filter_iterator概要

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
};

如果Iterator模型 Readable Lvalue Iterator 和 Bidirectional Traversal Iterator，然后iterator_category可转换为std::bidirectional_iterator_tag。否则，如果Iterator模型 Readable Lvalue Iterator 和 Forward Traversal Iterator，然后iterator_category可转换为std::forward_iterator_tag否则iterator_category可转换为std::input_iterator_tag.

filter_iteratorrequirements

该Iterator参数必须满足 Readable Iterator 和 Single Pass Iterator 的要求，或者满足 Input Iterator 的要求。

该谓词参数必须是 Assignable、Copy Constructible，并且表达式p(x)必须是有效的，其中p是一个对象，类型为谓词, x是一个对象，类型为iterator_traits<Iterator>::value_type的 const 对象，其中p(x)必须可转换为bool.

filter_iterator模型

模型filter_iterator的模型取决于Iterator参数模型，具体如下表所示。

filter_iterator<P1, X>与filter_iterator<P2, Y>当且仅当X与Y.

filter_iterator操作

除了概念所要求的那些操作外，filter_iterator满足，filter_iterator还提供了以下操作。

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++();

template <class Predicate, class Iterator>
filter_iterator<Predicate,Iterator>
make_filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());

template <class Predicate, class Iterator>
filter_iterator<Predicate,Iterator>
make_filter_iterator(Iterator x, Iterator end = Iterator());

示例

本示例使用了filter_iterator然后make_filter_iterator来仅输出整数数组中的正整数。然后make_filter_iterator它被用于输出大于-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;
}

找到。输出是

4 5 8
4 5 8
0 -1 4 5 8

本示例的源代码可以在这里找到。