator_tag.

`filter_iterator` 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.

`filter_iterator` models

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

If `Iterator` models	then `filter_iterator` models
Single Pass Iterator	Single Pass Iterator
Forward Traversal Iterator	Forward Traversal Iterator

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

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

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

`filter_iterator` 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`

Iterator 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`

filter_iterator requirements

filter_iterator models

filter_iterator operations

`filter_iterator` requirements

`filter_iterator` models

`filter_iterator` operations