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### Iterator Adaptor

The `iterator_adaptor` class template adapts some `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.

.. 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.

#### Reference

### 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:

### Requirements

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

.. _base_parameters:

### 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:

       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

### Operations

#### Public

    iterator_adaptor();

Requires: The `Base` type must be Default Constructible.
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`

#### Protected

    Base const& base_reference() const;

Returns: A const reference to `m_iterator`.

    Base& base_reference();

Returns: A non-const reference to `m_iterator`.

#### 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`

#### 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

`node_base*` really is an iterator
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` _ 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