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7<title>Iterator Facade</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." />
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14<body>
15<div class="document" id="iterator-facade">
16<h1 class="title">Iterator Facade</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"><!-- Copyright David Abrahams 2006. Distributed under the Boost -->
43<!-- Software License, Version 1.0. (See accompanying -->
44<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
45<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is a base class template that implements the
46interface of standard iterators in terms of a few core functions
47and associated types, to be supplied by a derived iterator class.</td>
48</tr>
49</tbody>
50</table>
51<div class="contents topic" id="table-of-contents">
52<p class="topic-title first">Table of Contents</p>
53<ul class="simple">
54<li><a class="reference internal" href="#overview" id="id23">Overview</a><ul>
55<li><a class="reference internal" href="#usage" id="id24">Usage</a></li>
56<li><a class="reference internal" href="#iterator-core-access" id="id25">Iterator Core Access</a></li>
57<li><a class="reference internal" href="#operator" id="id26"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></li>
58<li><a class="reference internal" href="#id2" id="id27"><tt class="docutils literal"><span class="pre">operator-&gt;</span></tt></a></li>
59</ul>
60</li>
61<li><a class="reference internal" href="#reference" id="id28">Reference</a><ul>
62<li><a class="reference internal" href="#iterator-facade-requirements" id="id29"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Requirements</a></li>
63<li><a class="reference internal" href="#iterator-facade-operations" id="id30"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> operations</a></li>
64</ul>
65</li>
66<li><a class="reference internal" href="#tutorial-example" id="id31">Tutorial Example</a><ul>
67<li><a class="reference internal" href="#the-problem" id="id32">The Problem</a></li>
68<li><a class="reference internal" href="#a-basic-iterator-using-iterator-facade" id="id33">A Basic Iterator Using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a><ul>
69<li><a class="reference internal" href="#template-arguments-for-iterator-facade" id="id34">Template Arguments for <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a><ul>
70<li><a class="reference internal" href="#derived" id="id35"><tt class="docutils literal"><span class="pre">Derived</span></tt></a></li>
71<li><a class="reference internal" href="#value" id="id36"><tt class="docutils literal"><span class="pre">Value</span></tt></a></li>
72<li><a class="reference internal" href="#categoryortraversal" id="id37"><tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt></a></li>
73<li><a class="reference internal" href="#id12" id="id38"><tt class="docutils literal"><span class="pre">Reference</span></tt></a></li>
74<li><a class="reference internal" href="#difference" id="id39"><tt class="docutils literal"><span class="pre">Difference</span></tt></a></li>
75</ul>
76</li>
77<li><a class="reference internal" href="#constructors-and-data-members" id="id40">Constructors and Data Members</a></li>
78<li><a class="reference internal" href="#implementing-the-core-operations" id="id41">Implementing the Core Operations</a></li>
79</ul>
80</li>
81<li><a class="reference internal" href="#a-constant-node-iterator" id="id42">A constant <tt class="docutils literal"><span class="pre">node_iterator</span></tt></a></li>
82<li><a class="reference internal" href="#interoperability" id="id43">Interoperability</a></li>
83<li><a class="reference internal" href="#telling-the-truth" id="id44">Telling the Truth</a></li>
84<li><a class="reference internal" href="#wrap-up" id="id45">Wrap Up</a></li>
85</ul>
86</li>
87</ul>
88</div>
89<div class="section" id="overview">
90<h1><a class="toc-backref" href="#id23">Overview</a></h1>
91<!-- Distributed under the Boost -->
92<!-- Software License, Version 1.0. (See accompanying -->
93<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
94<!-- Version 1.1 of this ReStructuredText document corresponds to
95n1530_, the paper accepted by the LWG for TR1. -->
96<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
97<p>While the iterator interface is rich, there is a core subset of the
98interface that is necessary for all the functionality. We have
99identified the following core behaviors for iterators:</p>
100<ul class="simple">
101<li>dereferencing</li>
102<li>incrementing</li>
103<li>decrementing</li>
104<li>equality comparison</li>
105<li>random-access motion</li>
106<li>distance measurement</li>
107</ul>
108<p>In addition to the behaviors listed above, the core interface elements
109include the associated types exposed through iterator traits:
110<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">difference_type</span></tt>, and
111<tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p>
112<p>Iterator facade uses the Curiously Recurring Template
113Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id1">[Cop95]</a> so that the user can specify the behavior
114of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> in a derived class. Former designs used
115policy objects to specify the behavior, but that approach was
116discarded for several reasons:</p>
117<blockquote>
118<ol class="arabic simple">
119<li>the creation and eventual copying of the policy object may create
120overhead that can be avoided with the current approach.</li>
121<li>The policy object approach does not allow for custom constructors
122on the created iterator types, an essential feature if
123<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> should be used in other library
124implementations.</li>
125<li>Without the use of CRTP, the standard requirement that an
126iterator's <tt class="docutils literal"><span class="pre">operator++</span></tt> returns the iterator type itself
127would mean that all iterators built with the library would
128have to be specializations of <tt class="docutils literal"><span class="pre">iterator_facade&lt;...&gt;</span></tt>, rather
129than something more descriptive like
130<tt class="docutils literal"><span class="pre">indirect_iterator&lt;T*&gt;</span></tt>. Cumbersome type generator
131metafunctions would be needed to build new parameterized
132iterators, and a separate <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> layer would be
133impossible.</li>
134</ol>
135</blockquote>
136<div class="section" id="usage">
137<h2><a class="toc-backref" href="#id24">Usage</a></h2>
138<p>The user of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> derives his iterator class from a
139specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and passes the derived
140iterator class as <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s first template parameter.
141The order of the other template parameters have been carefully
142chosen to take advantage of useful defaults. For example, when
143defining a constant lvalue iterator, the user can pass a
144const-qualified version of the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt> as
145<tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s <tt class="docutils literal"><span class="pre">Value</span></tt> parameter and omit the
146<tt class="docutils literal"><span class="pre">Reference</span></tt> parameter which follows.</p>
147<p>The derived iterator class must define member functions implementing
148the iterator's core behaviors. The following table describes
149expressions which are required to be valid depending on the category
150of the derived iterator type. These member functions are described
151briefly below and in more detail in the iterator facade
152requirements.</p>
153<blockquote>
154<table border="1" class="docutils">
155<colgroup>
156<col width="44%" />
157<col width="56%" />
158</colgroup>
159<thead valign="bottom">
160<tr><th class="head">Expression</th>
161<th class="head">Effects</th>
162</tr>
163</thead>
164<tbody valign="top">
165<tr><td><tt class="docutils literal"><span class="pre">i.dereference()</span></tt></td>
166<td>Access the value referred to</td>
167</tr>
168<tr><td><tt class="docutils literal"><span class="pre">i.equal(j)</span></tt></td>
169<td>Compare for equality with <tt class="docutils literal"><span class="pre">j</span></tt></td>
170</tr>
171<tr><td><tt class="docutils literal"><span class="pre">i.increment()</span></tt></td>
172<td>Advance by one position</td>
173</tr>
174<tr><td><tt class="docutils literal"><span class="pre">i.decrement()</span></tt></td>
175<td>Retreat by one position</td>
176</tr>
177<tr><td><tt class="docutils literal"><span class="pre">i.advance(n)</span></tt></td>
178<td>Advance by <tt class="docutils literal"><span class="pre">n</span></tt> positions</td>
179</tr>
180<tr><td><tt class="docutils literal"><span class="pre">i.distance_to(j)</span></tt></td>
181<td>Measure the distance to <tt class="docutils literal"><span class="pre">j</span></tt></td>
182</tr>
183</tbody>
184</table>
185</blockquote>
186<!-- Should we add a comment that a zero overhead implementation of iterator_facade
187is possible with proper inlining? -->
188<p>In addition to implementing the core interface functions, an iterator
189derived from <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> typically defines several
190constructors. To model any of the standard iterator concepts, the
191iterator must at least have a copy constructor. Also, if the iterator
192type <tt class="docutils literal"><span class="pre">X</span></tt> is meant to be automatically interoperate with another
193iterator type <tt class="docutils literal"><span class="pre">Y</span></tt> (as with constant and mutable iterators) then
194there must be an implicit conversion from <tt class="docutils literal"><span class="pre">X</span></tt> to <tt class="docutils literal"><span class="pre">Y</span></tt> or from <tt class="docutils literal"><span class="pre">Y</span></tt>
195to <tt class="docutils literal"><span class="pre">X</span></tt> (but not both), typically implemented as a conversion
196constructor. Finally, if the iterator is to model Forward Traversal
197Iterator or a more-refined iterator concept, a default constructor is
198required.</p>
199</div>
200<div class="section" id="iterator-core-access">
201<h2><a class="toc-backref" href="#id25">Iterator Core Access</a></h2>
202<p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and the operator implementations need to be able
203to access the core member functions in the derived class. Making the
204core member functions public would expose an implementation detail to
205the user. The design used here ensures that implementation details do
206not appear in the public interface of the derived iterator type.</p>
207<p>Preventing direct access to the core member functions has two
208advantages. First, there is no possibility for the user to accidently
209use a member function of the iterator when a member of the value_type
210was intended. This has been an issue with smart pointer
211implementations in the past. The second and main advantage is that
212library implementers can freely exchange a hand-rolled iterator
213implementation for one based on <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> without fear of
214breaking code that was accessing the public core member functions
215directly.</p>
216<p>In a naive implementation, keeping the derived class' core member
217functions private would require it to grant friendship to
218<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and each of the seven operators. In order to
219reduce the burden of limiting access, <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> is
220provided, a class that acts as a gateway to the core member functions
221in the derived iterator class. The author of the derived class only
222needs to grant friendship to <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> to make his core
223member functions available to the library.</p>
224<!-- This is no long uptodate -thw -->
225<!-- Yes it is; I made sure of it! -DWA -->
226<p><tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> will be typically implemented as an empty
227class containing only private static member functions which invoke the
228iterator core member functions. There is, however, no need to
229standardize the gateway protocol. Note that even if
230<tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> used public member functions it would not
231open a safety loophole, as every core member function preserves the
232invariants of the iterator.</p>
233</div>
234<div class="section" id="operator">
235<h2><a class="toc-backref" href="#id26"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></h2>
236<p>The indexing operator for a generalized iterator presents special
237challenges. A random access iterator's <tt class="docutils literal"><span class="pre">operator[]</span></tt> is only
238required to return something convertible to its <tt class="docutils literal"><span class="pre">value_type</span></tt>.
239Requiring that it return an lvalue would rule out currently-legal
240random-access iterators which hold the referenced value in a data
241member (e.g. <a class="reference external" href="counting_iterator.html"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a>), because <tt class="docutils literal"><span class="pre">*(p+n)</span></tt> is a reference
242into the temporary iterator <tt class="docutils literal"><span class="pre">p+n</span></tt>, which is destroyed when
243<tt class="docutils literal"><span class="pre">operator[]</span></tt> returns.</p>
244<p>Writable iterators built with <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implement the
245semantics required by the preferred resolution to <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#299">issue 299</a> and
246adopted by proposal <a class="reference external" href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2003/n1550.htm">n1550</a>: the result of <tt class="docutils literal"><span class="pre">p[n]</span></tt> is an object
247convertible to the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt>, and <tt class="docutils literal"><span class="pre">p[n]</span> <span class="pre">=</span> <span class="pre">x</span></tt> is
248equivalent to <tt class="docutils literal"><span class="pre">*(p</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">x</span></tt> (Note: This result object may be
249implemented as a proxy containing a copy of <tt class="docutils literal"><span class="pre">p+n</span></tt>). This approach
250will work properly for any random-access iterator regardless of the
251other details of its implementation. A user who knows more about
252the implementation of her iterator is free to implement an
253<tt class="docutils literal"><span class="pre">operator[]</span></tt> that returns an lvalue in the derived iterator
254class; it will hide the one supplied by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> from
255clients of her iterator.</p>
256</div>
257<div class="section" id="id2">
258<span id="operator-arrow"></span><h2><a class="toc-backref" href="#id27"><tt class="docutils literal"><span class="pre">operator-&gt;</span></tt></a></h2>
259<p>The <tt class="docutils literal"><span class="pre">reference</span></tt> type of a readable iterator (and today's input
260iterator) need not in fact be a reference, so long as it is
261convertible to the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt>. When the <tt class="docutils literal"><span class="pre">value_type</span></tt>
262is a class, however, it must still be possible to access members
263through <tt class="docutils literal"><span class="pre">operator-&gt;</span></tt>. Therefore, an iterator whose <tt class="docutils literal"><span class="pre">reference</span></tt>
264type is not in fact a reference must return a proxy containing a copy
265of the referenced value from its <tt class="docutils literal"><span class="pre">operator-&gt;</span></tt>.</p>
266<p>The return types for <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s <tt class="docutils literal"><span class="pre">operator-&gt;</span></tt> and
267<tt class="docutils literal"><span class="pre">operator[]</span></tt> are not explicitly specified. Instead, those types
268are described in terms of a set of requirements, which must be
269satisfied by the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implementation.</p>
270<table class="docutils citation" frame="void" id="cop95" rules="none">
271<colgroup><col class="label" /><col /></colgroup>
272<tbody valign="top">
273<tr><td class="label">[Cop95]</td><td><em>(<a class="fn-backref" href="#id1">1</a>, <a class="fn-backref" href="#id10">2</a>)</em> [Coplien, 1995] Coplien, J., Curiously Recurring Template
274Patterns, C++ Report, February 1995, pp. 24-27.</td></tr>
275</tbody>
276</table>
277</div>
278</div>
279<div class="section" id="reference">
280<h1><a class="toc-backref" href="#id28">Reference</a></h1>
281<!-- Distributed under the Boost -->
282<!-- Software License, Version 1.0. (See accompanying -->
283<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
284<!-- Version 1.3 of this ReStructuredText document corresponds to
285n1530_, the paper accepted by the LWG for TR1. -->
286<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
287<pre class="literal-block">
288template &lt;
289 class Derived
290 , class Value
291 , class CategoryOrTraversal
292 , class Reference = Value&amp;
293 , class Difference = ptrdiff_t
294&gt;
295class iterator_facade {
296 public:
297 typedef remove_const&lt;Value&gt;::type value_type;
298 typedef Reference reference;
299 typedef Value* pointer;
300 typedef Difference difference_type;
301 typedef /* see <a class="reference internal" href="#iterator-category">below</a> */ iterator_category;
302
303 reference operator*() const;
304 /* see <a class="reference internal" href="#operator-arrow">below</a> */ operator-&gt;() const;
305 /* see <a class="reference internal" href="#brackets">below</a> */ operator[](difference_type n) const;
306 Derived&amp; operator++();
307 Derived operator++(int);
308 Derived&amp; operator--();
309 Derived operator--(int);
310 Derived&amp; operator+=(difference_type n);
311 Derived&amp; operator-=(difference_type n);
312 Derived operator-(difference_type n) const;
313 protected:
314 typedef iterator_facade iterator_facade_;
315};
316
317// Comparison operators
318template &lt;class Dr1, class V1, class TC1, class R1, class D1,
319 class Dr2, class V2, class TC2, class R2, class D2&gt;
320typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type // exposition
321operator ==(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
322 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
323
324template &lt;class Dr1, class V1, class TC1, class R1, class D1,
325 class Dr2, class V2, class TC2, class R2, class D2&gt;
326typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
327operator !=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
328 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
329
330template &lt;class Dr1, class V1, class TC1, class R1, class D1,
331 class Dr2, class V2, class TC2, class R2, class D2&gt;
332typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
333operator &lt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
334 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
335
336template &lt;class Dr1, class V1, class TC1, class R1, class D1,
337 class Dr2, class V2, class TC2, class R2, class D2&gt;
338typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
339operator &lt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
340 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
341
342template &lt;class Dr1, class V1, class TC1, class R1, class D1,
343 class Dr2, class V2, class TC2, class R2, class D2&gt;
344typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
345operator &gt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
346 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
347
348template &lt;class Dr1, class V1, class TC1, class R1, class D1,
349 class Dr2, class V2, class TC2, class R2, class D2&gt;
350typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
351operator &gt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
352 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
353
354// Iterator difference
355template &lt;class Dr1, class V1, class TC1, class R1, class D1,
356 class Dr2, class V2, class TC2, class R2, class D2&gt;
357/* see <a class="reference internal" href="#minus">below</a> */
358operator-(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
359 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
360
361// Iterator addition
362template &lt;class Dr, class V, class TC, class R, class D&gt;
363Derived operator+ (iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;,
364 typename Derived::difference_type n);
365
366template &lt;class Dr, class V, class TC, class R, class D&gt;
367Derived operator+ (typename Derived::difference_type n,
368 iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;);
369</pre>
370<p id="iterator-category">The <tt class="docutils literal"><span class="pre">iterator_category</span></tt> member of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is</p>
371<pre class="literal-block">
372<em>iterator-category</em>(CategoryOrTraversal, value_type, reference)
373</pre>
374<p>where <em>iterator-category</em> is defined as follows:</p>
375<pre class="literal-block" id="id7">
376<em>iterator-category</em>(C,R,V) :=
377 if (C is convertible to std::input_iterator_tag
378 || C is convertible to std::output_iterator_tag
379 )
380 return C
381
382 else if (C is not convertible to incrementable_traversal_tag)
383 <em>the program is ill-formed</em>
384
385 else return a type X satisfying the following two constraints:
386
387 1. X is convertible to X1, and not to any more-derived
388 type, where X1 is defined by:
389
390 if (R is a reference type
391 &amp;&amp; C is convertible to forward_traversal_tag)
392 {
393 if (C is convertible to random_access_traversal_tag)
394 X1 = random_access_iterator_tag
395 else if (C is convertible to bidirectional_traversal_tag)
396 X1 = bidirectional_iterator_tag
397 else
398 X1 = forward_iterator_tag
399 }
400 else
401 {
402 if (C is convertible to single_pass_traversal_tag
403 &amp;&amp; R is convertible to V)
404 X1 = input_iterator_tag
405 else
406 X1 = C
407 }
408
409 2. <a class="reference external" href="new-iter-concepts.html#category-to-traversal"><em>category-to-traversal</em></a>(X) is convertible to the most
410 derived traversal tag type to which X is also
411 convertible, and not to any more-derived traversal tag
412 type.
413</pre>
414<p>[Note: the intention is to allow <tt class="docutils literal"><span class="pre">iterator_category</span></tt> to be one of
415the five original category tags when convertibility to one of the
416traversal tags would add no information]</p>
417<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
418<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
419<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
420<p>The <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> template used above is for exposition
421purposes. The member operators should only be in an overload set
422provided the derived types <tt class="docutils literal"><span class="pre">Dr1</span></tt> and <tt class="docutils literal"><span class="pre">Dr2</span></tt> are interoperable,
423meaning that at least one of the types is convertible to the other. The
424<tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> approach uses SFINAE to take the operators
425out of the overload set when the types are not interoperable.
426The operators should behave <em>as-if</em> <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt>
427were defined to be:</p>
428<pre class="literal-block">
429template &lt;bool, typename&gt; enable_if_interoperable_impl
430{};
431
432template &lt;typename T&gt; enable_if_interoperable_impl&lt;true,T&gt;
433{ typedef T type; };
434
435template&lt;typename Dr1, typename Dr2, typename T&gt;
436struct enable_if_interoperable
437 : enable_if_interoperable_impl&lt;
438 is_convertible&lt;Dr1,Dr2&gt;::value || is_convertible&lt;Dr2,Dr1&gt;::value
439 , T
440 &gt;
441{};
442</pre>
443<div class="section" id="iterator-facade-requirements">
444<h2><a class="toc-backref" href="#id29"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Requirements</a></h2>
445<p>The following table describes the typical valid expressions on
446<tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s <tt class="docutils literal"><span class="pre">Derived</span></tt> parameter, depending on the
447iterator concept(s) it will model. The operations in the first
448column must be made accessible to member functions of class
449<tt class="docutils literal"><span class="pre">iterator_core_access</span></tt>. In addition,
450<tt class="docutils literal"><span class="pre">static_cast&lt;Derived*&gt;(iterator_facade*)</span></tt> shall be well-formed.</p>
451<p>In the table below, <tt class="docutils literal"><span class="pre">F</span></tt> is <tt class="docutils literal"><span class="pre">iterator_facade&lt;X,V,C,R,D&gt;</span></tt>, <tt class="docutils literal"><span class="pre">a</span></tt> is an
452object of type <tt class="docutils literal"><span class="pre">X</span></tt>, <tt class="docutils literal"><span class="pre">b</span></tt> and <tt class="docutils literal"><span class="pre">c</span></tt> are objects of type <tt class="docutils literal"><span class="pre">const</span> <span class="pre">X</span></tt>,
453<tt class="docutils literal"><span class="pre">n</span></tt> is an object of <tt class="docutils literal"><span class="pre">F::difference_type</span></tt>, <tt class="docutils literal"><span class="pre">y</span></tt> is a constant
454object of a single pass iterator type interoperable with <tt class="docutils literal"><span class="pre">X</span></tt>, and <tt class="docutils literal"><span class="pre">z</span></tt>
455is a constant object of a random access traversal iterator type
456interoperable with <tt class="docutils literal"><span class="pre">X</span></tt>.</p>
457<div class="topic" id="core-operations">
458<p class="topic-title first"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Core Operations</p>
459<table border="1" class="docutils">
460<colgroup>
461<col width="21%" />
462<col width="23%" />
463<col width="27%" />
464<col width="29%" />
465</colgroup>
466<thead valign="bottom">
467<tr><th class="head">Expression</th>
468<th class="head">Return Type</th>
469<th class="head">Assertion/Note</th>
470<th class="head">Used to implement Iterator
471Concept(s)</th>
472</tr>
473</thead>
474<tbody valign="top">
475<tr><td><tt class="docutils literal"><span class="pre">c.dereference()</span></tt></td>
476<td><tt class="docutils literal"><span class="pre">F::reference</span></tt></td>
477<td>&nbsp;</td>
478<td>Readable Iterator, Writable
479Iterator</td>
480</tr>
481<tr><td><tt class="docutils literal"><span class="pre">c.equal(y)</span></tt></td>
482<td>convertible to bool</td>
483<td>true iff <tt class="docutils literal"><span class="pre">c</span></tt> and <tt class="docutils literal"><span class="pre">y</span></tt>
484refer to the same
485position.</td>
486<td>Single Pass Iterator</td>
487</tr>
488<tr><td><tt class="docutils literal"><span class="pre">a.increment()</span></tt></td>
489<td>unused</td>
490<td>&nbsp;</td>
491<td>Incrementable Iterator</td>
492</tr>
493<tr><td><tt class="docutils literal"><span class="pre">a.decrement()</span></tt></td>
494<td>unused</td>
495<td>&nbsp;</td>
496<td>Bidirectional Traversal
497Iterator</td>
498</tr>
499<tr><td><tt class="docutils literal"><span class="pre">a.advance(n)</span></tt></td>
500<td>unused</td>
501<td>&nbsp;</td>
502<td>Random Access Traversal
503Iterator</td>
504</tr>
505<tr><td><tt class="docutils literal"><span class="pre">c.distance_to(z)</span></tt></td>
506<td>convertible to
507<tt class="docutils literal"><span class="pre">F::difference_type</span></tt></td>
508<td>equivalent to
509<tt class="docutils literal"><span class="pre">distance(c,</span> <span class="pre">X(z))</span></tt>.</td>
510<td>Random Access Traversal
511Iterator</td>
512</tr>
513</tbody>
514</table>
515</div>
516</div>
517<div class="section" id="iterator-facade-operations">
518<h2><a class="toc-backref" href="#id30"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> operations</a></h2>
519<p>The operations in this section are described in terms of operations on
520the core interface of <tt class="docutils literal"><span class="pre">Derived</span></tt> which may be inaccessible
521(i.e. private). The implementation should access these operations
522through member functions of class <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt>.</p>
523<p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
524<table class="docutils field-list" frame="void" rules="none">
525<col class="field-name" />
526<col class="field-body" />
527<tbody valign="top">
528<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">static_cast&lt;Derived</span> <span class="pre">const*&gt;(this)-&gt;dereference()</span></tt></td>
529</tr>
530</tbody>
531</table>
532<p><tt class="docutils literal"><span class="pre">operator-&gt;()</span> <span class="pre">const;</span></tt> (see <a class="reference internal" href="#operator-arrow">below</a>)</p>
533<table class="docutils field-list" frame="void" rules="none">
534<col class="field-name" />
535<col class="field-body" />
536<tbody valign="top">
537<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">If <tt class="docutils literal"><span class="pre">reference</span></tt> is a reference type, an object
538of type <tt class="docutils literal"><span class="pre">pointer</span></tt> equal to:</p>
539<pre class="literal-block">
540&amp;static_cast&lt;Derived const*&gt;(this)-&gt;dereference()
541</pre>
542<p class="last">Otherwise returns an object of unspecified type such that,
543<tt class="docutils literal"><span class="pre">(*static_cast&lt;Derived</span> <span class="pre">const*&gt;(this))-&gt;m</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">(w</span> <span class="pre">=</span> <span class="pre">**static_cast&lt;Derived</span> <span class="pre">const*&gt;(this),</span>
544<span class="pre">w.m)</span></tt> for some temporary object <tt class="docutils literal"><span class="pre">w</span></tt> of type <tt class="docutils literal"><span class="pre">value_type</span></tt>.</p>
545</td>
546</tr>
547</tbody>
548</table>
549<p id="brackets"><em>unspecified</em> <tt class="docutils literal"><span class="pre">operator[](difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p>
550<table class="docutils field-list" frame="void" rules="none">
551<col class="field-name" />
552<col class="field-body" />
553<tbody valign="top">
554<tr class="field"><th class="field-name">Returns:</th><td class="field-body">an object convertible to <tt class="docutils literal"><span class="pre">value_type</span></tt>. For constant
555objects <tt class="docutils literal"><span class="pre">v</span></tt> of type <tt class="docutils literal"><span class="pre">value_type</span></tt>, and <tt class="docutils literal"><span class="pre">n</span></tt> of type
556<tt class="docutils literal"><span class="pre">difference_type</span></tt>, <tt class="docutils literal"><span class="pre">(*this)[n]</span> <span class="pre">=</span> <span class="pre">v</span></tt> is equivalent to
557<tt class="docutils literal"><span class="pre">*(*this</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">v</span></tt>, and <tt class="docutils literal"><span class="pre">static_cast&lt;value_type</span>
558<span class="pre">const&amp;&gt;((*this)[n])</span></tt> is equivalent to
559<tt class="docutils literal"><span class="pre">static_cast&lt;value_type</span> <span class="pre">const&amp;&gt;(*(*this</span> <span class="pre">+</span> <span class="pre">n))</span></tt></td>
560</tr>
561</tbody>
562</table>
563<p><tt class="docutils literal"><span class="pre">Derived&amp;</span> <span class="pre">operator++();</span></tt></p>
564<table class="docutils field-list" frame="void" rules="none">
565<col class="field-name" />
566<col class="field-body" />
567<tbody valign="top">
568<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
569static_cast&lt;Derived*&gt;(this)-&gt;increment();
570return *static_cast&lt;Derived*&gt;(this);
571</pre>
572</td>
573</tr>
574</tbody>
575</table>
576<p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator++(int);</span></tt></p>
577<table class="docutils field-list" frame="void" rules="none">
578<col class="field-name" />
579<col class="field-body" />
580<tbody valign="top">
581<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
582Derived tmp(static_cast&lt;Derived const*&gt;(this));
583++*this;
584return tmp;
585</pre>
586</td>
587</tr>
588</tbody>
589</table>
590<p><tt class="docutils literal"><span class="pre">Derived&amp;</span> <span class="pre">operator--();</span></tt></p>
591<table class="docutils field-list" frame="void" rules="none">
592<col class="field-name" />
593<col class="field-body" />
594<tbody valign="top">
595<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
596static_cast&lt;Derived*&gt;(this)-&gt;decrement();
597return *static_cast&lt;Derived*&gt;(this);
598</pre>
599</td>
600</tr>
601</tbody>
602</table>
603<p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator--(int);</span></tt></p>
604<table class="docutils field-list" frame="void" rules="none">
605<col class="field-name" />
606<col class="field-body" />
607<tbody valign="top">
608<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
609Derived tmp(static_cast&lt;Derived const*&gt;(this));
610--*this;
611return tmp;
612</pre>
613</td>
614</tr>
615</tbody>
616</table>
617<p><tt class="docutils literal"><span class="pre">Derived&amp;</span> <span class="pre">operator+=(difference_type</span> <span class="pre">n);</span></tt></p>
618<table class="docutils field-list" frame="void" rules="none">
619<col class="field-name" />
620<col class="field-body" />
621<tbody valign="top">
622<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
623static_cast&lt;Derived*&gt;(this)-&gt;advance(n);
624return *static_cast&lt;Derived*&gt;(this);
625</pre>
626</td>
627</tr>
628</tbody>
629</table>
630<p><tt class="docutils literal"><span class="pre">Derived&amp;</span> <span class="pre">operator-=(difference_type</span> <span class="pre">n);</span></tt></p>
631<table class="docutils field-list" frame="void" rules="none">
632<col class="field-name" />
633<col class="field-body" />
634<tbody valign="top">
635<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
636static_cast&lt;Derived*&gt;(this)-&gt;advance(-n);
637return *static_cast&lt;Derived*&gt;(this);
638</pre>
639</td>
640</tr>
641</tbody>
642</table>
643<p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator-(difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p>
644<table class="docutils field-list" frame="void" rules="none">
645<col class="field-name" />
646<col class="field-body" />
647<tbody valign="top">
648<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
649Derived tmp(static_cast&lt;Derived const*&gt;(this));
650return tmp -= n;
651</pre>
652</td>
653</tr>
654</tbody>
655</table>
656<pre class="literal-block">
657template &lt;class Dr, class V, class TC, class R, class D&gt;
658Derived operator+ (iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;,
659 typename Derived::difference_type n);
660
661template &lt;class Dr, class V, class TC, class R, class D&gt;
662Derived operator+ (typename Derived::difference_type n,
663 iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;);
664</pre>
665<table class="docutils field-list" frame="void" rules="none">
666<col class="field-name" />
667<col class="field-body" />
668<tbody valign="top">
669<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
670Derived tmp(static_cast&lt;Derived const*&gt;(this));
671return tmp += n;
672</pre>
673</td>
674</tr>
675</tbody>
676</table>
677<pre class="literal-block">
678template &lt;class Dr1, class V1, class TC1, class R1, class D1,
679 class Dr2, class V2, class TC2, class R2, class D2&gt;
680typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
681operator ==(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
682 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
683</pre>
684<table class="docutils field-list" frame="void" rules="none">
685<col class="field-name" />
686<col class="field-body" />
687<tbody valign="top">
688<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
689<dl class="last docutils">
690<dt>then</dt>
691<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&amp;)lhs).equal((Dr2</span> <span class="pre">const&amp;)rhs)</span></tt>.</p>
692</dd>
693<dt>Otherwise,</dt>
694<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).equal((Dr1</span> <span class="pre">const&amp;)lhs)</span></tt>.</p>
695</dd>
696</dl>
697</td>
698</tr>
699</tbody>
700</table>
701<pre class="literal-block">
702template &lt;class Dr1, class V1, class TC1, class R1, class D1,
703 class Dr2, class V2, class TC2, class R2, class D2&gt;
704typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
705operator !=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
706 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
707</pre>
708<table class="docutils field-list" frame="void" rules="none">
709<col class="field-name" />
710<col class="field-body" />
711<tbody valign="top">
712<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
713<dl class="last docutils">
714<dt>then</dt>
715<dd><p class="first last"><tt class="docutils literal"><span class="pre">!((Dr1</span> <span class="pre">const&amp;)lhs).equal((Dr2</span> <span class="pre">const&amp;)rhs)</span></tt>.</p>
716</dd>
717<dt>Otherwise,</dt>
718<dd><p class="first last"><tt class="docutils literal"><span class="pre">!((Dr2</span> <span class="pre">const&amp;)rhs).equal((Dr1</span> <span class="pre">const&amp;)lhs)</span></tt>.</p>
719</dd>
720</dl>
721</td>
722</tr>
723</tbody>
724</table>
725<pre class="literal-block">
726template &lt;class Dr1, class V1, class TC1, class R1, class D1,
727 class Dr2, class V2, class TC2, class R2, class D2&gt;
728typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
729operator &lt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
730 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
731</pre>
732<table class="docutils field-list" frame="void" rules="none">
733<col class="field-name" />
734<col class="field-body" />
735<tbody valign="top">
736<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
737<dl class="last docutils">
738<dt>then</dt>
739<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&amp;)lhs).distance_to((Dr2</span> <span class="pre">const&amp;)rhs)</span> <span class="pre">&lt;</span> <span class="pre">0</span></tt>.</p>
740</dd>
741<dt>Otherwise,</dt>
742<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).distance_to((Dr1</span> <span class="pre">const&amp;)lhs)</span> <span class="pre">&gt;</span> <span class="pre">0</span></tt>.</p>
743</dd>
744</dl>
745</td>
746</tr>
747</tbody>
748</table>
749<pre class="literal-block">
750template &lt;class Dr1, class V1, class TC1, class R1, class D1,
751 class Dr2, class V2, class TC2, class R2, class D2&gt;
752typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
753operator &lt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
754 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
755</pre>
756<table class="docutils field-list" frame="void" rules="none">
757<col class="field-name" />
758<col class="field-body" />
759<tbody valign="top">
760<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
761<dl class="last docutils">
762<dt>then</dt>
763<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&amp;)lhs).distance_to((Dr2</span> <span class="pre">const&amp;)rhs)</span> <span class="pre">&lt;=</span> <span class="pre">0</span></tt>.</p>
764</dd>
765<dt>Otherwise,</dt>
766<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).distance_to((Dr1</span> <span class="pre">const&amp;)lhs)</span> <span class="pre">&gt;=</span> <span class="pre">0</span></tt>.</p>
767</dd>
768</dl>
769</td>
770</tr>
771</tbody>
772</table>
773<pre class="literal-block">
774template &lt;class Dr1, class V1, class TC1, class R1, class D1,
775 class Dr2, class V2, class TC2, class R2, class D2&gt;
776typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
777operator &gt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
778 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
779</pre>
780<table class="docutils field-list" frame="void" rules="none">
781<col class="field-name" />
782<col class="field-body" />
783<tbody valign="top">
784<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
785<dl class="last docutils">
786<dt>then</dt>
787<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&amp;)lhs).distance_to((Dr2</span> <span class="pre">const&amp;)rhs)</span> <span class="pre">&gt;</span> <span class="pre">0</span></tt>.</p>
788</dd>
789<dt>Otherwise,</dt>
790<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).distance_to((Dr1</span> <span class="pre">const&amp;)lhs)</span> <span class="pre">&lt;</span> <span class="pre">0</span></tt>.</p>
791</dd>
792</dl>
793</td>
794</tr>
795</tbody>
796</table>
797<pre class="literal-block">
798template &lt;class Dr1, class V1, class TC1, class R1, class D1,
799 class Dr2, class V2, class TC2, class R2, class D2&gt;
800typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
801operator &gt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
802 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
803</pre>
804<table class="docutils field-list" frame="void" rules="none">
805<col class="field-name" />
806<col class="field-body" />
807<tbody valign="top">
808<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
809<dl class="last docutils">
810<dt>then</dt>
811<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&amp;)lhs).distance_to((Dr2</span> <span class="pre">const&amp;)rhs)</span> <span class="pre">&gt;=</span> <span class="pre">0</span></tt>.</p>
812</dd>
813<dt>Otherwise,</dt>
814<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).distance_to((Dr1</span> <span class="pre">const&amp;)lhs)</span> <span class="pre">&lt;=</span> <span class="pre">0</span></tt>.</p>
815</dd>
816</dl>
817</td>
818</tr>
819</tbody>
820</table>
821<pre class="literal-block" id="minus">
822template &lt;class Dr1, class V1, class TC1, class R1, class D1,
823 class Dr2, class V2, class TC2, class R2, class D2&gt;
824typename enable_if_interoperable&lt;Dr1,Dr2,difference&gt;::type
825operator -(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
826 iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
827</pre>
828<table class="docutils field-list" frame="void" rules="none">
829<col class="field-name" />
830<col class="field-body" />
831<tbody valign="top">
832<tr class="field"><th class="field-name">Return Type:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
833<blockquote>
834<dl class="docutils">
835<dt>then</dt>
836<dd><p class="first last"><tt class="docutils literal"><span class="pre">difference</span></tt> shall be
837<tt class="docutils literal"><span class="pre">iterator_traits&lt;Dr1&gt;::difference_type</span></tt>.</p>
838</dd>
839<dt>Otherwise</dt>
840<dd><p class="first last"><tt class="docutils literal"><span class="pre">difference</span></tt> shall be <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dr2&gt;::difference_type</span></tt></p>
841</dd>
842</dl>
843</blockquote>
844</td>
845</tr>
846<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
847<dl class="last docutils">
848<dt>then</dt>
849<dd><p class="first last"><tt class="docutils literal"><span class="pre">-((Dr1</span> <span class="pre">const&amp;)lhs).distance_to((Dr2</span> <span class="pre">const&amp;)rhs)</span></tt>.</p>
850</dd>
851<dt>Otherwise,</dt>
852<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).distance_to((Dr1</span> <span class="pre">const&amp;)lhs)</span></tt>.</p>
853</dd>
854</dl>
855</td>
856</tr>
857</tbody>
858</table>
859</div>
860</div>
861<div class="section" id="tutorial-example">
862<h1><a class="toc-backref" href="#id31">Tutorial Example</a></h1>
863<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
864<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
865<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
866<p>In this section we'll walk through the implementation of a few
867iterators using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, based around the simple
868example of a linked list of polymorphic objects. This example was
869inspired by a <a class="reference external" href="http://thread.gmane.org/gmane.comp.lib.boost.user/5100">posting</a> by Keith Macdonald on the <a class="reference external" href="http://www.boost.org/more/mailing_lists.htm#users">Boost-Users</a>
870mailing list.</p>
871<div class="section" id="the-problem">
872<h2><a class="toc-backref" href="#id32">The Problem</a></h2>
873<p>Say we've written a polymorphic linked list node base class:</p>
874<pre class="literal-block">
875# include &lt;iostream&gt;
876
877struct node_base
878{
879 node_base() : m_next(0) {}
880
881 // Each node manages all of its tail nodes
882 virtual ~node_base() { delete m_next; }
883
884 // Access the rest of the list
885 node_base* next() const { return m_next; }
886
887 // print to the stream
888 virtual void print(std::ostream&amp; s) const = 0;
889
890 // double the value
891 virtual void double_me() = 0;
892
893 void append(node_base* p)
894 {
895 if (m_next)
896 m_next-&gt;append(p);
897 else
898 m_next = p;
899 }
900
901 private:
902 node_base* m_next;
903};
904</pre>
905<p>Lists can hold objects of different types by linking together
906specializations of the following template:</p>
907<pre class="literal-block">
908template &lt;class T&gt;
909struct node : node_base
910{
911 node(T x)
912 : m_value(x)
913 {}
914
915 void print(std::ostream&amp; s) const { s &lt;&lt; this-&gt;m_value; }
916 void double_me() { m_value += m_value; }
917
918 private:
919 T m_value;
920};
921</pre>
922<p>And we can print any node using the following streaming operator:</p>
923<pre class="literal-block">
924inline std::ostream&amp; operator&lt;&lt;(std::ostream&amp; s, node_base const&amp; n)
925{
926 n.print(s);
927 return s;
928}
929</pre>
930<p>Our first challenge is to build an appropriate iterator over these
931lists.</p>
932</div>
933<div class="section" id="a-basic-iterator-using-iterator-facade">
934<h2><a class="toc-backref" href="#id33">A Basic Iterator Using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a></h2>
935<p>We will construct a <tt class="docutils literal"><span class="pre">node_iterator</span></tt> class using inheritance from
936<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> to implement most of the iterator's operations.</p>
937<pre class="literal-block">
938# include &quot;node.hpp&quot;
939# include &lt;boost/iterator/iterator_facade.hpp&gt;
940
941class node_iterator
942 : public boost::iterator_facade&lt;...&gt;
943{
944 ...
945};
946</pre>
947<div class="section" id="template-arguments-for-iterator-facade">
948<h3><a class="toc-backref" href="#id34">Template Arguments for <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a></h3>
949<p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> has several template parameters, so we must decide
950what types to use for the arguments. The parameters are <tt class="docutils literal"><span class="pre">Derived</span></tt>,
951<tt class="docutils literal"><span class="pre">Value</span></tt>, <tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt>, <tt class="docutils literal"><span class="pre">Reference</span></tt>, and <tt class="docutils literal"><span class="pre">Difference</span></tt>.</p>
952<div class="section" id="derived">
953<h4><a class="toc-backref" href="#id35"><tt class="docutils literal"><span class="pre">Derived</span></tt></a></h4>
954<p>Because <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is meant to be used with the CRTP
955<a class="citation-reference" href="#cop95" id="id10">[Cop95]</a> the first parameter is the iterator class name itself,
956<tt class="docutils literal"><span class="pre">node_iterator</span></tt>.</p>
957</div>
958<div class="section" id="value">
959<h4><a class="toc-backref" href="#id36"><tt class="docutils literal"><span class="pre">Value</span></tt></a></h4>
960<p>The <tt class="docutils literal"><span class="pre">Value</span></tt> parameter determines the <tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s
961<tt class="docutils literal"><span class="pre">value_type</span></tt>. In this case, we are iterating over <tt class="docutils literal"><span class="pre">node_base</span></tt>
962objects, so <tt class="docutils literal"><span class="pre">Value</span></tt> will be <tt class="docutils literal"><span class="pre">node_base</span></tt>.</p>
963</div>
964<div class="section" id="categoryortraversal">
965<h4><a class="toc-backref" href="#id37"><tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt></a></h4>
966<p>Now we have to determine which <a class="reference external" href="new-iter-concepts.html#iterator-traversal-concepts-lib-iterator-traversal">iterator traversal concept</a> our
967<tt class="docutils literal"><span class="pre">node_iterator</span></tt> is going to model. Singly-linked lists only have
968forward links, so our iterator can't can't be a <a class="reference external" href="new-iter-concepts.html#bidirectional-traversal-iterators-lib-bidirectional-traversal-iterators">bidirectional
969traversal iterator</a>. Our iterator should be able to make multiple
970passes over the same linked list (unlike, say, an
971<tt class="docutils literal"><span class="pre">istream_iterator</span></tt> which consumes the stream it traverses), so it
972must be a <a class="reference external" href="new-iter-concepts.html#forward-traversal-iterators-lib-forward-traversal-iterators">forward traversal iterator</a>. Therefore, we'll pass
973<tt class="docutils literal"><span class="pre">boost::forward_traversal_tag</span></tt> in this position<a class="footnote-reference" href="#category" id="id11"><sup>1</sup></a>.</p>
974<table class="docutils footnote" frame="void" id="category" rules="none">
975<colgroup><col class="label" /><col /></colgroup>
976<tbody valign="top">
977<tr><td class="label"><a class="fn-backref" href="#id11">[1]</a></td><td><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> also supports old-style category
978tags, so we could have passed <tt class="docutils literal"><span class="pre">std::forward_iterator_tag</span></tt> here;
979either way, the resulting iterator's <tt class="docutils literal"><span class="pre">iterator_category</span></tt> will
980end up being <tt class="docutils literal"><span class="pre">std::forward_iterator_tag</span></tt>.</td></tr>
981</tbody>
982</table>
983</div>
984<div class="section" id="id12">
985<h4><a class="toc-backref" href="#id38"><tt class="docutils literal"><span class="pre">Reference</span></tt></a></h4>
986<p>The <tt class="docutils literal"><span class="pre">Reference</span></tt> argument becomes the type returned by
987<tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s dereference operation, and will also be the
988same as <tt class="docutils literal"><span class="pre">std::iterator_traits&lt;node_iterator&gt;::reference</span></tt>. The
989library's default for this parameter is <tt class="docutils literal"><span class="pre">Value&amp;</span></tt>; since
990<tt class="docutils literal"><span class="pre">node_base&amp;</span></tt> is a good choice for the iterator's <tt class="docutils literal"><span class="pre">reference</span></tt>
991type, we can omit this argument, or pass <tt class="docutils literal"><span class="pre">use_default</span></tt>.</p>
992</div>
993<div class="section" id="difference">
994<h4><a class="toc-backref" href="#id39"><tt class="docutils literal"><span class="pre">Difference</span></tt></a></h4>
995<p>The <tt class="docutils literal"><span class="pre">Difference</span></tt> argument determines how the distance between
996two <tt class="docutils literal"><span class="pre">node_iterator</span></tt>s will be measured and will also be the
997same as <tt class="docutils literal"><span class="pre">std::iterator_traits&lt;node_iterator&gt;::difference_type</span></tt>.
998The library's default for <tt class="docutils literal"><span class="pre">Difference</span></tt> is <tt class="docutils literal"><span class="pre">std::ptrdiff_t</span></tt>, an
999appropriate type for measuring the distance between any two
1000addresses in memory, and one that works for almost any iterator,
1001so we can omit this argument, too.</p>
1002<p>The declaration of <tt class="docutils literal"><span class="pre">node_iterator</span></tt> will therefore look something
1003like:</p>
1004<pre class="literal-block">
1005# include &quot;node.hpp&quot;
1006# include &lt;boost/iterator/iterator_facade.hpp&gt;
1007
1008class node_iterator
1009 : public boost::iterator_facade&lt;
1010 node_iterator
1011 , node_base
1012 , boost::forward_traversal_tag
1013 &gt;
1014{
1015 ...
1016};
1017</pre>
1018</div>
1019</div>
1020<div class="section" id="constructors-and-data-members">
1021<h3><a class="toc-backref" href="#id40">Constructors and Data Members</a></h3>
1022<p>Next we need to decide how to represent the iterator's position.
1023This representation will take the form of data members, so we'll
1024also need to write constructors to initialize them. The
1025<tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s position is quite naturally represented using
1026a pointer to a <tt class="docutils literal"><span class="pre">node_base</span></tt>. We'll need a constructor to build an
1027iterator from a <tt class="docutils literal"><span class="pre">node_base*</span></tt>, and a default constructor to
1028satisfy the <a class="reference external" href="new-iter-concepts.html#forward-traversal-iterators-lib-forward-traversal-iterators">forward traversal iterator</a> requirements<a class="footnote-reference" href="#default" id="id13"><sup>2</sup></a>.
1029Our <tt class="docutils literal"><span class="pre">node_iterator</span></tt> then becomes:</p>
1030<pre class="literal-block">
1031# include &quot;node.hpp&quot;
1032# include &lt;boost/iterator/iterator_facade.hpp&gt;
1033
1034class node_iterator
1035 : public boost::iterator_facade&lt;
1036 node_iterator
1037 , node_base
1038 , boost::forward_traversal_tag
1039 &gt;
1040{
1041 public:
1042 node_iterator()
1043 : m_node(0)
1044 {}
1045
1046 explicit node_iterator(node_base* p)
1047 : m_node(p)
1048 {}
1049
1050 private:
1051 ...
1052 node_base* m_node;
1053};
1054</pre>
1055<table class="docutils footnote" frame="void" id="default" rules="none">
1056<colgroup><col class="label" /><col /></colgroup>
1057<tbody valign="top">
1058<tr><td class="label"><a class="fn-backref" href="#id13">[2]</a></td><td>Technically, the C++ standard places almost no
1059requirements on a default-constructed iterator, so if we were
1060really concerned with efficiency, we could've written the
1061default constructor to leave <tt class="docutils literal"><span class="pre">m_node</span></tt> uninitialized.</td></tr>
1062</tbody>
1063</table>
1064</div>
1065<div class="section" id="implementing-the-core-operations">
1066<h3><a class="toc-backref" href="#id41">Implementing the Core Operations</a></h3>
1067<p>The last step is to implement the <a class="reference internal" href="#core-operations">core operations</a> required by
1068the concepts we want our iterator to model. Referring to the
1069<a class="reference internal" href="#core-operations">table</a>, we can see that the first three rows are applicable
1070because <tt class="docutils literal"><span class="pre">node_iterator</span></tt> needs to satisfy the requirements for
1071<a class="reference external" href="new-iter-concepts.html#readable-iterators-lib-readable-iterators">readable iterator</a>, <a class="reference external" href="new-iter-concepts.html#single-pass-iterators-lib-single-pass-iterators">single pass iterator</a>, and <a class="reference external" href="new-iter-concepts.html#incrementable-iterators-lib-incrementable-iterators">incrementable
1072iterator</a>.</p>
1073<p>We therefore need to supply <tt class="docutils literal"><span class="pre">dereference</span></tt>,
1074<tt class="docutils literal"><span class="pre">equal</span></tt>, and <tt class="docutils literal"><span class="pre">increment</span></tt> members. We don't want these members
1075to become part of <tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s public interface, so we can
1076make them private and grant friendship to
1077<tt class="docutils literal"><span class="pre">boost::iterator_core_access</span></tt>, a &quot;back-door&quot; that
1078<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> uses to get access to the core operations:</p>
1079<pre class="literal-block">
1080# include &quot;node.hpp&quot;
1081# include &lt;boost/iterator/iterator_facade.hpp&gt;
1082
1083class node_iterator
1084 : public boost::iterator_facade&lt;
1085 node_iterator
1086 , node_base
1087 , boost::forward_traversal_tag
1088 &gt;
1089{
1090 public:
1091 node_iterator()
1092 : m_node(0) {}
1093
1094 explicit node_iterator(node_base* p)
1095 : m_node(p) {}
1096
1097 private:
1098 friend class boost::iterator_core_access;
1099
1100 void increment() { m_node = m_node-&gt;next(); }
1101
1102 bool equal(node_iterator const&amp; other) const
1103 {
1104 return this-&gt;m_node == other.m_node;
1105 }
1106
1107 node_base&amp; dereference() const { return *m_node; }
1108
1109 node_base* m_node;
1110};
1111</pre>
1112<p>Voilà; a complete and conforming readable, forward-traversal
1113iterator! For a working example of its use, see <a class="reference external" href="../example/node_iterator1.cpp">this program</a>.</p>
1114</div>
1115</div>
1116<div class="section" id="a-constant-node-iterator">
1117<h2><a class="toc-backref" href="#id42">A constant <tt class="docutils literal"><span class="pre">node_iterator</span></tt></a></h2>
1118<div class="sidebar">
1119<p class="first sidebar-title">Constant and Mutable iterators</p>
1120<p>The term <strong>mutable iterator</strong> means an iterator through which
1121the object it references (its &quot;referent&quot;) can be modified. A
1122<strong>constant iterator</strong> is one which doesn't allow modification of
1123its referent.</p>
1124<p>The words <em>constant</em> and <em>mutable</em> don't refer to the ability to
1125modify the iterator itself. For example, an <tt class="docutils literal"><span class="pre">int</span> <span class="pre">const*</span></tt> is a
1126non-<tt class="docutils literal"><span class="pre">const</span></tt> <em>constant iterator</em>, which can be incremented
1127but doesn't allow modification of its referent, and <tt class="docutils literal"><span class="pre">int*</span>
1128<span class="pre">const</span></tt> is a <tt class="docutils literal"><span class="pre">const</span></tt> <em>mutable iterator</em>, which cannot be
1129modified but which allows modification of its referent.</p>
1130<p class="last">Confusing? We agree, but those are the standard terms. It
1131probably doesn't help much that a container's constant iterator
1132is called <tt class="docutils literal"><span class="pre">const_iterator</span></tt>.</p>
1133</div>
1134<p>Now, our <tt class="docutils literal"><span class="pre">node_iterator</span></tt> gives clients access to both <tt class="docutils literal"><span class="pre">node</span></tt>'s <tt class="docutils literal"><span class="pre">print(std::ostream&amp;)</span> <span class="pre">const</span></tt> member function, but also its
1135mutating <tt class="docutils literal"><span class="pre">double_me()</span></tt> member. If we wanted to build a
1136<em>constant</em> <tt class="docutils literal"><span class="pre">node_iterator</span></tt>, we'd only have to make three
1137changes:</p>
1138<pre class="literal-block">
1139class const_node_iterator
1140 : public boost::iterator_facade&lt;
1141 const_node_iterator
1142 , node_base <strong>const</strong>
1143 , boost::forward_traversal_tag
1144 &gt;
1145{
1146 public:
1147 const_node_iterator()
1148 : m_node(0) {}
1149
1150 explicit const_node_iterator(node_base* p)
1151 : m_node(p) {}
1152
1153 private:
1154 friend class boost::iterator_core_access;
1155
1156 void increment() { m_node = m_node-&gt;next(); }
1157
1158 bool equal(const_node_iterator const&amp; other) const
1159 {
1160 return this-&gt;m_node == other.m_node;
1161 }
1162
1163 node_base <strong>const</strong>&amp; dereference() const { return *m_node; }
1164
1165 node_base <strong>const</strong>* m_node;
1166};
1167</pre>
1168<div class="sidebar">
1169<p class="first sidebar-title"><tt class="docutils literal"><span class="pre">const</span></tt> and an iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt></p>
1170<p class="last">The C++ standard requires an iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt> <em>not</em> be
1171<tt class="docutils literal"><span class="pre">const</span></tt>-qualified, so <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> strips the
1172<tt class="docutils literal"><span class="pre">const</span></tt> from its <tt class="docutils literal"><span class="pre">Value</span></tt> parameter in order to produce the
1173iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt>. Making the <tt class="docutils literal"><span class="pre">Value</span></tt> argument
1174<tt class="docutils literal"><span class="pre">const</span></tt> provides a useful hint to <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> that the
1175iterator is a <em>constant iterator</em>, and the default <tt class="docutils literal"><span class="pre">Reference</span></tt>
1176argument will be correct for all lvalue iterators.</p>
1177</div>
1178<p>As a matter of fact, <tt class="docutils literal"><span class="pre">node_iterator</span></tt> and <tt class="docutils literal"><span class="pre">const_node_iterator</span></tt>
1179are so similar that it makes sense to factor the common code out
1180into a template as follows:</p>
1181<pre class="literal-block">
1182template &lt;class Value&gt;
1183class node_iter
1184 : public boost::iterator_facade&lt;
1185 node_iter&lt;Value&gt;
1186 , Value
1187 , boost::forward_traversal_tag
1188 &gt;
1189{
1190 public:
1191 node_iter()
1192 : m_node(0) {}
1193
1194 explicit node_iter(Value* p)
1195 : m_node(p) {}
1196
1197 private:
1198 friend class boost::iterator_core_access;
1199
1200 bool equal(node_iter&lt;Value&gt; const&amp; other) const
1201 {
1202 return this-&gt;m_node == other.m_node;
1203 }
1204
1205 void increment()
1206 { m_node = m_node-&gt;next(); }
1207
1208 Value&amp; dereference() const
1209 { return *m_node; }
1210
1211 Value* m_node;
1212};
1213typedef node_iter&lt;node_base&gt; node_iterator;
1214typedef node_iter&lt;node_base const&gt; node_const_iterator;
1215</pre>
1216</div>
1217<div class="section" id="interoperability">
1218<h2><a class="toc-backref" href="#id43">Interoperability</a></h2>
1219<p>Our <tt class="docutils literal"><span class="pre">const_node_iterator</span></tt> works perfectly well on its own, but
1220taken together with <tt class="docutils literal"><span class="pre">node_iterator</span></tt> it doesn't quite meet
1221expectations. For example, we'd like to be able to pass a
1222<tt class="docutils literal"><span class="pre">node_iterator</span></tt> where a <tt class="docutils literal"><span class="pre">node_const_iterator</span></tt> was expected,
1223just as you can with <tt class="docutils literal"><span class="pre">std::list&lt;int&gt;</span></tt>'s <tt class="docutils literal"><span class="pre">iterator</span></tt> and
1224<tt class="docutils literal"><span class="pre">const_iterator</span></tt>. Furthermore, given a <tt class="docutils literal"><span class="pre">node_iterator</span></tt> and a
1225<tt class="docutils literal"><span class="pre">node_const_iterator</span></tt> into the same list, we should be able to
1226compare them for equality.</p>
1227<p>This expected ability to use two different iterator types together
1228is known as <a class="reference external" href="new-iter-concepts.html#interoperable-iterators-lib-interoperable-iterators"><strong>interoperability</strong></a>. Achieving interoperability in
1229our case is as simple as templatizing the <tt class="docutils literal"><span class="pre">equal</span></tt> function and
1230adding a templatized converting constructor<a class="footnote-reference" href="#broken" id="id16"><sup>3</sup></a><a class="footnote-reference" href="#random" id="id17"><sup>4</sup></a>:</p>
1231<pre class="literal-block">
1232template &lt;class Value&gt;
1233class node_iter
1234 : public boost::iterator_facade&lt;
1235 node_iter&lt;Value&gt;
1236 , Value
1237 , boost::forward_traversal_tag
1238 &gt;
1239{
1240 public:
1241 node_iter()
1242 : m_node(0) {}
1243
1244 explicit node_iter(Value* p)
1245 : m_node(p) {}
1246
1247 template &lt;class OtherValue&gt;
1248 node_iter(node_iter&lt;OtherValue&gt; const&amp; other)
1249 : m_node(other.m_node) {}
1250
1251 private:
1252 friend class boost::iterator_core_access;
1253 template &lt;class&gt; friend class node_iter;
1254
1255 template &lt;class OtherValue&gt;
1256 bool equal(node_iter&lt;OtherValue&gt; const&amp; other) const
1257 {
1258 return this-&gt;m_node == other.m_node;
1259 }
1260
1261 void increment()
1262 { m_node = m_node-&gt;next(); }
1263
1264 Value&amp; dereference() const
1265 { return *m_node; }
1266
1267 Value* m_node;
1268};
1269typedef impl::node_iterator&lt;node_base&gt; node_iterator;
1270typedef impl::node_iterator&lt;node_base const&gt; node_const_iterator;
1271</pre>
1272<table class="docutils footnote" frame="void" id="broken" rules="none">
1273<colgroup><col class="label" /><col /></colgroup>
1274<tbody valign="top">
1275<tr><td class="label"><a class="fn-backref" href="#id16">[3]</a></td><td>If you're using an older compiler and it can't handle
1276this example, see the <a class="reference external" href="../example/node_iterator2.hpp">example code</a> for workarounds.</td></tr>
1277</tbody>
1278</table>
1279<table class="docutils footnote" frame="void" id="random" rules="none">
1280<colgroup><col class="label" /><col /></colgroup>
1281<tbody valign="top">
1282<tr><td class="label"><a class="fn-backref" href="#id17">[4]</a></td><td>If <tt class="docutils literal"><span class="pre">node_iterator</span></tt> had been a <a class="reference external" href="new-iter-concepts.html#random-access-traversal-iterators-lib-random-access-traversal-iterators">random access
1283traversal iterator</a>, we'd have had to templatize its
1284<tt class="docutils literal"><span class="pre">distance_to</span></tt> function as well.</td></tr>
1285</tbody>
1286</table>
1287<p>You can see an example program which exercises our interoperable
1288iterators <a class="reference external" href="../example/node_iterator2.cpp">here</a>.</p>
1289</div>
1290<div class="section" id="telling-the-truth">
1291<h2><a class="toc-backref" href="#id44">Telling the Truth</a></h2>
1292<p>Now <tt class="docutils literal"><span class="pre">node_iterator</span></tt> and <tt class="docutils literal"><span class="pre">node_const_iterator</span></tt> behave exactly as
1293you'd expect... almost. We can compare them and we can convert in
1294one direction: from <tt class="docutils literal"><span class="pre">node_iterator</span></tt> to <tt class="docutils literal"><span class="pre">node_const_iterator</span></tt>.
1295If we try to convert from <tt class="docutils literal"><span class="pre">node_const_iterator</span></tt> to
1296<tt class="docutils literal"><span class="pre">node_iterator</span></tt>, we'll get an error when the converting
1297constructor tries to initialize <tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s <tt class="docutils literal"><span class="pre">m_node</span></tt>, a
1298<tt class="docutils literal"><span class="pre">node*</span></tt> with a <tt class="docutils literal"><span class="pre">node</span> <span class="pre">const*</span></tt>. So what's the problem?</p>
1299<p>The problem is that
1300<tt class="docutils literal"><span class="pre">boost::</span></tt><a class="reference external" href="../../type_traits/index.html#relationships"><tt class="docutils literal"><span class="pre">is_convertible</span></tt></a><tt class="docutils literal"><span class="pre">&lt;node_const_iterator,node_iterator&gt;::value</span></tt>
1301will be <tt class="docutils literal"><span class="pre">true</span></tt>, but it should be <tt class="docutils literal"><span class="pre">false</span></tt>. <a class="reference external" href="../../type_traits/index.html#relationships"><tt class="docutils literal"><span class="pre">is_convertible</span></tt></a>
1302lies because it can only see as far as the <em>declaration</em> of
1303<tt class="docutils literal"><span class="pre">node_iter</span></tt>'s converting constructor, but can't look inside at
1304the <em>definition</em> to make sure it will compile. A perfect solution
1305would make <tt class="docutils literal"><span class="pre">node_iter</span></tt>'s converting constructor disappear when
1306the <tt class="docutils literal"><span class="pre">m_node</span></tt> conversion would fail.</p>
1307<p>In fact, that sort of magic is possible using
1308<a class="reference external" href="../../utility/enable_if.html"><tt class="docutils literal"><span class="pre">boost::enable_if</span></tt></a>. By rewriting the converting constructor as
1309follows, we can remove it from the overload set when it's not
1310appropriate:</p>
1311<pre class="literal-block">
1312#include &lt;boost/type_traits/is_convertible.hpp&gt;
1313#include &lt;boost/utility/enable_if.hpp&gt;
1314
1315 ...
1316
1317private:
1318 struct enabler {};
1319
1320public:
1321 template &lt;class OtherValue&gt;
1322 node_iter(
1323 node_iter&lt;OtherValue&gt; const&amp; other
1324 , typename boost::enable_if&lt;
1325 boost::is_convertible&lt;OtherValue*,Value*&gt;
1326 , enabler
1327 &gt;::type = enabler()
1328 )
1329 : m_node(other.m_node) {}
1330</pre>
1331</div>
1332<div class="section" id="wrap-up">
1333<h2><a class="toc-backref" href="#id45">Wrap Up</a></h2>
1334<p>This concludes our <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> tutorial, but before you
1335stop reading we urge you to take a look at <a class="reference external" href="iterator_adaptor.html"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt></a>.
1336There's another way to approach writing these iterators which might
1337even be superior.</p>
1338</div>
1339</div>
1340</div>
1341<div class="footer">
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