| |
| [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.\n |
| [*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).\n |
| [*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`.\n |
| [*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`.\n |
| [*Returns: ] `*this` |
| |
| |
| [endsect] |