[section:counting Counting Iterator]

A `counting_iterator` adapts an object by adding an `operator*` that
returns the current value of the object. All other iterator operations
are forwarded to the adapted object.


[h2 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:

[pre
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 [example_link counting_iterator_example.cpp..here].

[h2 Reference]


[h3 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&)

[blurb *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.]

[h3 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;


[h3 Concepts]


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`.


[h3 Operations]


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


  counting_iterator();

[*Requires: ] `Incrementable` is Default Constructible.[br]
[*Effects: ] Default construct the member `m_inc`.


  counting_iterator(counting_iterator const& rhs);

[*Effects: ] Construct member `m_inc` from `rhs.m_inc`.



  explicit counting_iterator(Incrementable x);

[*Effects: ] Construct member `m_inc` from `x`.


  reference operator*() const;

[*Returns: ] `m_inc`


  counting_iterator& operator++();

[*Effects: ] `++m_inc`[br]
[*Returns: ] `*this`


  counting_iterator& operator--();

[*Effects: ] `--m_inc`[br]
[*Returns: ] `*this`


  Incrementable const& base() const;

[*Returns: ] `m_inc`


[endsect]
