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- [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]
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