ceph/src/boost/libs/iterator/doc/iterator_facade.html

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   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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  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
  27 Lab</a>, University of Hanover <a class="last reference external" href="http://www.ive.uni-hannover.de">Institute for Transport
  28 Railway 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
  46 interface of standard iterators in terms of a few core functions
  47 and 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
  95 n1530_, 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
  98 interface that is necessary for all the functionality.  We have
  99 identified 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
 109 include 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
 113 Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id1">[Cop95]</a> so that the user can specify the behavior
 114 of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> in a derived class.  Former designs used
 115 policy objects to specify the behavior, but that approach was
 116 discarded for several reasons:</p>
 117 <blockquote>
 118 <ol class="arabic simple">
 119 <li>the creation and eventual copying of the policy object may create
 120 overhead that can be avoided with the current approach.</li>
 121 <li>The policy object approach does not allow for custom constructors
 122 on 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
 124 implementations.</li>
 125 <li>Without the use of CRTP, the standard requirement that an
 126 iterator's <tt class="docutils literal"><span class="pre">operator++</span></tt> returns the iterator type itself
 127 would mean that all iterators built with the library would
 128 have to be specializations of <tt class="docutils literal"><span class="pre">iterator_facade&lt;...&gt;</span></tt>, rather
 129 than something more descriptive like
 130 <tt class="docutils literal"><span class="pre">indirect_iterator&lt;T*&gt;</span></tt>.  Cumbersome type generator
 131 metafunctions would be needed to build new parameterized
 132 iterators, and a separate <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> layer would be
 133 impossible.</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
 139 specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and passes the derived
 140 iterator class as <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s first template parameter.
 141 The order of the other template parameters have been carefully
 142 chosen to take advantage of useful defaults.  For example, when
 143 defining a constant lvalue iterator, the user can pass a
 144 const-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
 148 the iterator's core behaviors.  The following table describes
 149 expressions which are required to be valid depending on the category
 150 of the derived iterator type.  These member functions are described
 151 briefly below and in more detail in the iterator facade
 152 requirements.</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
 187 is possible with proper inlining? -->
 188 <p>In addition to implementing the core interface functions, an iterator
 189 derived from <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> typically defines several
 190 constructors. To model any of the standard iterator concepts, the
 191 iterator must at least have a copy constructor. Also, if the iterator
 192 type <tt class="docutils literal"><span class="pre">X</span></tt> is meant to be automatically interoperate with another
 193 iterator type <tt class="docutils literal"><span class="pre">Y</span></tt> (as with constant and mutable iterators) then
 194 there 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>
 195 to <tt class="docutils literal"><span class="pre">X</span></tt> (but not both), typically implemented as a conversion
 196 constructor. Finally, if the iterator is to model Forward Traversal
 197 Iterator or a more-refined iterator concept, a default constructor is
 198 required.</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
 203 to access the core member functions in the derived class.  Making the
 204 core member functions public would expose an implementation detail to
 205 the user.  The design used here ensures that implementation details do
 206 not appear in the public interface of the derived iterator type.</p>
 207 <p>Preventing direct access to the core member functions has two
 208 advantages.  First, there is no possibility for the user to accidently
 209 use a member function of the iterator when a member of the value_type
 210 was intended.  This has been an issue with smart pointer
 211 implementations in the past.  The second and main advantage is that
 212 library implementers can freely exchange a hand-rolled iterator
 213 implementation for one based on <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> without fear of
 214 breaking code that was accessing the public core member functions
 215 directly.</p>
 216 <p>In a naive implementation, keeping the derived class' core member
 217 functions 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
 219 reduce the burden of limiting access, <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> is
 220 provided, a class that acts as a gateway to the core member functions
 221 in the derived iterator class.  The author of the derived class only
 222 needs to grant friendship to <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> to make his core
 223 member 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
 227 class containing only private static member functions which invoke the
 228 iterator core member functions. There is, however, no need to
 229 standardize 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
 231 open a safety loophole, as every core member function preserves the
 232 invariants 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
 237 challenges.  A random access iterator's <tt class="docutils literal"><span class="pre">operator[]</span></tt> is only
 238 required to return something convertible to its <tt class="docutils literal"><span class="pre">value_type</span></tt>.
 239 Requiring that it return an lvalue would rule out currently-legal
 240 random-access iterators which hold the referenced value in a data
 241 member (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
 242 into 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
 245 semantics 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
 246 adopted 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
 247 convertible 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
 248 equivalent 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
 249 implemented as a proxy containing a copy of <tt class="docutils literal"><span class="pre">p+n</span></tt>).  This approach
 250 will work properly for any random-access iterator regardless of the
 251 other details of its implementation.  A user who knows more about
 252 the 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
 254 class; it will hide the one supplied by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> from
 255 clients 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
 260 iterator) need not in fact be a reference, so long as it is
 261 convertible 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>
 262 is a class, however, it must still be possible to access members
 263 through <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>
 264 type is not in fact a reference must return a proxy containing a copy
 265 of 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
 268 are described in terms of a set of requirements, which must be
 269 satisfied 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
 274 Patterns, 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
 285 n1530_, the paper accepted by the LWG for TR1. -->
 286 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 287 <pre class="literal-block">
 288 template &lt;
 289     class Derived
 290   , class Value
 291   , class CategoryOrTraversal
 292   , class Reference  = Value&amp;
 293   , class Difference = ptrdiff_t
 294 &gt;
 295 class 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
 318 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 319           class Dr2, class V2, class TC2, class R2, class D2&gt;
 320 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type // exposition
 321 operator ==(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
 324 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 325           class Dr2, class V2, class TC2, class R2, class D2&gt;
 326 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 327 operator !=(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
 330 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 331           class Dr2, class V2, class TC2, class R2, class D2&gt;
 332 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 333 operator &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
 336 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 337           class Dr2, class V2, class TC2, class R2, class D2&gt;
 338 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 339 operator &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
 342 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 343           class Dr2, class V2, class TC2, class R2, class D2&gt;
 344 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 345 operator &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
 348 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 349           class Dr2, class V2, class TC2, class R2, class D2&gt;
 350 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 351 operator &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
 355 template &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> */
 358 operator-(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
 362 template &lt;class Dr, class V, class TC, class R, class D&gt;
 363 Derived operator+ (iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;,
 364                    typename Derived::difference_type n);
 365
 366 template &lt;class Dr, class V, class TC, class R, class D&gt;
 367 Derived 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
 415 the five original category tags when convertibility to one of the
 416 traversal 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
 421 purposes.  The member operators should only be in an overload set
 422 provided 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,
 423 meaning 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
 425 out of the overload set when the types are not interoperable.
 426 The operators should behave <em>as-if</em> <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt>
 427 were defined to be:</p>
 428 <pre class="literal-block">
 429 template &lt;bool, typename&gt; enable_if_interoperable_impl
 430 {};
 431
 432 template &lt;typename T&gt; enable_if_interoperable_impl&lt;true,T&gt;
 433 { typedef T type; };
 434
 435 template&lt;typename Dr1, typename Dr2, typename T&gt;
 436 struct 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
 447 iterator concept(s) it will model.  The operations in the first
 448 column 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
 452 object 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
 454 object 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>
 455 is a constant object of a random access traversal iterator type
 456 interoperable 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
 471 Concept(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
 479 Iterator</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>
 484 refer to the same
 485 position.</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
 497 Iterator</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
 503 Iterator</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
 511 Iterator</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
 520 the 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
 522 through 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
 538 of 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
 555 objects <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">
 569 static_cast&lt;Derived*&gt;(this)-&gt;increment();
 570 return *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">
 582 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 583 ++*this;
 584 return 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">
 596 static_cast&lt;Derived*&gt;(this)-&gt;decrement();
 597 return *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">
 609 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 610 --*this;
 611 return 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">
 623 static_cast&lt;Derived*&gt;(this)-&gt;advance(n);
 624 return *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">
 636 static_cast&lt;Derived*&gt;(this)-&gt;advance(-n);
 637 return *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">
 649 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 650 return tmp -= n;
 651 </pre>
 652 </td>
 653 </tr>
 654 </tbody>
 655 </table>
 656 <pre class="literal-block">
 657 template &lt;class Dr, class V, class TC, class R, class D&gt;
 658 Derived operator+ (iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;,
 659                    typename Derived::difference_type n);
 660
 661 template &lt;class Dr, class V, class TC, class R, class D&gt;
 662 Derived 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">
 670 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 671 return tmp += n;
 672 </pre>
 673 </td>
 674 </tr>
 675 </tbody>
 676 </table>
 677 <pre class="literal-block">
 678 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 679           class Dr2, class V2, class TC2, class R2, class D2&gt;
 680 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 681 operator ==(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">
 702 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 703           class Dr2, class V2, class TC2, class R2, class D2&gt;
 704 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 705 operator !=(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">
 726 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 727           class Dr2, class V2, class TC2, class R2, class D2&gt;
 728 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 729 operator &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">
 750 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 751           class Dr2, class V2, class TC2, class R2, class D2&gt;
 752 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 753 operator &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">
 774 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 775           class Dr2, class V2, class TC2, class R2, class D2&gt;
 776 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 777 operator &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">
 798 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 799           class Dr2, class V2, class TC2, class R2, class D2&gt;
 800 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 801 operator &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">
 822 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 823           class Dr2, class V2, class TC2, class R2, class D2&gt;
 824 typename enable_if_interoperable&lt;Dr1,Dr2,difference&gt;::type
 825 operator -(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
 867 iterators using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, based around the simple
 868 example of a linked list of polymorphic objects.  This example was
 869 inspired 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>
 870 mailing 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
 877 struct 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
 906 specializations of the following template:</p>
 907 <pre class="literal-block">
 908 template &lt;class T&gt;
 909 struct 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">
 924 inline 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
 931 lists.</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
 941 class 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
 950 what 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>
 962 objects, 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
 968 forward 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
 969 traversal iterator</a>.  Our iterator should be able to make multiple
 970 passes 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
 972 must 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
 978 tags, so we could have passed <tt class="docutils literal"><span class="pre">std::forward_iterator_tag</span></tt> here;
 979 either way, the resulting iterator's <tt class="docutils literal"><span class="pre">iterator_category</span></tt> will
 980 end 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
 988 same as <tt class="docutils literal"><span class="pre">std::iterator_traits&lt;node_iterator&gt;::reference</span></tt>.  The
 989 library'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>
 991 type, 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
 996 two <tt class="docutils literal"><span class="pre">node_iterator</span></tt>s will be measured and will also be the
 997 same as <tt class="docutils literal"><span class="pre">std::iterator_traits&lt;node_iterator&gt;::difference_type</span></tt>.
 998 The 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
 999 appropriate type for measuring the distance between any two
1000 addresses in memory, and one that works for almost any iterator,
1001 so 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
1003 like:</p>
1004 <pre class="literal-block">
1005 # include &quot;node.hpp&quot;
1006 # include &lt;boost/iterator/iterator_facade.hpp&gt;
1007
1008 class 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.
1023 This representation will take the form of data members, so we'll
1024 also 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
1026 a pointer to a <tt class="docutils literal"><span class="pre">node_base</span></tt>.  We'll need a constructor to build an
1027 iterator from a <tt class="docutils literal"><span class="pre">node_base*</span></tt>, and a default constructor to
1028 satisfy 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>.
1029 Our <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
1034 class 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
1059 requirements on a default-constructed iterator, so if we were
1060 really concerned with efficiency, we could've written the
1061 default 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
1068 the 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
1070 because <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
1072 iterator</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
1075 to become part of <tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s public interface, so we can
1076 make 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
1083 class 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
1113 iterator!  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
1121 the object it references (its &quot;referent&quot;) can be modified.  A
1122 <strong>constant iterator</strong> is one which doesn't allow modification of
1123 its referent.</p>
1124 <p>The words <em>constant</em> and <em>mutable</em> don't refer to the ability to
1125 modify the iterator itself.  For example, an <tt class="docutils literal"><span class="pre">int</span> <span class="pre">const*</span></tt> is a
1126 non-<tt class="docutils literal"><span class="pre">const</span></tt> <em>constant iterator</em>, which can be incremented
1127 but 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
1129 modified but which allows modification of its referent.</p>
1130 <p class="last">Confusing?  We agree, but those are the standard terms.  It
1131 probably doesn't help much that a container's constant iterator
1132 is 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
1135 mutating <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
1137 changes:</p>
1138 <pre class="literal-block">
1139 class 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
1173 iterator'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
1175 iterator is a <em>constant iterator</em>, and the default <tt class="docutils literal"><span class="pre">Reference</span></tt>
1176 argument 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>
1179 are so similar that it makes sense to factor the common code out
1180 into a template as follows:</p>
1181 <pre class="literal-block">
1182 template &lt;class Value&gt;
1183 class 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 };
1213 typedef node_iter&lt;node_base&gt; node_iterator;
1214 typedef 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
1220 taken together with <tt class="docutils literal"><span class="pre">node_iterator</span></tt> it doesn't quite meet
1221 expectations.  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,
1223 just 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
1226 compare them for equality.</p>
1227 <p>This expected ability to use two different iterator types together
1228 is known as <a class="reference external" href="new-iter-concepts.html#interoperable-iterators-lib-interoperable-iterators"><strong>interoperability</strong></a>.  Achieving interoperability in
1229 our case is as simple as templatizing the <tt class="docutils literal"><span class="pre">equal</span></tt> function and
1230 adding 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">
1232 template &lt;class Value&gt;
1233 class 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 };
1269 typedef impl::node_iterator&lt;node_base&gt; node_iterator;
1270 typedef 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
1276 this 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
1283 traversal 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
1288 iterators <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
1293 you'd expect... almost.  We can compare them and we can convert in
1294 one 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>.
1295 If 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
1297 constructor 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>
1301 will 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>
1302 lies 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
1304 the <em>definition</em> to make sure it will compile.  A perfect solution
1305 would make <tt class="docutils literal"><span class="pre">node_iter</span></tt>'s converting constructor disappear when
1306 the <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
1309 follows, we can remove it from the overload set when it's not
1310 appropriate:</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
1317 private:
1318   struct enabler {};
1319
1320 public:
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
1335 stop 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>.
1336 There's another way to approach writing these iterators which might
1337 even be superior.</p>
1338 </div>
1339 </div>
1340 </div>
1341 <div class="footer">
1342 <hr class="footer" />
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