    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`