iterator/doc/quickbook/adaptor.qbk

[section:adaptor Iterator Adaptor]

The `iterator_adaptor` class template adapts some `Base` [#base]_
type to create a new iterator.  Instantiations of `iterator_adaptor`
are derived from a corresponding instantiation of `iterator_facade`
and implement the core behaviors in terms of the `Base` type. In
essence, `iterator_adaptor` merely forwards all operations to an
instance of the `Base` type, which it stores as a member.

.. [#base] The term "Base" here does not refer to a base class and is
   not meant to imply the use of derivation. We have followed the lead
   of the standard library, which provides a base() function to access
   the underlying iterator object of a `reverse_iterator` adaptor.

The user of `iterator_adaptor` creates a class derived from an
instantiation of `iterator_adaptor` and then selectively
redefines some of the core member functions described in the
`iterator_facade` core requirements table. The `Base` type need
not meet the full requirements for an iterator; it need only
support the operations used by the core interface functions of
`iterator_adaptor` that have not been redefined in the user's
derived class.

Several of the template parameters of `iterator_adaptor` default
to `use_default`. This allows the
user to make use of a default parameter even when she wants to
specify a parameter later in the parameter list.  Also, the
defaults for the corresponding associated types are somewhat
complicated, so metaprogramming is required to compute them, and
`use_default` can help to simplify the implementation.  Finally,
the identity of the `use_default` type is not left unspecified
because specification helps to highlight that the `Reference`
template parameter may not always be identical to the iterator's
`reference` type, and will keep users from making mistakes based on
that assumption.

[section:adaptor_reference Reference]

[h2 Synopsis]

  template <
      class Derived
    , class Base
    , class Value               = use_default
    , class CategoryOrTraversal = use_default
    , class Reference           = use_default
    , class Difference = use_default
  >
  class iterator_adaptor 
    : public iterator_facade<Derived, *V'*, *C'*, *R'*, *D'*> // see details
  {
      friend class iterator_core_access;
   public:
      iterator_adaptor();
      explicit iterator_adaptor(Base const& iter);
      typedef Base base_type;
      Base const& base() const;
   protected:
      typedef iterator_adaptor iterator_adaptor\_;
      Base const& base_reference() const;
      Base& base_reference();
   private: // Core iterator interface for iterator_facade.  
      typename iterator_adaptor::reference dereference() const;

      template <
      class OtherDerived, class OtherIterator, class V, class C, class R, class D
      >   
      bool equal(iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& x) const;
  
      void advance(typename iterator_adaptor::difference_type n);
      void increment();
      void decrement();

      template <
          class OtherDerived, class OtherIterator, class V, class C, class R, class D
      >   
      typename iterator_adaptor::difference_type distance_to(
          iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& y) const;

   private:
      Base m_iterator; // exposition only
  };

__ base_parameters_

.. _requirements:

[h2 Requirements]

`static_cast<Derived*>(iterator_adaptor*)` shall be well-formed.
The `Base` argument shall be Assignable and Copy Constructible.


.. _base_parameters:

[h2 Base Class Parameters]

The *V'*, *C'*, *R'*, and *D'* parameters of the `iterator_facade`
used as a base class in the summary of `iterator_adaptor`
above are defined as follows:

[pre
   *V'* = if (Value is use_default)
             return iterator_traits<Base>::value_type
         else
             return Value

   *C'* = if (CategoryOrTraversal is use_default)
             return iterator_traversal<Base>::type
         else
             return CategoryOrTraversal

   *R'* = if (Reference is use_default)
             if (Value is use_default)
                 return iterator_traits<Base>::reference
             else
                 return Value&
         else
             return Reference

   *D'* = if (Difference is use_default)
             return iterator_traits<Base>::difference_type
         else
             return Difference
]

[h2 Operations]

[h3 Public]


  iterator_adaptor();

[*Requires:] The `Base` type must be Default Constructible.[br]
[*Returns:] An instance of `iterator_adaptor` with 
    `m_iterator` default constructed.


  explicit iterator_adaptor(Base const& iter);

[*Returns:] An instance of `iterator_adaptor` with
    `m_iterator` copy constructed from `iter`.

  Base const& base() const;

[*Returns:] `m_iterator`


[h3 Protected]

  Base const& base_reference() const;

[*Returns:] A const reference to `m_iterator`.


  Base& base_reference();

[*Returns:] A non-const reference to `m_iterator`.

[h3 Private]

  typename iterator_adaptor::reference dereference() const;

[*Returns:] `*m_iterator`


  template <
  class OtherDerived, class OtherIterator, class V, class C, class R, class D
  >   
  bool equal(iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& x) const;

[*Returns:] `m_iterator == x.base()`


  void advance(typename iterator_adaptor::difference_type n);

[*Effects:] `m_iterator += n;`

  void increment();

[*Effects:] `++m_iterator;`

  void decrement();

[*Effects:] `--m_iterator;`


  template <
      class OtherDerived, class OtherIterator, class V, class C, class R, class D
  >   
  typename iterator_adaptor::difference_type distance_to(
      iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& y) const;

[*Returns:] `y.base() - m_iterator`

[endsect]

[section:adaptor_tutorial Tutorial]

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

[blurb [*`node_base*` really *is* an iterator][br][br]
  It's not really a very interesting iterator, since `node_base`
  is an abstract class: a pointer to a `node_base` just points
  at some base subobject of an instance of some other class, and
  incrementing a `node_base*` moves it past this base subobject
  to who-knows-where?  The most we can do with that incremented
  position is to compare another `node_base*` to it.  In other
  words, the original iterator traverses a one-element array.
]

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` [#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 
[@../example/node_iterator3.cpp `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


[endsect]

[endsect]