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  1<?xml version="1.0" encoding="utf-8" ?>
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  6<meta name="generator" content="Docutils 0.5: http://docutils.sourceforge.net/" />
  7<title>Iterator Adaptor</title>
  8<meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
  9<meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" />
 10<meta name="date" content="2006-09-11" />
 11<meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." />
 12<link rel="stylesheet" href="../../../rst.css" type="text/css" />
 13</head>
 14<body>
 15<div class="document" id="iterator-adaptor">
 16<h1 class="title">Iterator Adaptor</h1>
 17<table class="docinfo" frame="void" rules="none">
 18<col class="docinfo-name" />
 19<col class="docinfo-content" />
 20<tbody valign="top">
 21<tr><th class="docinfo-name">Author:</th>
 22<td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr>
 23<tr><th class="docinfo-name">Contact:</th>
 24<td><a class="first reference external" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek&#64;osl.iu.edu">jsiek&#64;osl.iu.edu</a>, <a class="last reference external" href="mailto:witt&#64;ive.uni-hannover.de">witt&#64;ive.uni-hannover.de</a></td></tr>
 25<tr><th class="docinfo-name">Organization:</th>
 26<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
 27Lab</a>, University of Hanover <a class="last reference external" href="http://www.ive.uni-hannover.de">Institute for Transport
 28Railway Operation and Construction</a></td></tr>
 29<tr><th class="docinfo-name">Date:</th>
 30<td>2006-09-11</td></tr>
 31<tr><th class="docinfo-name">Copyright:</th>
 32<td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr>
 33</tbody>
 34</table>
 35<!-- Distributed under the Boost -->
 36<!-- Software License, Version 1.0. (See accompanying -->
 37<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 38<table class="docutils field-list" frame="void" rules="none">
 39<col class="field-name" />
 40<col class="field-body" />
 41<tbody valign="top">
 42<tr class="field"><th class="field-name">abstract:</th><td class="field-body"></td>
 43</tr>
 44</tbody>
 45</table>
 46<!-- Distributed under the Boost -->
 47<!-- Software License, Version 1.0. (See accompanying -->
 48<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 49<!-- Version 1.1 of this ReStructuredText document corresponds to
 50n1530_, the paper accepted by the LWG. -->
 51<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 52<p>Each specialization of the <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template is derived from
 53a specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>. The core interface functions
 54expected by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> are implemented in terms of the
 55<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
 56from <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> typically redefines some of the core
 57interface functions to adapt the behavior of the <tt class="docutils literal"><span class="pre">Base</span></tt> type.
 58Whether the derived class models any of the standard iterator concepts
 59depends on the operations supported by the <tt class="docutils literal"><span class="pre">Base</span></tt> type and which
 60core interface functions of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> are redefined in the
 61<tt class="docutils literal"><span class="pre">Derived</span></tt> class.</p>
 62<div class="contents topic" id="table-of-contents">
 63<p class="topic-title first">Table of Contents</p>
 64<ul class="simple">
 65<li><a class="reference internal" href="#overview" id="id6">Overview</a></li>
 66<li><a class="reference internal" href="#reference" id="id7">Reference</a><ul>
 67<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>
 68<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>
 69<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>
 70<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>
 71<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>
 72</ul>
 73</li>
 74<li><a class="reference internal" href="#tutorial-example" id="id13">Tutorial Example</a></li>
 75</ul>
 76</div>
 77<div class="section" id="overview">
 78<h1><a class="toc-backref" href="#id6">Overview</a></h1>
 79<!-- Distributed under the Boost -->
 80<!-- Software License, Version 1.0. (See accompanying -->
 81<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 82<!-- Version 1.2 of this ReStructuredText document corresponds to
 83n1530_, the paper accepted by the LWG for TR1. -->
 84<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 85<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>
 86type to create a new iterator.  Instantiations of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>
 87are derived from a corresponding instantiation of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>
 88and implement the core behaviors in terms of the <tt class="docutils literal"><span class="pre">Base</span></tt> type. In
 89essence, <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> merely forwards all operations to an
 90instance of the <tt class="docutils literal"><span class="pre">Base</span></tt> type, which it stores as a member.</p>
 91<table class="docutils footnote" frame="void" id="base" rules="none">
 92<colgroup><col class="label" /><col /></colgroup>
 93<tbody valign="top">
 94<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 &quot;Base&quot; here does not refer to a base class and is
 95not meant to imply the use of derivation. We have followed the lead
 96of the standard library, which provides a base() function to access
 97the underlying iterator object of a <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> adaptor.</td></tr>
 98</tbody>
 99</table>
100<p>The user of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> creates a class derived from an
101instantiation of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> and then selectively
102redefines some of the core member functions described in the
103<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
104not meet the full requirements for an iterator; it need only
105support the operations used by the core interface functions of
106<tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> that have not been redefined in the user's
107derived class.</p>
108<p>Several of the template parameters of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> default
109to <tt class="docutils literal"><span class="pre">use_default</span></tt>. This allows the
110user to make use of a default parameter even when she wants to
111specify a parameter later in the parameter list.  Also, the
112defaults for the corresponding associated types are somewhat
113complicated, so metaprogramming is required to compute them, and
114<tt class="docutils literal"><span class="pre">use_default</span></tt> can help to simplify the implementation.  Finally,
115the identity of the <tt class="docutils literal"><span class="pre">use_default</span></tt> type is not left unspecified
116because specification helps to highlight that the <tt class="docutils literal"><span class="pre">Reference</span></tt>
117template parameter may not always be identical to the iterator's
118<tt class="docutils literal"><span class="pre">reference</span></tt> type, and will keep users from making mistakes based on
119that assumption.</p>
120</div>
121<div class="section" id="reference">
122<h1><a class="toc-backref" href="#id7">Reference</a></h1>
123<!-- Distributed under the Boost -->
124<!-- Software License, Version 1.0. (See accompanying -->
125<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
126<!-- Version 1.4 of this ReStructuredText document corresponds to
127n1530_, the paper accepted by the LWG for TR1. -->
128<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
129<pre class="literal-block">
130template &lt;
131    class Derived
132  , class Base
133  , class Value               = use_default
134  , class CategoryOrTraversal = use_default
135  , class Reference           = use_default
136  , class Difference = use_default
137&gt;
138class iterator_adaptor
139  : public iterator_facade&lt;Derived, <em>V'</em>, <em>C'</em>, <em>R'</em>, <em>D'</em>&gt; // see <a class="reference internal" href="#base-parameters">details</a>
140{
141    friend class iterator_core_access;
142 public:
143    iterator_adaptor();
144    explicit iterator_adaptor(Base const&amp; iter);
145    typedef Base base_type;
146    Base const&amp; base() const;
147 protected:
148    typedef iterator_adaptor iterator_adaptor_;
149    Base const&amp; base_reference() const;
150    Base&amp; base_reference();
151 private: // Core iterator interface for iterator_facade.
152    typename iterator_adaptor::reference dereference() const;
153
154    template &lt;
155    class OtherDerived, class OtherIterator, class V, class C, class R, class D
156    &gt;
157    bool equal(iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; x) const;
158
159    void advance(typename iterator_adaptor::difference_type n);
160    void increment();
161    void decrement();
162
163    template &lt;
164        class OtherDerived, class OtherIterator, class V, class C, class R, class D
165    &gt;
166    typename iterator_adaptor::difference_type distance_to(
167        iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; y) const;
168
169 private:
170    Base m_iterator; // exposition only
171};
172</pre>
173<div class="section" id="iterator-adaptor-requirements">
174<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>
175<p><tt class="docutils literal"><span class="pre">static_cast&lt;Derived*&gt;(iterator_adaptor*)</span></tt> shall be well-formed.
176The <tt class="docutils literal"><span class="pre">Base</span></tt> argument shall be Assignable and Copy Constructible.</p>
177</div>
178<div class="section" id="iterator-adaptor-base-class-parameters">
179<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>
180<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>
181used as a base class in the summary of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>
182above are defined as follows:</p>
183<pre class="literal-block">
184<em>V'</em> = if (Value is use_default)
185          return iterator_traits&lt;Base&gt;::value_type
186      else
187          return Value
188
189<em>C'</em> = if (CategoryOrTraversal is use_default)
190          return iterator_traversal&lt;Base&gt;::type
191      else
192          return CategoryOrTraversal
193
194<em>R'</em> = if (Reference is use_default)
195          if (Value is use_default)
196              return iterator_traits&lt;Base&gt;::reference
197          else
198              return Value&amp;
199      else
200          return Reference
201
202<em>D'</em> = if (Difference is use_default)
203          return iterator_traits&lt;Base&gt;::difference_type
204      else
205          return Difference
206</pre>
207<!-- ``iterator_adaptor`` models
208- - - - - - - - - - - - - - - - - - - - - - - - - - -
209
210In order for ``Derived`` to model the iterator concepts corresponding
211to ``iterator_traits<Derived>::iterator_category``, the expressions
212involving ``m_iterator`` in the specifications of those private member
213functions of ``iterator_adaptor`` that may be called by
214``iterator_facade<Derived, V, C, R, D>`` in evaluating any valid
215expression involving ``Derived`` in those concepts' requirements. -->
216<!-- The above is confusing and needs a rewrite. -JGS -->
217<!-- That's why it's removed.  We're embracing inheritance, remember? -->
218</div>
219<div class="section" id="iterator-adaptor-public-operations">
220<h2><a class="toc-backref" href="#id10"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></h2>
221<p><tt class="docutils literal"><span class="pre">iterator_adaptor();</span></tt></p>
222<table class="docutils field-list" frame="void" rules="none">
223<col class="field-name" />
224<col class="field-body" />
225<tbody valign="top">
226<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>
227</tr>
228<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
229<tt class="docutils literal"><span class="pre">m_iterator</span></tt> default constructed.</td>
230</tr>
231</tbody>
232</table>
233<p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">iterator_adaptor(Base</span> <span class="pre">const&amp;</span> <span class="pre">iter);</span></tt></p>
234<table class="docutils field-list" frame="void" rules="none">
235<col class="field-name" />
236<col class="field-body" />
237<tbody valign="top">
238<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
239<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>
240</tr>
241</tbody>
242</table>
243<p><tt class="docutils literal"><span class="pre">Base</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
244<table class="docutils field-list" frame="void" rules="none">
245<col class="field-name" />
246<col class="field-body" />
247<tbody valign="top">
248<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>
249</tr>
250</tbody>
251</table>
252</div>
253<div class="section" id="iterator-adaptor-protected-member-functions">
254<h2><a class="toc-backref" href="#id11"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></h2>
255<p><tt class="docutils literal"><span class="pre">Base</span> <span class="pre">const&amp;</span> <span class="pre">base_reference()</span> <span class="pre">const;</span></tt></p>
256<table class="docutils field-list" frame="void" rules="none">
257<col class="field-name" />
258<col class="field-body" />
259<tbody valign="top">
260<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>
261</tr>
262</tbody>
263</table>
264<p><tt class="docutils literal"><span class="pre">Base&amp;</span> <span class="pre">base_reference();</span></tt></p>
265<table class="docutils field-list" frame="void" rules="none">
266<col class="field-name" />
267<col class="field-body" />
268<tbody valign="top">
269<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>
270</tr>
271</tbody>
272</table>
273</div>
274<div class="section" id="iterator-adaptor-private-member-functions">
275<h2><a class="toc-backref" href="#id12"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></h2>
276<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>
277<table class="docutils field-list" frame="void" rules="none">
278<col class="field-name" />
279<col class="field-body" />
280<tbody valign="top">
281<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>
282</tr>
283</tbody>
284</table>
285<pre class="literal-block">
286template &lt;
287class OtherDerived, class OtherIterator, class V, class C, class R, class D
288&gt;
289bool equal(iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; x) const;
290</pre>
291<table class="docutils field-list" frame="void" rules="none">
292<col class="field-name" />
293<col class="field-body" />
294<tbody valign="top">
295<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>
296</tr>
297</tbody>
298</table>
299<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>
300<table class="docutils field-list" frame="void" rules="none">
301<col class="field-name" />
302<col class="field-body" />
303<tbody valign="top">
304<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>
305</tr>
306</tbody>
307</table>
308<p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">increment();</span></tt></p>
309<table class="docutils field-list" frame="void" rules="none">
310<col class="field-name" />
311<col class="field-body" />
312<tbody valign="top">
313<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>
314</tr>
315</tbody>
316</table>
317<p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">decrement();</span></tt></p>
318<table class="docutils field-list" frame="void" rules="none">
319<col class="field-name" />
320<col class="field-body" />
321<tbody valign="top">
322<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>
323</tr>
324</tbody>
325</table>
326<pre class="literal-block">
327template &lt;
328    class OtherDerived, class OtherIterator, class V, class C, class R, class D
329&gt;
330typename iterator_adaptor::difference_type distance_to(
331    iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; y) const;
332</pre>
333<table class="docutils field-list" frame="void" rules="none">
334<col class="field-name" />
335<col class="field-body" />
336<tbody valign="top">
337<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>
338</tr>
339</tbody>
340</table>
341</div>
342</div>
343<div class="section" id="tutorial-example">
344<h1><a class="toc-backref" href="#id13">Tutorial Example</a></h1>
345<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
346<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
347<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
348<p>In this section we'll further refine the <tt class="docutils literal"><span class="pre">node_iter</span></tt> class
349template 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
350read that material, you should go back now and check it out because
351we're going to pick up right where it left off.</p>
352<div class="sidebar">
353<p class="first sidebar-title"><tt class="docutils literal"><span class="pre">node_base*</span></tt> really <em>is</em> an iterator</p>
354<p class="last">It's not really a very interesting iterator, since <tt class="docutils literal"><span class="pre">node_base</span></tt>
355is an abstract class: a pointer to a <tt class="docutils literal"><span class="pre">node_base</span></tt> just points
356at some base subobject of an instance of some other class, and
357incrementing a <tt class="docutils literal"><span class="pre">node_base*</span></tt> moves it past this base subobject
358to who-knows-where?  The most we can do with that incremented
359position is to compare another <tt class="docutils literal"><span class="pre">node_base*</span></tt> to it.  In other
360words, the original iterator traverses a one-element array.</p>
361</div>
362<p>You probably didn't think of it this way, but the <tt class="docutils literal"><span class="pre">node_base*</span></tt>
363object that underlies <tt class="docutils literal"><span class="pre">node_iterator</span></tt> is itself an iterator,
364just like all other pointers.  If we examine that pointer closely
365from an iterator perspective, we can see that it has much in common
366with the <tt class="docutils literal"><span class="pre">node_iterator</span></tt> we're building.  First, they share most
367of 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>,
368<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
369core 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
370the <tt class="docutils literal"><span class="pre">node_iterator</span></tt> return the result of invoking the same
371operations on the underlying pointer, via the <tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s
372<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
373between <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
374they are incremented: <tt class="docutils literal"><span class="pre">node_iterator</span></tt> follows the
375<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>
376<p>It turns out that the pattern of building an iterator on another
377iterator-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
378just a few aspects of the underlying type's behavior is an
379extremely common one, and it's the pattern addressed by
380<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
381using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, but because iterator_adaptor tries to
382mimic as much of the <tt class="docutils literal"><span class="pre">Base</span></tt> type's behavior as possible, we
383neither have to supply a <tt class="docutils literal"><span class="pre">Value</span></tt> argument, nor implement any core
384behaviors other than <tt class="docutils literal"><span class="pre">increment</span></tt>.  The implementation of
385<tt class="docutils literal"><span class="pre">node_iter</span></tt> is thus reduced to:</p>
386<pre class="literal-block">
387template &lt;class Value&gt;
388class node_iter
389  : public boost::iterator_adaptor&lt;
390        node_iter&lt;Value&gt;                // Derived
391      , Value*                          // Base
392      , boost::use_default              // Value
393      , boost::forward_traversal_tag    // CategoryOrTraversal
394    &gt;
395{
396 private:
397    struct enabler {};  // a private type avoids misuse
398
399 public:
400    node_iter()
401      : node_iter::iterator_adaptor_(0) {}
402
403    explicit node_iter(Value* p)
404      : node_iter::iterator_adaptor_(p) {}
405
406    template &lt;class OtherValue&gt;
407    node_iter(
408        node_iter&lt;OtherValue&gt; const&amp; other
409      , typename boost::enable_if&lt;
410            boost::is_convertible&lt;OtherValue*,Value*&gt;
411          , enabler
412        &gt;::type = enabler()
413    )
414      : node_iter::iterator_adaptor_(other.base()) {}
415
416 private:
417    friend class boost::iterator_core_access;
418    void increment() { this-&gt;base_reference() = this-&gt;base()-&gt;next(); }
419};
420</pre>
421<p>Note the use of <tt class="docutils literal"><span class="pre">node_iter::iterator_adaptor_</span></tt> here: because
422<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
423that refers to itself, that gives us a convenient way to refer to
424the complicated base class type of <tt class="docutils literal"><span class="pre">node_iter&lt;Value&gt;</span></tt>. [Note:
425this technique is known not to work with Borland C++ 5.6.4 and
426Metrowerks CodeWarrior versions prior to 9.0]</p>
427<p>You can see an example program that exercises this version of the
428node iterators <a class="reference external" href="../example/node_iterator3.cpp">here</a>.</p>
429<p>In the case of <tt class="docutils literal"><span class="pre">node_iter</span></tt>, it's not very compelling to pass
430<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>
431argument; 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>
432along to <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>, and that'd even be shorter!  Most
433iterator class templates built with <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> are
434parameterized on another iterator type, rather than on its
435<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
436iterator type argument and reverses its direction of traversal,
437since the original iterator and the reversed one have all the same
438associated types, <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s delegation of default
439types to its <tt class="docutils literal"><span class="pre">Base</span></tt> saves the implementor of
440<tt class="docutils literal"><span class="pre">boost::reverse_iterator</span></tt> from writing:</p>
441<pre class="literal-block">
442std::iterator_traits&lt;Iterator&gt;::<em>some-associated-type</em>
443</pre>
444<p>at least four times.</p>
445<p>We urge you to review the documentation and implementations of
446<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
447adaptors</a> to get an idea of the sorts of things you can do with
448<tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>.  In particular, have a look at
449<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
450adaptor, 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
451<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>
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