counting_iterator synopsis

template <
    class Incrementable
  , class CategoryOrTraversal = use_default
  , class Difference = use_default
>
class counting_iterator
{
public:
    typedef Incrementable value_type;
    typedef const Incrementable& reference;
    typedef const Incrementable* pointer;
    typedef /* see below */ difference_type;
    typedef /* see below */ iterator_category;

    counting_iterator();
    counting_iterator(counting_iterator const& rhs);
    explicit counting_iterator(Incrementable x);
    Incrementable const& base() const;
    reference operator*() const;
    counting_iterator& operator++();
    counting_iterator& operator--();
private:
    Incrementable m_inc; // exposition
};

If the Difference argument is use_default then difference_type is an unspecified signed integral type. Otherwise difference_type is Difference.

iterator_category is determined according to the following algorithm:

if (CategoryOrTraversal is not use_default)
    return CategoryOrTraversal
else if (numeric_limits<Incrementable>::is_specialized)
    return iterator-category(
        random_access_traversal_tag, Incrementable, const Incrementable&)
else
    return iterator-category(
         iterator_traversal<Incrementable>::type, 
         Incrementable, const Incrementable&)

[Note: implementers are encouraged to provide an implementation of

operator- and a difference_type that avoids overflows in the cases where std::numeric_limits<Incrementable>::is_specialized is true.]

counting_iterator requirements

The Incrementable argument shall be Copy Constructible and Assignable.

If iterator_category is convertible to forward_iterator_tag or forward_traversal_tag, the following must be well-formed:

Incrementable i, j;
++i;         // pre-increment
i == j;      // operator equal

If iterator_category is convertible to bidirectional_iterator_tag or bidirectional_traversal_tag, the following expression must also be well-formed:

--i

If iterator_category is convertible to random_access_iterator_tag or random_access_traversal_tag, the following must must also be valid:

counting_iterator::difference_type n;
i += n;
n = i - j;
i < j;

counting_iterator models

Specializations of counting_iterator model Readable Lvalue Iterator. In addition, they model the concepts corresponding to the iterator tags to which their iterator_category is convertible. Also, if CategoryOrTraversal is not use_default then counting_iterator models the concept corresponding to the iterator tag CategoryOrTraversal. Otherwise, if numeric_limits<Incrementable>::is_specialized, then counting_iterator models Random Access Traversal Iterator. Otherwise, counting_iterator models the same iterator traversal concepts modeled by Incrementable.

counting_iterator<X,C1,D1> is interoperable with counting_iterator<Y,C2,D2> if and only if X is interoperable with Y.

counting_iterator operations

In addition to the operations required by the concepts modeled by counting_iterator, counting_iterator provides the following operations.

counting_iterator();

counting_iterator(counting_iterator const& rhs);

explicit counting_iterator(Incrementable x);

reference operator*() const;

counting_iterator& operator++();

counting_iterator& operator--();

Incrementable const& base() const;

template <class Incrementable>
counting_iterator<Incrementable> make_counting_iterator(Incrementable x);

Example

This example fills an array with numbers and a second array with pointers into the first array, using counting_iterator for both tasks. Finally indirect_iterator is used to print out the numbers into the first array via indirection through the second array.

int N = 7;
std::vector<int> numbers;
typedef std::vector<int>::iterator n_iter;
std::copy(boost::counting_iterator<int>(0),
         boost::counting_iterator<int>(N),
         std::back_inserter(numbers));

std::vector<std::vector<int>::iterator> pointers;
std::copy(boost::make_counting_iterator(numbers.begin()),
          boost::make_counting_iterator(numbers.end()),
          std::back_inserter(pointers));

std::cout << "indirectly printing out the numbers from 0 to " 
          << N << std::endl;
std::copy(boost::make_indirect_iterator(pointers.begin()),
          boost::make_indirect_iterator(pointers.end()),
          std::ostream_iterator<int>(std::cout, " "));
std::cout << std::endl;

The output is:

indirectly printing out the numbers from 0 to 7
0 1 2 3 4 5 6 

The source code for this example can be found here.