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

If the Difference argument is use_default then the difference_type member is an implementation defined signed integral type. Otherwise difference_type is Difference.

If CategoryOrTraversal is not use_default then the member iterator_category is CategoryOrTraversal. Otherwise, if Incrementable is a numeric type then iterator_category is a type convertible to random_access_iterator_tag. If Incrementable is not a numeric type then iterator_category is iterator_traversal<Incrementable>::type.

[Note: implementers are encouraged to provide an implementation of

operator- and a difference_type that avoids overflows in the cases when the Incrementable type is a numeric type.]

counting_iterator requirements

The Incrementable type must be Default Constructible, Copy Constructible, and Assignable.

If iterator_category is convertible to forward_iterator_tag or forward_traversal_tag then the following expressions must be valid:

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

If iterator_category is convertible to bidirectional_iterator_tag or bidirectional_traversal_tag then pre-decrement is required:

--i

If iterator_category is convertible to random_access_iterator_tag or random_access_traversal_tag then these additional expressions are also required:

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

counting_iterator models

counting_iterator models Readable Lvalue Iterator. In addition, counting_iterator models the concepts corresponding to the iterator tags that counting_iterator::iterator_category is convertible to.

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 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.