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- <title>Iterator Adaptor</title>
- <meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
- <meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" />
- <meta name="date" content="2006-09-11" />
- <meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." />
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- <body>
- <div class="document" id="iterator-adaptor">
- <h1 class="title">Iterator Adaptor</h1>
- <table class="docinfo" frame="void" rules="none">
- <col class="docinfo-name" />
- <col class="docinfo-content" />
- <tbody valign="top">
- <tr><th class="docinfo-name">Author:</th>
- <td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr>
- <tr><th class="docinfo-name">Contact:</th>
- <td><a class="first reference external" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>, <a class="last reference external" href="mailto:witt@ive.uni-hannover.de">witt@ive.uni-hannover.de</a></td></tr>
- <tr><th class="docinfo-name">Organization:</th>
- <td><a class="first reference external" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference external" href="http://www.osl.iu.edu">Open Systems
- Lab</a>, University of Hanover <a class="last reference external" href="http://www.ive.uni-hannover.de">Institute for Transport
- Railway Operation and Construction</a></td></tr>
- <tr><th class="docinfo-name">Date:</th>
- <td>2006-09-11</td></tr>
- <tr><th class="docinfo-name">Copyright:</th>
- <td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr>
- </tbody>
- </table>
- <!-- Distributed under the Boost -->
- <!-- Software License, Version 1.0. (See accompanying -->
- <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">abstract:</th><td class="field-body"></td>
- </tr>
- </tbody>
- </table>
- <!-- Distributed under the Boost -->
- <!-- Software License, Version 1.0. (See accompanying -->
- <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
- <!-- Version 1.1 of this ReStructuredText document corresponds to
- n1530_, the paper accepted by the LWG. -->
- <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
- <p>Each specialization of the <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template is derived from
- a specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>. The core interface functions
- expected by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> are implemented in terms of the
- <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="docutils literal"><span class="pre">Base</span></tt> template parameter. A class derived
- from <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> typically redefines some of the core
- interface functions to adapt the behavior of the <tt class="docutils literal"><span class="pre">Base</span></tt> type.
- Whether the derived class models any of the standard iterator concepts
- depends on the operations supported by the <tt class="docutils literal"><span class="pre">Base</span></tt> type and which
- core interface functions of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> are redefined in the
- <tt class="docutils literal"><span class="pre">Derived</span></tt> class.</p>
- <div class="contents topic" id="table-of-contents">
- <p class="topic-title first">Table of Contents</p>
- <ul class="simple">
- <li><a class="reference internal" href="#overview" id="id6">Overview</a></li>
- <li><a class="reference internal" href="#reference" id="id7">Reference</a><ul>
- <li><a class="reference internal" href="#iterator-adaptor-requirements" id="id8"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></li>
- <li><a class="reference internal" href="#iterator-adaptor-base-class-parameters" id="id9"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></li>
- <li><a class="reference internal" href="#iterator-adaptor-public-operations" id="id10"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></li>
- <li><a class="reference internal" href="#iterator-adaptor-protected-member-functions" id="id11"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></li>
- <li><a class="reference internal" href="#iterator-adaptor-private-member-functions" id="id12"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></li>
- </ul>
- </li>
- <li><a class="reference internal" href="#tutorial-example" id="id13">Tutorial Example</a></li>
- </ul>
- </div>
- <div class="section" id="overview">
- <h1><a class="toc-backref" href="#id6">Overview</a></h1>
- <!-- Distributed under the Boost -->
- <!-- Software License, Version 1.0. (See accompanying -->
- <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
- <!-- Version 1.2 of this ReStructuredText document corresponds to
- n1530_, the paper accepted by the LWG for TR1. -->
- <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
- <p>The <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template adapts some <tt class="docutils literal"><span class="pre">Base</span></tt><a class="footnote-reference" href="#base" id="id1"><sup>1</sup></a>
- type to create a new iterator. Instantiations of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>
- are derived from a corresponding instantiation of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>
- and implement the core behaviors in terms of the <tt class="docutils literal"><span class="pre">Base</span></tt> type. In
- essence, <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> merely forwards all operations to an
- instance of the <tt class="docutils literal"><span class="pre">Base</span></tt> type, which it stores as a member.</p>
- <table class="docutils footnote" frame="void" id="base" rules="none">
- <colgroup><col class="label" /><col /></colgroup>
- <tbody valign="top">
- <tr><td class="label">[1]</td><td><em>(<a class="fn-backref" href="#id1">1</a>, <a class="fn-backref" href="#id3">2</a>)</em> 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 <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> adaptor.</td></tr>
- </tbody>
- </table>
- <p>The user of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> creates a class derived from an
- instantiation of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> and then selectively
- redefines some of the core member functions described in the
- <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> core requirements table. The <tt class="docutils literal"><span class="pre">Base</span></tt> type need
- not meet the full requirements for an iterator; it need only
- support the operations used by the core interface functions of
- <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> that have not been redefined in the user's
- derived class.</p>
- <p>Several of the template parameters of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> default
- to <tt class="docutils literal"><span class="pre">use_default</span></tt>. 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
- <tt class="docutils literal"><span class="pre">use_default</span></tt> can help to simplify the implementation. Finally,
- the identity of the <tt class="docutils literal"><span class="pre">use_default</span></tt> type is not left unspecified
- because specification helps to highlight that the <tt class="docutils literal"><span class="pre">Reference</span></tt>
- template parameter may not always be identical to the iterator's
- <tt class="docutils literal"><span class="pre">reference</span></tt> type, and will keep users from making mistakes based on
- that assumption.</p>
- </div>
- <div class="section" id="reference">
- <h1><a class="toc-backref" href="#id7">Reference</a></h1>
- <!-- Distributed under the Boost -->
- <!-- Software License, Version 1.0. (See accompanying -->
- <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
- <!-- Version 1.4 of this ReStructuredText document corresponds to
- n1530_, the paper accepted by the LWG for TR1. -->
- <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
- <pre class="literal-block">
- 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, <em>V'</em>, <em>C'</em>, <em>R'</em>, <em>D'</em>> // see <a class="reference internal" href="#base-parameters">details</a>
- {
- 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
- };
- </pre>
- <div class="section" id="iterator-adaptor-requirements">
- <span id="requirements"></span><h2><a class="toc-backref" href="#id8"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></h2>
- <p><tt class="docutils literal"><span class="pre">static_cast<Derived*>(iterator_adaptor*)</span></tt> shall be well-formed.
- The <tt class="docutils literal"><span class="pre">Base</span></tt> argument shall be Assignable and Copy Constructible.</p>
- </div>
- <div class="section" id="iterator-adaptor-base-class-parameters">
- <span id="base-parameters"></span><h2><a class="toc-backref" href="#id9"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></h2>
- <p>The <em>V'</em>, <em>C'</em>, <em>R'</em>, and <em>D'</em> parameters of the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>
- used as a base class in the summary of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>
- above are defined as follows:</p>
- <pre class="literal-block">
- <em>V'</em> = if (Value is use_default)
- return iterator_traits<Base>::value_type
- else
- return Value
- <em>C'</em> = if (CategoryOrTraversal is use_default)
- return iterator_traversal<Base>::type
- else
- return CategoryOrTraversal
- <em>R'</em> = if (Reference is use_default)
- if (Value is use_default)
- return iterator_traits<Base>::reference
- else
- return Value&
- else
- return Reference
- <em>D'</em> = if (Difference is use_default)
- return iterator_traits<Base>::difference_type
- else
- return Difference
- </pre>
- <!-- ``iterator_adaptor`` models
- - - - - - - - - - - - - - - - - - - - - - - - - - - -
- In order for ``Derived`` to model the iterator concepts corresponding
- to ``iterator_traits<Derived>::iterator_category``, the expressions
- involving ``m_iterator`` in the specifications of those private member
- functions of ``iterator_adaptor`` that may be called by
- ``iterator_facade<Derived, V, C, R, D>`` in evaluating any valid
- expression involving ``Derived`` in those concepts' requirements. -->
- <!-- The above is confusing and needs a rewrite. -JGS -->
- <!-- That's why it's removed. We're embracing inheritance, remember? -->
- </div>
- <div class="section" id="iterator-adaptor-public-operations">
- <h2><a class="toc-backref" href="#id10"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></h2>
- <p><tt class="docutils literal"><span class="pre">iterator_adaptor();</span></tt></p>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Requires:</th><td class="field-body">The <tt class="docutils literal"><span class="pre">Base</span></tt> type must be Default Constructible.</td>
- </tr>
- <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> with
- <tt class="docutils literal"><span class="pre">m_iterator</span></tt> default constructed.</td>
- </tr>
- </tbody>
- </table>
- <p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">iterator_adaptor(Base</span> <span class="pre">const&</span> <span class="pre">iter);</span></tt></p>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> with
- <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">iter</span></tt>.</td>
- </tr>
- </tbody>
- </table>
- <p><tt class="docutils literal"><span class="pre">Base</span> <span class="pre">const&</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
- </tr>
- </tbody>
- </table>
- </div>
- <div class="section" id="iterator-adaptor-protected-member-functions">
- <h2><a class="toc-backref" href="#id11"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></h2>
- <p><tt class="docutils literal"><span class="pre">Base</span> <span class="pre">const&</span> <span class="pre">base_reference()</span> <span class="pre">const;</span></tt></p>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Returns:</th><td class="field-body">A const reference to <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td>
- </tr>
- </tbody>
- </table>
- <p><tt class="docutils literal"><span class="pre">Base&</span> <span class="pre">base_reference();</span></tt></p>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Returns:</th><td class="field-body">A non-const reference to <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td>
- </tr>
- </tbody>
- </table>
- </div>
- <div class="section" id="iterator-adaptor-private-member-functions">
- <h2><a class="toc-backref" href="#id12"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></h2>
- <p><tt class="docutils literal"><span class="pre">typename</span> <span class="pre">iterator_adaptor::reference</span> <span class="pre">dereference()</span> <span class="pre">const;</span></tt></p>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*m_iterator</span></tt></td>
- </tr>
- </tbody>
- </table>
- <pre class="literal-block">
- 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;
- </pre>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span> <span class="pre">==</span> <span class="pre">x.base()</span></tt></td>
- </tr>
- </tbody>
- </table>
- <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">advance(typename</span> <span class="pre">iterator_adaptor::difference_type</span> <span class="pre">n);</span></tt></p>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span> <span class="pre">+=</span> <span class="pre">n;</span></tt></td>
- </tr>
- </tbody>
- </table>
- <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">increment();</span></tt></p>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator;</span></tt></td>
- </tr>
- </tbody>
- </table>
- <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">decrement();</span></tt></p>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator;</span></tt></td>
- </tr>
- </tbody>
- </table>
- <pre class="literal-block">
- 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;
- </pre>
- <table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">y.base()</span> <span class="pre">-</span> <span class="pre">m_iterator</span></tt></td>
- </tr>
- </tbody>
- </table>
- </div>
- </div>
- <div class="section" id="tutorial-example">
- <h1><a class="toc-backref" href="#id13">Tutorial Example</a></h1>
- <!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
- <!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
- <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
- <p>In this section we'll further refine the <tt class="docutils literal"><span class="pre">node_iter</span></tt> class
- template we developed in the <a class="reference external" href="iterator_facade.html#tutorial-example"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> tutorial</a>. 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.</p>
- <div class="sidebar">
- <p class="first sidebar-title"><tt class="docutils literal"><span class="pre">node_base*</span></tt> really <em>is</em> an iterator</p>
- <p class="last">It's not really a very interesting iterator, since <tt class="docutils literal"><span class="pre">node_base</span></tt>
- is an abstract class: a pointer to a <tt class="docutils literal"><span class="pre">node_base</span></tt> just points
- at some base subobject of an instance of some other class, and
- incrementing a <tt class="docutils literal"><span class="pre">node_base*</span></tt> moves it past this base subobject
- to who-knows-where? The most we can do with that incremented
- position is to compare another <tt class="docutils literal"><span class="pre">node_base*</span></tt> to it. In other
- words, the original iterator traverses a one-element array.</p>
- </div>
- <p>You probably didn't think of it this way, but the <tt class="docutils literal"><span class="pre">node_base*</span></tt>
- object that underlies <tt class="docutils literal"><span class="pre">node_iterator</span></tt> 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 <tt class="docutils literal"><span class="pre">node_iterator</span></tt> we're building. First, they share most
- of the same associated types (<tt class="docutils literal"><span class="pre">value_type</span></tt>, <tt class="docutils literal"><span class="pre">reference</span></tt>,
- <tt class="docutils literal"><span class="pre">pointer</span></tt>, and <tt class="docutils literal"><span class="pre">difference_type</span></tt>). Second, even some of the
- core functionality is the same: <tt class="docutils literal"><span class="pre">operator*</span></tt> and <tt class="docutils literal"><span class="pre">operator==</span></tt> on
- the <tt class="docutils literal"><span class="pre">node_iterator</span></tt> return the result of invoking the same
- operations on the underlying pointer, via the <tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s
- <a class="reference external" href="iterator_facade.html#implementing-the-core-operations"><tt class="docutils literal"><span class="pre">dereference</span></tt> and <tt class="docutils literal"><span class="pre">equal</span></tt> member functions</a>). The only real behavioral difference
- between <tt class="docutils literal"><span class="pre">node_base*</span></tt> and <tt class="docutils literal"><span class="pre">node_iterator</span></tt> can be observed when
- they are incremented: <tt class="docutils literal"><span class="pre">node_iterator</span></tt> follows the
- <tt class="docutils literal"><span class="pre">m_next</span></tt> pointer, while <tt class="docutils literal"><span class="pre">node_base*</span></tt> just applies an address offset.</p>
- <p>It turns out that the pattern of building an iterator on another
- iterator-like type (the <tt class="docutils literal"><span class="pre">Base</span></tt><a class="footnote-reference" href="#base" id="id3"><sup>1</sup></a> 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
- <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>. Using <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> is very much like
- using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, but because iterator_adaptor tries to
- mimic as much of the <tt class="docutils literal"><span class="pre">Base</span></tt> type's behavior as possible, we
- neither have to supply a <tt class="docutils literal"><span class="pre">Value</span></tt> argument, nor implement any core
- behaviors other than <tt class="docutils literal"><span class="pre">increment</span></tt>. The implementation of
- <tt class="docutils literal"><span class="pre">node_iter</span></tt> is thus reduced to:</p>
- <pre class="literal-block">
- 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(); }
- };
- </pre>
- <p>Note the use of <tt class="docutils literal"><span class="pre">node_iter::iterator_adaptor_</span></tt> here: because
- <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> defines a nested <tt class="docutils literal"><span class="pre">iterator_adaptor_</span></tt> type
- that refers to itself, that gives us a convenient way to refer to
- the complicated base class type of <tt class="docutils literal"><span class="pre">node_iter<Value></span></tt>. [Note:
- this technique is known not to work with Borland C++ 5.6.4 and
- Metrowerks CodeWarrior versions prior to 9.0]</p>
- <p>You can see an example program that exercises this version of the
- node iterators <a class="reference external" href="../example/node_iterator3.cpp">here</a>.</p>
- <p>In the case of <tt class="docutils literal"><span class="pre">node_iter</span></tt>, it's not very compelling to pass
- <tt class="docutils literal"><span class="pre">boost::use_default</span></tt> as <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="docutils literal"><span class="pre">Value</span></tt>
- argument; we could have just passed <tt class="docutils literal"><span class="pre">node_iter</span></tt>'s <tt class="docutils literal"><span class="pre">Value</span></tt>
- along to <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>, and that'd even be shorter! Most
- iterator class templates built with <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> are
- parameterized on another iterator type, rather than on its
- <tt class="docutils literal"><span class="pre">value_type</span></tt>. For example, <tt class="docutils literal"><span class="pre">boost::reverse_iterator</span></tt> 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, <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s delegation of default
- types to its <tt class="docutils literal"><span class="pre">Base</span></tt> saves the implementor of
- <tt class="docutils literal"><span class="pre">boost::reverse_iterator</span></tt> from writing:</p>
- <pre class="literal-block">
- std::iterator_traits<Iterator>::<em>some-associated-type</em>
- </pre>
- <p>at least four times.</p>
- <p>We urge you to review the documentation and implementations of
- <a class="reference external" href="reverse_iterator.html"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt></a> and the other Boost <a class="reference external" href="index.html#specialized-adaptors">specialized iterator
- adaptors</a> to get an idea of the sorts of things you can do with
- <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>. In particular, have a look at
- <a class="reference external" href="transform_iterator.html"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt></a>, which is perhaps the most straightforward
- adaptor, and also <a class="reference external" href="counting_iterator.html"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a>, which demonstrates that
- <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="docutils literal"><span class="pre">Base</span></tt> type needn't be an iterator.</p>
- </div>
- </div>
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