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In this section we'll further refine the node_iter class template we developed in the |fac_tut|_. If you haven't already read that material, you should go back now and check it out because we're going to pick up right where it left off.

.. |fac_tut| replace:: iterator_facade tutorial .. _fac_tut: iterator_facade.html#tutorial-example

You probably didn't think of it this way, but the node_base* object that underlies node_iterator is itself an iterator, just like all other pointers. If we examine that pointer closely from an iterator perspective, we can see that it has much in common with the node_iterator we're building. First, they share most of the same associated types (value_type, reference, pointer, and difference_type). Second, even some of the core functionality is the same: operator* and operator== on the node_iterator return the result of invoking the same operations on the underlying pointer, via the node_iterator's |dereference_and_equal|_). The only real behavioral difference between node_base* and node_iterator can be observed when they are incremented: node_iterator follows the m_next pointer, while node_base* just applies an address offset.

.. |dereference_and_equal| replace:: dereference and equal member functions .. _dereference_and_equal: iterator_facade.html#implementing-the-core-operations

It turns out that the pattern of building an iterator on another iterator-like type (the Base _ type) while modifying just a few aspects of the underlying type's behavior is an extremely common one, and it's the pattern addressed by iterator_adaptor. Using iterator_adaptor is very much like using iterator_facade, but because iterator_adaptor tries to mimic as much of the Base type's behavior as possible, we neither have to supply a Value argument, nor implement any core behaviors other than increment. The implementation of node_iter is thus reduced to:

template <class Value>
class node_iter
  : public boost::iterator_adaptor<
        node_iter<Value>                // Derived
      , Value*                          // Base
      , boost::use_default              // Value
      , boost::forward_traversal_tag    // CategoryOrTraversal
    >
{
 private:
    struct enabler {};  // a private type avoids misuse

 public:
    node_iter()
      : node_iter::iterator_adaptor_(0) {}

    explicit node_iter(Value* p)
      : node_iter::iterator_adaptor_(p) {}

    template <class OtherValue>
    node_iter(
        node_iter<OtherValue> const& other
      , typename boost::enable_if<
            boost::is_convertible<OtherValue*,Value*>
          , enabler
        >::type = enabler()
    )
      : node_iter::iterator_adaptor_(other.base()) {}

 private:
    friend class boost::iterator_core_access;
    void increment() { this->base_reference() = this->base()->next(); }
};

Note the use of node_iter::iterator_adaptor_ here: because iterator_adaptor defines a nested iterator_adaptor_ type that refers to itself, that gives us a convenient way to refer to the complicated base class type of node_iter<Value>. [Note: this technique is known not to work with Borland C++ 5.6.4 and Metrowerks CodeWarrior versions prior to 9.0]

You can see an example program that exercises this version of the node iterators here.

In the case of node_iter, it's not very compelling to pass boost::use_default as iterator_adaptor 's Value argument; we could have just passed node_iter 's Value along to iterator_adaptor, and that'd even be shorter! Most iterator class templates built with iterator_adaptor are parameterized on another iterator type, rather than on its value_type. For example, boost::reverse_iterator takes an iterator type argument and reverses its direction of traversal, since the original iterator and the reversed one have all the same associated types, iterator_adaptor 's delegation of default types to its Base saves the implementor of boost::reverse_iterator from writing:

std::iterator_traits<Iterator>::*some-associated-type*

at least four times.

We urge you to review the documentation and implementations of |reverse_iterator|_ and the other Boost specialized iterator adaptors__ to get an idea of the sorts of things you can do with iterator_adaptor. In particular, have a look at |transform_iterator|, which is perhaps the most straightforward adaptor, and also |counting_iterator|, which demonstrates that iterator_adaptor's Base type needn't be an iterator.

.. |reverse_iterator| replace:: reverse_iterator .. _reverse_iterator: reverse_iterator.html

.. |counting_iterator| replace:: counting_iterator .. _counting_iterator: counting_iterator.html

.. |transform_iterator| replace:: transform_iterator .. _transform_iterator: transform_iterator.html

__ index.html#specialized-adaptors


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