ceph/src/boost/libs/iterator/doc/facade-and-adaptor.html

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   8 <meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
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  10 <meta name="date" content="2006-09-11" />
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  14 <div class="document" id="iterator-facade-and-adaptor">
  15 <h1 class="title">Iterator Facade and Adaptor</h1>
  16 <table class="docinfo" frame="void" rules="none">
  17 <col class="docinfo-name" />
  18 <col class="docinfo-content" />
  19 <tbody valign="top">
  20 <tr><th class="docinfo-name">Author:</th>
  21 <td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr>
  22 <tr><th class="docinfo-name">Contact:</th>
  23 <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;styleadvisor.com">witt&#64;styleadvisor.com</a></td></tr>
  24 <tr><th class="docinfo-name">Organization:</th>
  25 <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
  26 Lab</a>, <a class="last reference external" href="http://www.styleadvisor.com">Zephyr Associates, Inc.</a></td></tr>
  27 <tr><th class="docinfo-name">Date:</th>
  28 <td>2006-09-11</td></tr>
  29 <tr class="field"><th class="docinfo-name">Number:</th><td class="field-body">This is a revised version of <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1530.html">N1530</a>=03-0113, which was
  30 accepted for Technical Report 1 by the C++ standard
  31 committee's library working group.</td>
  32 </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 <!-- Version 1.9 of this ReStructuredText document corresponds to
  39 n1530_, the paper accepted by the LWG. -->
  40 <table class="docutils field-list" frame="void" rules="none">
  41 <col class="field-name" />
  42 <col class="field-body" />
  43 <tbody valign="top">
  44 <tr class="field"><th class="field-name">copyright:</th><td class="field-body">Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td>
  45 </tr>
  46 </tbody>
  47 </table>
  48 <table class="docutils field-list" frame="void" rules="none">
  49 <col class="field-name" />
  50 <col class="field-body" />
  51 <tbody valign="top">
  52 <tr class="field"><th class="field-name">abstract:</th><td class="field-body">We propose a set of class templates that help programmers
  53 build standard-conforming iterators, both from scratch and
  54 by adapting other iterators.</td>
  55 </tr>
  56 </tbody>
  57 </table>
  58 <div class="contents topic" id="table-of-contents">
  59 <p class="topic-title first">Table of Contents</p>
  60 <ul class="simple">
  61 <li><a class="reference internal" href="#motivation" id="id15">Motivation</a></li>
  62 <li><a class="reference internal" href="#impact-on-the-standard" id="id16">Impact on the Standard</a></li>
  63 <li><a class="reference internal" href="#design" id="id17">Design</a><ul>
  64 <li><a class="reference internal" href="#iterator-concepts" id="id18">Iterator Concepts</a></li>
  65 <li><a class="reference internal" href="#interoperability" id="id19">Interoperability</a></li>
  66 <li><a class="reference internal" href="#iterator-facade" id="id20">Iterator Facade</a><ul>
  67 <li><a class="reference internal" href="#usage" id="id21">Usage</a></li>
  68 <li><a class="reference internal" href="#iterator-core-access" id="id22">Iterator Core Access</a></li>
  69 <li><a class="reference internal" href="#operator" id="id23"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></li>
  70 <li><a class="reference internal" href="#id6" id="id24"><tt class="docutils literal"><span class="pre">operator-&gt;</span></tt></a></li>
  71 </ul>
  72 </li>
  73 <li><a class="reference internal" href="#iterator-adaptor" id="id25">Iterator Adaptor</a></li>
  74 <li><a class="reference internal" href="#specialized-adaptors" id="id26">Specialized Adaptors</a></li>
  75 </ul>
  76 </li>
  77 <li><a class="reference internal" href="#proposed-text" id="id27">Proposed Text</a><ul>
  78 <li><a class="reference internal" href="#header-iterator-helper-synopsis-lib-iterator-helper-synopsis" id="id28">Header <tt class="docutils literal"><span class="pre">&lt;iterator_helper&gt;</span></tt> synopsis    [lib.iterator.helper.synopsis]</a></li>
  79 <li><a class="reference internal" href="#iterator-facade-lib-iterator-facade" id="id29">Iterator facade [lib.iterator.facade]</a><ul>
  80 <li><a class="reference internal" href="#class-template-iterator-facade" id="id30">Class template <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a></li>
  81 <li><a class="reference internal" href="#iterator-facade-requirements" id="id31"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Requirements</a></li>
  82 <li><a class="reference internal" href="#iterator-facade-operations" id="id32"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> operations</a></li>
  83 </ul>
  84 </li>
  85 <li><a class="reference internal" href="#iterator-adaptor-lib-iterator-adaptor" id="id33">Iterator adaptor [lib.iterator.adaptor]</a><ul>
  86 <li><a class="reference internal" href="#class-template-iterator-adaptor" id="id34">Class template <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt></a></li>
  87 <li><a class="reference internal" href="#iterator-adaptor-requirements" id="id35"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></li>
  88 <li><a class="reference internal" href="#iterator-adaptor-base-class-parameters" id="id36"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></li>
  89 <li><a class="reference internal" href="#iterator-adaptor-public-operations" id="id37"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></li>
  90 <li><a class="reference internal" href="#iterator-adaptor-protected-member-functions" id="id38"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></li>
  91 <li><a class="reference internal" href="#iterator-adaptor-private-member-functions" id="id39"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></li>
  92 </ul>
  93 </li>
  94 <li><a class="reference internal" href="#specialized-adaptors-lib-iterator-special-adaptors" id="id40">Specialized adaptors [lib.iterator.special.adaptors]</a><ul>
  95 <li><a class="reference internal" href="#indirect-iterator" id="id41">Indirect iterator</a><ul>
  96 <li><a class="reference internal" href="#class-template-pointee" id="id42">Class template <tt class="docutils literal"><span class="pre">pointee</span></tt></a></li>
  97 <li><a class="reference internal" href="#class-template-indirect-reference" id="id43">Class template <tt class="docutils literal"><span class="pre">indirect_reference</span></tt></a></li>
  98 <li><a class="reference internal" href="#class-template-indirect-iterator" id="id44">Class template <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt></a></li>
  99 <li><a class="reference internal" href="#indirect-iterator-requirements" id="id45"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> requirements</a></li>
 100 <li><a class="reference internal" href="#indirect-iterator-models" id="id46"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models</a></li>
 101 <li><a class="reference internal" href="#indirect-iterator-operations" id="id47"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> operations</a></li>
 102 </ul>
 103 </li>
 104 <li><a class="reference internal" href="#reverse-iterator" id="id48">Reverse iterator</a><ul>
 105 <li><a class="reference internal" href="#class-template-reverse-iterator" id="id49">Class template <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt></a></li>
 106 <li><a class="reference internal" href="#reverse-iterator-requirements" id="id50"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> requirements</a></li>
 107 <li><a class="reference internal" href="#reverse-iterator-models" id="id51"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> models</a></li>
 108 <li><a class="reference internal" href="#reverse-iterator-operations" id="id52"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> operations</a></li>
 109 </ul>
 110 </li>
 111 <li><a class="reference internal" href="#transform-iterator" id="id53">Transform iterator</a><ul>
 112 <li><a class="reference internal" href="#class-template-transform-iterator" id="id54">Class template <tt class="docutils literal"><span class="pre">transform_iterator</span></tt></a></li>
 113 <li><a class="reference internal" href="#transform-iterator-requirements" id="id55"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> requirements</a></li>
 114 <li><a class="reference internal" href="#transform-iterator-models" id="id56"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models</a></li>
 115 <li><a class="reference internal" href="#transform-iterator-operations" id="id57"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> operations</a></li>
 116 </ul>
 117 </li>
 118 <li><a class="reference internal" href="#filter-iterator" id="id58">Filter iterator</a><ul>
 119 <li><a class="reference internal" href="#class-template-filter-iterator" id="id59">Class template <tt class="docutils literal"><span class="pre">filter_iterator</span></tt></a></li>
 120 <li><a class="reference internal" href="#filter-iterator-requirements" id="id60"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt> requirements</a></li>
 121 <li><a class="reference internal" href="#filter-iterator-models" id="id61"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models</a></li>
 122 <li><a class="reference internal" href="#filter-iterator-operations" id="id62"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt> operations</a></li>
 123 </ul>
 124 </li>
 125 <li><a class="reference internal" href="#counting-iterator" id="id63">Counting iterator</a><ul>
 126 <li><a class="reference internal" href="#class-template-counting-iterator" id="id64">Class template <tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a></li>
 127 <li><a class="reference internal" href="#counting-iterator-requirements" id="id65"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> requirements</a></li>
 128 <li><a class="reference internal" href="#counting-iterator-models" id="id66"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models</a></li>
 129 <li><a class="reference internal" href="#counting-iterator-operations" id="id67"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> operations</a></li>
 130 </ul>
 131 </li>
 132 <li><a class="reference internal" href="#function-output-iterator" id="id68">Function output iterator</a><ul>
 133 <li><a class="reference internal" href="#class-template-function-output-iterator" id="id69">Class template <tt class="docutils literal"><span class="pre">function_output_iterator</span></tt></a></li>
 134 <li><a class="reference internal" href="#header" id="id70">Header</a></li>
 135 <li><a class="reference internal" href="#function-output-iterator-requirements" id="id71"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> requirements</a></li>
 136 <li><a class="reference internal" href="#function-output-iterator-models" id="id72"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> models</a></li>
 137 <li><a class="reference internal" href="#function-output-iterator-operations" id="id73"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> operations</a></li>
 138 </ul>
 139 </li>
 140 </ul>
 141 </li>
 142 </ul>
 143 </li>
 144 </ul>
 145 </div>
 146 <div class="section" id="motivation">
 147 <h1><a class="toc-backref" href="#id15">Motivation</a></h1>
 148 <p>Iterators play an important role in modern C++ programming. The
 149 iterator is the central abstraction of the algorithms of the Standard
 150 Library, allowing algorithms to be re-used in in a wide variety of
 151 contexts.  The C++ Standard Library contains a wide variety of useful
 152 iterators. Every one of the standard containers comes with constant
 153 and mutable iterators<a class="footnote-reference" href="#mutable" id="id1"><sup>2</sup></a>, and also reverse versions of those
 154 same iterators which traverse the container in the opposite direction.
 155 The Standard also supplies <tt class="docutils literal"><span class="pre">istream_iterator</span></tt> and
 156 <tt class="docutils literal"><span class="pre">ostream_iterator</span></tt> for reading from and writing to streams,
 157 <tt class="docutils literal"><span class="pre">insert_iterator</span></tt>, <tt class="docutils literal"><span class="pre">front_insert_iterator</span></tt> and
 158 <tt class="docutils literal"><span class="pre">back_insert_iterator</span></tt> for inserting elements into containers, and
 159 <tt class="docutils literal"><span class="pre">raw_storage_iterator</span></tt> for initializing raw memory [7].</p>
 160 <p>Despite the many iterators supplied by the Standard Library, obvious
 161 and useful iterators are missing, and creating new iterator types is
 162 still a common task for C++ programmers.  The literature documents
 163 several of these, for example line_iterator [3] and Constant_iterator
 164 [9].  The iterator abstraction is so powerful that we expect
 165 programmers will always need to invent new iterator types.</p>
 166 <p>Although it is easy to create iterators that <em>almost</em> conform to the
 167 standard, the iterator requirements contain subtleties which can make
 168 creating an iterator which <em>actually</em> conforms quite difficult.
 169 Further, the iterator interface is rich, containing many operators
 170 that are technically redundant and tedious to implement.  To automate
 171 the repetitive work of constructing iterators, we propose
 172 <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, an iterator base class template which provides
 173 the rich interface of standard iterators and delegates its
 174 implementation to member functions of the derived class.  In addition
 175 to reducing the amount of code necessary to create an iterator, the
 176 <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> also provides compile-time error detection.
 177 Iterator implementation mistakes that often go unnoticed are turned
 178 into compile-time errors because the derived class implementation must
 179 match the expectations of the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>.</p>
 180 <p>A common pattern of iterator construction is the adaptation of one
 181 iterator to form a new one.  The functionality of an iterator is
 182 composed of four orthogonal aspects: traversal, indirection, equality
 183 comparison and distance measurement.  Adapting an old iterator to
 184 create a new one often saves work because one can reuse one aspect of
 185 functionality while redefining the other.  For example, the Standard
 186 provides <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt>, which adapts any Bidirectional Iterator
 187 by inverting its direction of traversal.  As with plain iterators,
 188 iterator adaptors defined outside the Standard have become commonplace
 189 in the literature:</p>
 190 <ul class="simple">
 191 <li>Checked iter[13] adds bounds-checking to an existing iterator.</li>
 192 <li>The iterators of the View Template Library[14], which adapts
 193 containers, are themselves adaptors over the underlying iterators.</li>
 194 <li>Smart iterators [5] adapt an iterator's dereferencing behavior by
 195 applying a function object to the object being referenced and
 196 returning the result.</li>
 197 <li>Custom iterators [4], in which a variety of adaptor types are enumerated.</li>
 198 <li>Compound iterators [1], which access a slice out of a container of containers.</li>
 199 <li>Several iterator adaptors from the MTL [12].  The MTL contains a
 200 strided iterator, where each call to <tt class="docutils literal"><span class="pre">operator++()</span></tt> moves the
 201 iterator ahead by some constant factor, and a scaled iterator, which
 202 multiplies the dereferenced value by some constant.</li>
 203 </ul>
 204 <table class="docutils footnote" frame="void" id="concept" rules="none">
 205 <colgroup><col class="label" /><col /></colgroup>
 206 <tbody valign="top">
 207 <tr><td class="label">[1]</td><td>We use the term concept to mean a set of requirements
 208 that a type must satisfy to be used with a particular template
 209 parameter.</td></tr>
 210 </tbody>
 211 </table>
 212 <table class="docutils footnote" frame="void" id="mutable" rules="none">
 213 <colgroup><col class="label" /><col /></colgroup>
 214 <tbody valign="top">
 215 <tr><td class="label"><a class="fn-backref" href="#id1">[2]</a></td><td>The term mutable iterator refers to iterators over objects that
 216 can be changed by assigning to the dereferenced iterator, while
 217 constant iterator refers to iterators over objects that cannot be
 218 modified.</td></tr>
 219 </tbody>
 220 </table>
 221 <p>To fulfill the need for constructing adaptors, we propose the
 222 <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template.  Instantiations of
 223 <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> serve as a base classes for new iterators,
 224 providing the default behavior of forwarding all operations to the
 225 underlying iterator.  The user can selectively replace these features
 226 in the derived iterator class.  This proposal also includes a number
 227 of more specialized adaptors, such as the <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> that
 228 applies some user-specified function during the dereference of the
 229 iterator.</p>
 230 </div>
 231 <div class="section" id="impact-on-the-standard">
 232 <h1><a class="toc-backref" href="#id16">Impact on the Standard</a></h1>
 233 <p>This proposal is purely an addition to the C++ standard library.
 234 However, note that this proposal relies on the proposal for New
 235 Iterator Concepts.</p>
 236 </div>
 237 <div class="section" id="design">
 238 <h1><a class="toc-backref" href="#id17">Design</a></h1>
 239 <div class="section" id="iterator-concepts">
 240 <h2><a class="toc-backref" href="#id18">Iterator Concepts</a></h2>
 241 <p>This proposal is formulated in terms of the new <tt class="docutils literal"><span class="pre">iterator</span> <span class="pre">concepts</span></tt>
 242 as proposed in <a class="reference external" href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2003/n1550.htm">n1550</a>, since user-defined and especially adapted
 243 iterators suffer from the well known categorization problems that are
 244 inherent to the current iterator categories.</p>
 245 <p>This proposal does not strictly depend on proposal <a class="reference external" href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2003/n1550.htm">n1550</a>, as there
 246 is a direct mapping between new and old categories. This proposal
 247 could be reformulated using this mapping if <a class="reference external" href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2003/n1550.htm">n1550</a> was not accepted.</p>
 248 </div>
 249 <div class="section" id="interoperability">
 250 <h2><a class="toc-backref" href="#id19">Interoperability</a></h2>
 251 <p>The question of iterator interoperability is poorly addressed in the
 252 current standard.  There are currently two defect reports that are
 253 concerned with interoperability issues.</p>
 254 <p>Issue <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#179">179</a> concerns the fact that mutable container iterator types
 255 are only required to be convertible to the corresponding constant
 256 iterator types, but objects of these types are not required to
 257 interoperate in comparison or subtraction expressions.  This situation
 258 is tedious in practice and out of line with the way built in types
 259 work.  This proposal implements the proposed resolution to issue
 260 <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#179">179</a>, as most standard library implementations do nowadays. In other
 261 words, if an iterator type A has an implicit or user defined
 262 conversion to an iterator type B, the iterator types are interoperable
 263 and the usual set of operators are available.</p>
 264 <p>Issue <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#280">280</a> concerns the current lack of interoperability between
 265 reverse iterator types. The proposed new reverse_iterator template
 266 fixes the issues raised in 280. It provides the desired
 267 interoperability without introducing unwanted overloads.</p>
 268 </div>
 269 <div class="section" id="iterator-facade">
 270 <h2><a class="toc-backref" href="#id20">Iterator Facade</a></h2>
 271 <!-- Distributed under the Boost -->
 272 <!-- Software License, Version 1.0. (See accompanying -->
 273 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 274 <!-- Version 1.1 of this ReStructuredText document corresponds to
 275 n1530_, the paper accepted by the LWG for TR1. -->
 276 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 277 <p>While the iterator interface is rich, there is a core subset of the
 278 interface that is necessary for all the functionality.  We have
 279 identified the following core behaviors for iterators:</p>
 280 <ul class="simple">
 281 <li>dereferencing</li>
 282 <li>incrementing</li>
 283 <li>decrementing</li>
 284 <li>equality comparison</li>
 285 <li>random-access motion</li>
 286 <li>distance measurement</li>
 287 </ul>
 288 <p>In addition to the behaviors listed above, the core interface elements
 289 include the associated types exposed through iterator traits:
 290 <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
 291 <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p>
 292 <p>Iterator facade uses the Curiously Recurring Template
 293 Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id4">[Cop95]</a> so that the user can specify the behavior
 294 of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> in a derived class.  Former designs used
 295 policy objects to specify the behavior, but that approach was
 296 discarded for several reasons:</p>
 297 <blockquote>
 298 <ol class="arabic simple">
 299 <li>the creation and eventual copying of the policy object may create
 300 overhead that can be avoided with the current approach.</li>
 301 <li>The policy object approach does not allow for custom constructors
 302 on the created iterator types, an essential feature if
 303 <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> should be used in other library
 304 implementations.</li>
 305 <li>Without the use of CRTP, the standard requirement that an
 306 iterator's <tt class="docutils literal"><span class="pre">operator++</span></tt> returns the iterator type itself
 307 would mean that all iterators built with the library would
 308 have to be specializations of <tt class="docutils literal"><span class="pre">iterator_facade&lt;...&gt;</span></tt>, rather
 309 than something more descriptive like
 310 <tt class="docutils literal"><span class="pre">indirect_iterator&lt;T*&gt;</span></tt>.  Cumbersome type generator
 311 metafunctions would be needed to build new parameterized
 312 iterators, and a separate <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> layer would be
 313 impossible.</li>
 314 </ol>
 315 </blockquote>
 316 <div class="section" id="usage">
 317 <h3><a class="toc-backref" href="#id21">Usage</a></h3>
 318 <p>The user of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> derives his iterator class from a
 319 specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and passes the derived
 320 iterator class as <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s first template parameter.
 321 The order of the other template parameters have been carefully
 322 chosen to take advantage of useful defaults.  For example, when
 323 defining a constant lvalue iterator, the user can pass a
 324 const-qualified version of the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt> as
 325 <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
 326 <tt class="docutils literal"><span class="pre">Reference</span></tt> parameter which follows.</p>
 327 <p>The derived iterator class must define member functions implementing
 328 the iterator's core behaviors.  The following table describes
 329 expressions which are required to be valid depending on the category
 330 of the derived iterator type.  These member functions are described
 331 briefly below and in more detail in the iterator facade
 332 requirements.</p>
 333 <blockquote>
 334 <table border="1" class="docutils">
 335 <colgroup>
 336 <col width="44%" />
 337 <col width="56%" />
 338 </colgroup>
 339 <thead valign="bottom">
 340 <tr><th class="head">Expression</th>
 341 <th class="head">Effects</th>
 342 </tr>
 343 </thead>
 344 <tbody valign="top">
 345 <tr><td><tt class="docutils literal"><span class="pre">i.dereference()</span></tt></td>
 346 <td>Access the value referred to</td>
 347 </tr>
 348 <tr><td><tt class="docutils literal"><span class="pre">i.equal(j)</span></tt></td>
 349 <td>Compare for equality with <tt class="docutils literal"><span class="pre">j</span></tt></td>
 350 </tr>
 351 <tr><td><tt class="docutils literal"><span class="pre">i.increment()</span></tt></td>
 352 <td>Advance by one position</td>
 353 </tr>
 354 <tr><td><tt class="docutils literal"><span class="pre">i.decrement()</span></tt></td>
 355 <td>Retreat by one position</td>
 356 </tr>
 357 <tr><td><tt class="docutils literal"><span class="pre">i.advance(n)</span></tt></td>
 358 <td>Advance by <tt class="docutils literal"><span class="pre">n</span></tt> positions</td>
 359 </tr>
 360 <tr><td><tt class="docutils literal"><span class="pre">i.distance_to(j)</span></tt></td>
 361 <td>Measure the distance to <tt class="docutils literal"><span class="pre">j</span></tt></td>
 362 </tr>
 363 </tbody>
 364 </table>
 365 </blockquote>
 366 <!-- Should we add a comment that a zero overhead implementation of iterator_facade
 367 is possible with proper inlining? -->
 368 <p>In addition to implementing the core interface functions, an iterator
 369 derived from <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> typically defines several
 370 constructors. To model any of the standard iterator concepts, the
 371 iterator must at least have a copy constructor. Also, if the iterator
 372 type <tt class="docutils literal"><span class="pre">X</span></tt> is meant to be automatically interoperate with another
 373 iterator type <tt class="docutils literal"><span class="pre">Y</span></tt> (as with constant and mutable iterators) then
 374 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>
 375 to <tt class="docutils literal"><span class="pre">X</span></tt> (but not both), typically implemented as a conversion
 376 constructor. Finally, if the iterator is to model Forward Traversal
 377 Iterator or a more-refined iterator concept, a default constructor is
 378 required.</p>
 379 </div>
 380 <div class="section" id="iterator-core-access">
 381 <h3><a class="toc-backref" href="#id22">Iterator Core Access</a></h3>
 382 <p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and the operator implementations need to be able
 383 to access the core member functions in the derived class.  Making the
 384 core member functions public would expose an implementation detail to
 385 the user.  The design used here ensures that implementation details do
 386 not appear in the public interface of the derived iterator type.</p>
 387 <p>Preventing direct access to the core member functions has two
 388 advantages.  First, there is no possibility for the user to accidently
 389 use a member function of the iterator when a member of the value_type
 390 was intended.  This has been an issue with smart pointer
 391 implementations in the past.  The second and main advantage is that
 392 library implementers can freely exchange a hand-rolled iterator
 393 implementation for one based on <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> without fear of
 394 breaking code that was accessing the public core member functions
 395 directly.</p>
 396 <p>In a naive implementation, keeping the derived class' core member
 397 functions private would require it to grant friendship to
 398 <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and each of the seven operators.  In order to
 399 reduce the burden of limiting access, <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> is
 400 provided, a class that acts as a gateway to the core member functions
 401 in the derived iterator class.  The author of the derived class only
 402 needs to grant friendship to <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> to make his core
 403 member functions available to the library.</p>
 404 <!-- This is no long uptodate -thw -->
 405 <!-- Yes it is; I made sure of it! -DWA -->
 406 <p><tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> will be typically implemented as an empty
 407 class containing only private static member functions which invoke the
 408 iterator core member functions. There is, however, no need to
 409 standardize the gateway protocol.  Note that even if
 410 <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> used public member functions it would not
 411 open a safety loophole, as every core member function preserves the
 412 invariants of the iterator.</p>
 413 </div>
 414 <div class="section" id="operator">
 415 <h3><a class="toc-backref" href="#id23"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></h3>
 416 <p>The indexing operator for a generalized iterator presents special
 417 challenges.  A random access iterator's <tt class="docutils literal"><span class="pre">operator[]</span></tt> is only
 418 required to return something convertible to its <tt class="docutils literal"><span class="pre">value_type</span></tt>.
 419 Requiring that it return an lvalue would rule out currently-legal
 420 random-access iterators which hold the referenced value in a data
 421 member (e.g. <a class="reference internal" href="#counting"><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
 422 into the temporary iterator <tt class="docutils literal"><span class="pre">p+n</span></tt>, which is destroyed when
 423 <tt class="docutils literal"><span class="pre">operator[]</span></tt> returns.</p>
 424 <p>Writable iterators built with <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implement the
 425 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
 426 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
 427 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
 428 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
 429 implemented as a proxy containing a copy of <tt class="docutils literal"><span class="pre">p+n</span></tt>).  This approach
 430 will work properly for any random-access iterator regardless of the
 431 other details of its implementation.  A user who knows more about
 432 the implementation of her iterator is free to implement an
 433 <tt class="docutils literal"><span class="pre">operator[]</span></tt> that returns an lvalue in the derived iterator
 434 class; it will hide the one supplied by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> from
 435 clients of her iterator.</p>
 436 </div>
 437 <div class="section" id="id6">
 438 <span id="operator-arrow"></span><h3><a class="toc-backref" href="#id24"><tt class="docutils literal"><span class="pre">operator-&gt;</span></tt></a></h3>
 439 <p>The <tt class="docutils literal"><span class="pre">reference</span></tt> type of a readable iterator (and today's input
 440 iterator) need not in fact be a reference, so long as it is
 441 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>
 442 is a class, however, it must still be possible to access members
 443 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>
 444 type is not in fact a reference must return a proxy containing a copy
 445 of the referenced value from its <tt class="docutils literal"><span class="pre">operator-&gt;</span></tt>.</p>
 446 <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
 447 <tt class="docutils literal"><span class="pre">operator[]</span></tt> are not explicitly specified. Instead, those types
 448 are described in terms of a set of requirements, which must be
 449 satisfied by the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implementation.</p>
 450 <table class="docutils citation" frame="void" id="cop95" rules="none">
 451 <colgroup><col class="label" /><col /></colgroup>
 452 <tbody valign="top">
 453 <tr><td class="label"><a class="fn-backref" href="#id4">[Cop95]</a></td><td>[Coplien, 1995] Coplien, J., Curiously Recurring Template
 454 Patterns, C++ Report, February 1995, pp. 24-27.</td></tr>
 455 </tbody>
 456 </table>
 457 </div>
 458 </div>
 459 <div class="section" id="iterator-adaptor">
 460 <h2><a class="toc-backref" href="#id25">Iterator Adaptor</a></h2>
 461 <!-- Distributed under the Boost -->
 462 <!-- Software License, Version 1.0. (See accompanying -->
 463 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 464 <!-- Version 1.2 of this ReStructuredText document corresponds to
 465 n1530_, the paper accepted by the LWG for TR1. -->
 466 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 467 <p>The <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template adapts some <tt class="docutils literal"><span class="pre">Base</span></tt><a class="footnote-reference" href="#base" id="id7"><sup>3</sup></a>
 468 type to create a new iterator.  Instantiations of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>
 469 are derived from a corresponding instantiation of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>
 470 and implement the core behaviors in terms of the <tt class="docutils literal"><span class="pre">Base</span></tt> type. In
 471 essence, <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> merely forwards all operations to an
 472 instance of the <tt class="docutils literal"><span class="pre">Base</span></tt> type, which it stores as a member.</p>
 473 <table class="docutils footnote" frame="void" id="base" rules="none">
 474 <colgroup><col class="label" /><col /></colgroup>
 475 <tbody valign="top">
 476 <tr><td class="label"><a class="fn-backref" href="#id7">[3]</a></td><td>The term &quot;Base&quot; here does not refer to a base class and is
 477 not meant to imply the use of derivation. We have followed the lead
 478 of the standard library, which provides a base() function to access
 479 the underlying iterator object of a <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> adaptor.</td></tr>
 480 </tbody>
 481 </table>
 482 <p>The user of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> creates a class derived from an
 483 instantiation of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> and then selectively
 484 redefines some of the core member functions described in the
 485 <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> core requirements table. The <tt class="docutils literal"><span class="pre">Base</span></tt> type need
 486 not meet the full requirements for an iterator; it need only
 487 support the operations used by the core interface functions of
 488 <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> that have not been redefined in the user's
 489 derived class.</p>
 490 <p>Several of the template parameters of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> default
 491 to <tt class="docutils literal"><span class="pre">use_default</span></tt>. This allows the
 492 user to make use of a default parameter even when she wants to
 493 specify a parameter later in the parameter list.  Also, the
 494 defaults for the corresponding associated types are somewhat
 495 complicated, so metaprogramming is required to compute them, and
 496 <tt class="docutils literal"><span class="pre">use_default</span></tt> can help to simplify the implementation.  Finally,
 497 the identity of the <tt class="docutils literal"><span class="pre">use_default</span></tt> type is not left unspecified
 498 because specification helps to highlight that the <tt class="docutils literal"><span class="pre">Reference</span></tt>
 499 template parameter may not always be identical to the iterator's
 500 <tt class="docutils literal"><span class="pre">reference</span></tt> type, and will keep users from making mistakes based on
 501 that assumption.</p>
 502 </div>
 503 <div class="section" id="specialized-adaptors">
 504 <h2><a class="toc-backref" href="#id26">Specialized Adaptors</a></h2>
 505 <p>This proposal also contains several examples of specialized adaptors
 506 which were easily implemented using <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>:</p>
 507 <ul class="simple">
 508 <li><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt>, which iterates over iterators, pointers,
 509 or smart pointers and applies an extra level of dereferencing.</li>
 510 <li>A new <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt>, which inverts the direction of a Base
 511 iterator's motion, while allowing adapted constant and mutable
 512 iterators to interact in the expected ways (unlike those in most
 513 implementations of C++98).</li>
 514 <li><tt class="docutils literal"><span class="pre">transform_iterator</span></tt>, which applies a user-defined function object
 515 to the underlying values when dereferenced.</li>
 516 <li><tt class="docutils literal"><span class="pre">filter_iterator</span></tt>, which provides a view of an iterator range in
 517 which some elements of the underlying range are skipped.</li>
 518 </ul>
 519 <ul class="simple" id="counting">
 520 <li><tt class="docutils literal"><span class="pre">counting_iterator</span></tt>, which adapts any incrementable type
 521 (e.g. integers, iterators) so that incrementing/decrementing the
 522 adapted iterator and dereferencing it produces successive values of
 523 the Base type.</li>
 524 <li><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt>, which makes it easier to create custom
 525 output iterators.</li>
 526 </ul>
 527 <p>Based on examples in the Boost library, users have generated many new
 528 adaptors, among them a permutation adaptor which applies some
 529 permutation to a random access iterator, and a strided adaptor, which
 530 adapts a random access iterator by multiplying its unit of motion by a
 531 constant factor.  In addition, the Boost Graph Library (BGL) uses
 532 iterator adaptors to adapt other graph libraries, such as LEDA [10]
 533 and Stanford GraphBase [8], to the BGL interface (which requires C++
 534 Standard compliant iterators).</p>
 535 </div>
 536 </div>
 537 <div class="section" id="proposed-text">
 538 <h1><a class="toc-backref" href="#id27">Proposed Text</a></h1>
 539 <div class="section" id="header-iterator-helper-synopsis-lib-iterator-helper-synopsis">
 540 <h2><a class="toc-backref" href="#id28">Header <tt class="docutils literal"><span class="pre">&lt;iterator_helper&gt;</span></tt> synopsis    [lib.iterator.helper.synopsis]</a></h2>
 541 <pre class="literal-block">
 542 struct use_default;
 543
 544 struct iterator_core_access { /* implementation detail */ };
 545
 546 template &lt;
 547     class Derived
 548   , class Value
 549   , class CategoryOrTraversal
 550   , class Reference  = Value&amp;
 551   , class Difference = ptrdiff_t
 552 &gt;
 553 class iterator_facade;
 554
 555 template &lt;
 556     class Derived
 557   , class Base
 558   , class Value      = use_default
 559   , class CategoryOrTraversal  = use_default
 560   , class Reference  = use_default
 561   , class Difference = use_default
 562 &gt;
 563 class iterator_adaptor;
 564
 565 template &lt;
 566     class Iterator
 567   , class Value = use_default
 568   , class CategoryOrTraversal = use_default
 569   , class Reference = use_default
 570   , class Difference = use_default
 571 &gt;
 572 class indirect_iterator;
 573
 574 template &lt;class Dereferenceable&gt;
 575 struct pointee;
 576
 577 template &lt;class Dereferenceable&gt;
 578 struct indirect_reference;
 579
 580 template &lt;class Iterator&gt;
 581 class reverse_iterator;
 582
 583 template &lt;
 584     class UnaryFunction
 585   , class Iterator
 586   , class Reference = use_default
 587   , class Value = use_default
 588 &gt;
 589 class transform_iterator;
 590
 591 template &lt;class Predicate, class Iterator&gt;
 592 class filter_iterator;
 593
 594 template &lt;
 595     class Incrementable
 596   , class CategoryOrTraversal  = use_default
 597   , class Difference = use_default
 598 &gt;
 599 class counting_iterator;
 600
 601 template &lt;class UnaryFunction&gt;
 602 class function_output_iterator;
 603 </pre>
 604 </div>
 605 <div class="section" id="iterator-facade-lib-iterator-facade">
 606 <h2><a class="toc-backref" href="#id29">Iterator facade [lib.iterator.facade]</a></h2>
 607 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 608 <!-- Software License, Version 1.0. (See accompanying -->
 609 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 610 <p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is a base class template that implements the
 611 interface of standard iterators in terms of a few core functions
 612 and associated types, to be supplied by a derived iterator class.</p>
 613 <div class="section" id="class-template-iterator-facade">
 614 <h3><a class="toc-backref" href="#id30">Class template <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a></h3>
 615 <!-- Distributed under the Boost -->
 616 <!-- Software License, Version 1.0. (See accompanying -->
 617 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 618 <!-- Version 1.3 of this ReStructuredText document corresponds to
 619 n1530_, the paper accepted by the LWG for TR1. -->
 620 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 621 <pre class="literal-block">
 622 template &lt;
 623     class Derived
 624   , class Value
 625   , class CategoryOrTraversal
 626   , class Reference  = Value&amp;
 627   , class Difference = ptrdiff_t
 628 &gt;
 629 class iterator_facade {
 630  public:
 631     typedef remove_const&lt;Value&gt;::type value_type;
 632     typedef Reference reference;
 633     typedef Value* pointer;
 634     typedef Difference difference_type;
 635     typedef /* see <a class="reference internal" href="#iterator-category">below</a> */ iterator_category;
 636
 637     reference operator*() const;
 638     /* see <a class="reference internal" href="#operator-arrow">below</a> */ operator-&gt;() const;
 639     /* see <a class="reference internal" href="#brackets">below</a> */ operator[](difference_type n) const;
 640     Derived&amp; operator++();
 641     Derived operator++(int);
 642     Derived&amp; operator--();
 643     Derived operator--(int);
 644     Derived&amp; operator+=(difference_type n);
 645     Derived&amp; operator-=(difference_type n);
 646     Derived operator-(difference_type n) const;
 647  protected:
 648     typedef iterator_facade iterator_facade_;
 649 };
 650
 651 // Comparison operators
 652 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 653           class Dr2, class V2, class TC2, class R2, class D2&gt;
 654 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type // exposition
 655 operator ==(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
 656             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 657
 658 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 659           class Dr2, class V2, class TC2, class R2, class D2&gt;
 660 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 661 operator !=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
 662             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 663
 664 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 665           class Dr2, class V2, class TC2, class R2, class D2&gt;
 666 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 667 operator &lt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
 668            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 669
 670 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 671           class Dr2, class V2, class TC2, class R2, class D2&gt;
 672 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 673 operator &lt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
 674             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 675
 676 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 677           class Dr2, class V2, class TC2, class R2, class D2&gt;
 678 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 679 operator &gt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
 680            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 681
 682 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 683           class Dr2, class V2, class TC2, class R2, class D2&gt;
 684 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 685 operator &gt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
 686             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 687
 688 // Iterator difference
 689 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
 690           class Dr2, class V2, class TC2, class R2, class D2&gt;
 691 /* see <a class="reference internal" href="#minus">below</a> */
 692 operator-(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
 693           iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 694
 695 // Iterator addition
 696 template &lt;class Dr, class V, class TC, class R, class D&gt;
 697 Derived operator+ (iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;,
 698                    typename Derived::difference_type n);
 699
 700 template &lt;class Dr, class V, class TC, class R, class D&gt;
 701 Derived operator+ (typename Derived::difference_type n,
 702                    iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;);
 703 </pre>
 704 <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>
 705 <pre class="literal-block">
 706 <em>iterator-category</em>(CategoryOrTraversal, value_type, reference)
 707 </pre>
 708 <p>where <em>iterator-category</em> is defined as follows:</p>
 709 <pre class="literal-block" id="id12">
 710 <em>iterator-category</em>(C,R,V) :=
 711    if (C is convertible to std::input_iterator_tag
 712        || C is convertible to std::output_iterator_tag
 713    )
 714        return C
 715
 716    else if (C is not convertible to incrementable_traversal_tag)
 717        <em>the program is ill-formed</em>
 718
 719    else return a type X satisfying the following two constraints:
 720
 721       1. X is convertible to X1, and not to any more-derived
 722          type, where X1 is defined by:
 723
 724            if (R is a reference type
 725                &amp;&amp; C is convertible to forward_traversal_tag)
 726            {
 727                if (C is convertible to random_access_traversal_tag)
 728                    X1 = random_access_iterator_tag
 729                else if (C is convertible to bidirectional_traversal_tag)
 730                    X1 = bidirectional_iterator_tag
 731                else
 732                    X1 = forward_iterator_tag
 733            }
 734            else
 735            {
 736                if (C is convertible to single_pass_traversal_tag
 737                    &amp;&amp; R is convertible to V)
 738                    X1 = input_iterator_tag
 739                else
 740                    X1 = C
 741            }
 742
 743       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
 744          derived traversal tag type to which X is also
 745          convertible, and not to any more-derived traversal tag
 746          type.
 747 </pre>
 748 <p>[Note: the intention is to allow <tt class="docutils literal"><span class="pre">iterator_category</span></tt> to be one of
 749 the five original category tags when convertibility to one of the
 750 traversal tags would add no information]</p>
 751 <!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
 752 <!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
 753 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 754 <p>The <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> template used above is for exposition
 755 purposes.  The member operators should only be in an overload set
 756 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,
 757 meaning that at least one of the types is convertible to the other.  The
 758 <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> approach uses SFINAE to take the operators
 759 out of the overload set when the types are not interoperable.
 760 The operators should behave <em>as-if</em> <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt>
 761 were defined to be:</p>
 762 <pre class="literal-block">
 763 template &lt;bool, typename&gt; enable_if_interoperable_impl
 764 {};
 765
 766 template &lt;typename T&gt; enable_if_interoperable_impl&lt;true,T&gt;
 767 { typedef T type; };
 768
 769 template&lt;typename Dr1, typename Dr2, typename T&gt;
 770 struct enable_if_interoperable
 771   : enable_if_interoperable_impl&lt;
 772         is_convertible&lt;Dr1,Dr2&gt;::value || is_convertible&lt;Dr2,Dr1&gt;::value
 773       , T
 774     &gt;
 775 {};
 776 </pre>
 777 </div>
 778 <div class="section" id="iterator-facade-requirements">
 779 <h3><a class="toc-backref" href="#id31"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Requirements</a></h3>
 780 <p>The following table describes the typical valid expressions on
 781 <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
 782 iterator concept(s) it will model.  The operations in the first
 783 column must be made accessible to member functions of class
 784 <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt>.  In addition,
 785 <tt class="docutils literal"><span class="pre">static_cast&lt;Derived*&gt;(iterator_facade*)</span></tt> shall be well-formed.</p>
 786 <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
 787 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>,
 788 <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
 789 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>
 790 is a constant object of a random access traversal iterator type
 791 interoperable with <tt class="docutils literal"><span class="pre">X</span></tt>.</p>
 792 <div class="topic" id="core-operations">
 793 <p class="topic-title first"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Core Operations</p>
 794 <table border="1" class="docutils">
 795 <colgroup>
 796 <col width="21%" />
 797 <col width="23%" />
 798 <col width="27%" />
 799 <col width="29%" />
 800 </colgroup>
 801 <thead valign="bottom">
 802 <tr><th class="head">Expression</th>
 803 <th class="head">Return Type</th>
 804 <th class="head">Assertion/Note</th>
 805 <th class="head">Used to implement Iterator
 806 Concept(s)</th>
 807 </tr>
 808 </thead>
 809 <tbody valign="top">
 810 <tr><td><tt class="docutils literal"><span class="pre">c.dereference()</span></tt></td>
 811 <td><tt class="docutils literal"><span class="pre">F::reference</span></tt></td>
 812 <td>&nbsp;</td>
 813 <td>Readable Iterator, Writable
 814 Iterator</td>
 815 </tr>
 816 <tr><td><tt class="docutils literal"><span class="pre">c.equal(y)</span></tt></td>
 817 <td>convertible to bool</td>
 818 <td>true iff <tt class="docutils literal"><span class="pre">c</span></tt> and <tt class="docutils literal"><span class="pre">y</span></tt>
 819 refer to the same
 820 position.</td>
 821 <td>Single Pass Iterator</td>
 822 </tr>
 823 <tr><td><tt class="docutils literal"><span class="pre">a.increment()</span></tt></td>
 824 <td>unused</td>
 825 <td>&nbsp;</td>
 826 <td>Incrementable Iterator</td>
 827 </tr>
 828 <tr><td><tt class="docutils literal"><span class="pre">a.decrement()</span></tt></td>
 829 <td>unused</td>
 830 <td>&nbsp;</td>
 831 <td>Bidirectional Traversal
 832 Iterator</td>
 833 </tr>
 834 <tr><td><tt class="docutils literal"><span class="pre">a.advance(n)</span></tt></td>
 835 <td>unused</td>
 836 <td>&nbsp;</td>
 837 <td>Random Access Traversal
 838 Iterator</td>
 839 </tr>
 840 <tr><td><tt class="docutils literal"><span class="pre">c.distance_to(z)</span></tt></td>
 841 <td>convertible to
 842 <tt class="docutils literal"><span class="pre">F::difference_type</span></tt></td>
 843 <td>equivalent to
 844 <tt class="docutils literal"><span class="pre">distance(c,</span> <span class="pre">X(z))</span></tt>.</td>
 845 <td>Random Access Traversal
 846 Iterator</td>
 847 </tr>
 848 </tbody>
 849 </table>
 850 </div>
 851 </div>
 852 <div class="section" id="iterator-facade-operations">
 853 <h3><a class="toc-backref" href="#id32"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> operations</a></h3>
 854 <p>The operations in this section are described in terms of operations on
 855 the core interface of <tt class="docutils literal"><span class="pre">Derived</span></tt> which may be inaccessible
 856 (i.e. private).  The implementation should access these operations
 857 through member functions of class <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt>.</p>
 858 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
 859 <table class="docutils field-list" frame="void" rules="none">
 860 <col class="field-name" />
 861 <col class="field-body" />
 862 <tbody valign="top">
 863 <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>
 864 </tr>
 865 </tbody>
 866 </table>
 867 <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>
 868 <table class="docutils field-list" frame="void" rules="none">
 869 <col class="field-name" />
 870 <col class="field-body" />
 871 <tbody valign="top">
 872 <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
 873 of type <tt class="docutils literal"><span class="pre">pointer</span></tt> equal to:</p>
 874 <pre class="literal-block">
 875 &amp;static_cast&lt;Derived const*&gt;(this)-&gt;dereference()
 876 </pre>
 877 <p class="last">Otherwise returns an object of unspecified type such that,
 878 <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>
 879 <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>
 880 </td>
 881 </tr>
 882 </tbody>
 883 </table>
 884 <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>
 885 <table class="docutils field-list" frame="void" rules="none">
 886 <col class="field-name" />
 887 <col class="field-body" />
 888 <tbody valign="top">
 889 <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
 890 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
 891 <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
 892 <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>
 893 <span class="pre">const&amp;&gt;((*this)[n])</span></tt> is equivalent to
 894 <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>
 895 </tr>
 896 </tbody>
 897 </table>
 898 <p><tt class="docutils literal"><span class="pre">Derived&amp;</span> <span class="pre">operator++();</span></tt></p>
 899 <table class="docutils field-list" frame="void" rules="none">
 900 <col class="field-name" />
 901 <col class="field-body" />
 902 <tbody valign="top">
 903 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 904 static_cast&lt;Derived*&gt;(this)-&gt;increment();
 905 return *static_cast&lt;Derived*&gt;(this);
 906 </pre>
 907 </td>
 908 </tr>
 909 </tbody>
 910 </table>
 911 <p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator++(int);</span></tt></p>
 912 <table class="docutils field-list" frame="void" rules="none">
 913 <col class="field-name" />
 914 <col class="field-body" />
 915 <tbody valign="top">
 916 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 917 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 918 ++*this;
 919 return tmp;
 920 </pre>
 921 </td>
 922 </tr>
 923 </tbody>
 924 </table>
 925 <p><tt class="docutils literal"><span class="pre">Derived&amp;</span> <span class="pre">operator--();</span></tt></p>
 926 <table class="docutils field-list" frame="void" rules="none">
 927 <col class="field-name" />
 928 <col class="field-body" />
 929 <tbody valign="top">
 930 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 931 static_cast&lt;Derived*&gt;(this)-&gt;decrement();
 932 return *static_cast&lt;Derived*&gt;(this);
 933 </pre>
 934 </td>
 935 </tr>
 936 </tbody>
 937 </table>
 938 <p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator--(int);</span></tt></p>
 939 <table class="docutils field-list" frame="void" rules="none">
 940 <col class="field-name" />
 941 <col class="field-body" />
 942 <tbody valign="top">
 943 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 944 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 945 --*this;
 946 return tmp;
 947 </pre>
 948 </td>
 949 </tr>
 950 </tbody>
 951 </table>
 952 <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>
 953 <table class="docutils field-list" frame="void" rules="none">
 954 <col class="field-name" />
 955 <col class="field-body" />
 956 <tbody valign="top">
 957 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 958 static_cast&lt;Derived*&gt;(this)-&gt;advance(n);
 959 return *static_cast&lt;Derived*&gt;(this);
 960 </pre>
 961 </td>
 962 </tr>
 963 </tbody>
 964 </table>
 965 <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>
 966 <table class="docutils field-list" frame="void" rules="none">
 967 <col class="field-name" />
 968 <col class="field-body" />
 969 <tbody valign="top">
 970 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 971 static_cast&lt;Derived*&gt;(this)-&gt;advance(-n);
 972 return *static_cast&lt;Derived*&gt;(this);
 973 </pre>
 974 </td>
 975 </tr>
 976 </tbody>
 977 </table>
 978 <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>
 979 <table class="docutils field-list" frame="void" rules="none">
 980 <col class="field-name" />
 981 <col class="field-body" />
 982 <tbody valign="top">
 983 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 984 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 985 return tmp -= n;
 986 </pre>
 987 </td>
 988 </tr>
 989 </tbody>
 990 </table>
 991 <pre class="literal-block">
 992 template &lt;class Dr, class V, class TC, class R, class D&gt;
 993 Derived operator+ (iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;,
 994                    typename Derived::difference_type n);
 995
 996 template &lt;class Dr, class V, class TC, class R, class D&gt;
 997 Derived operator+ (typename Derived::difference_type n,
 998                    iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;);
 999 </pre>
1000 <table class="docutils field-list" frame="void" rules="none">
1001 <col class="field-name" />
1002 <col class="field-body" />
1003 <tbody valign="top">
1004 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
1005 Derived tmp(static_cast&lt;Derived const*&gt;(this));
1006 return tmp += n;
1007 </pre>
1008 </td>
1009 </tr>
1010 </tbody>
1011 </table>
1012 <pre class="literal-block">
1013 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
1014           class Dr2, class V2, class TC2, class R2, class D2&gt;
1015 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
1016 operator ==(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
1017             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
1018 </pre>
1019 <table class="docutils field-list" frame="void" rules="none">
1020 <col class="field-name" />
1021 <col class="field-body" />
1022 <tbody valign="top">
1023 <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>
1024 <dl class="last docutils">
1025 <dt>then</dt>
1026 <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>
1027 </dd>
1028 <dt>Otherwise,</dt>
1029 <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>
1030 </dd>
1031 </dl>
1032 </td>
1033 </tr>
1034 </tbody>
1035 </table>
1036 <pre class="literal-block">
1037 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
1038           class Dr2, class V2, class TC2, class R2, class D2&gt;
1039 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
1040 operator !=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
1041             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
1042 </pre>
1043 <table class="docutils field-list" frame="void" rules="none">
1044 <col class="field-name" />
1045 <col class="field-body" />
1046 <tbody valign="top">
1047 <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>
1048 <dl class="last docutils">
1049 <dt>then</dt>
1050 <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>
1051 </dd>
1052 <dt>Otherwise,</dt>
1053 <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>
1054 </dd>
1055 </dl>
1056 </td>
1057 </tr>
1058 </tbody>
1059 </table>
1060 <pre class="literal-block">
1061 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
1062           class Dr2, class V2, class TC2, class R2, class D2&gt;
1063 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
1064 operator &lt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
1065            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
1066 </pre>
1067 <table class="docutils field-list" frame="void" rules="none">
1068 <col class="field-name" />
1069 <col class="field-body" />
1070 <tbody valign="top">
1071 <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>
1072 <dl class="last docutils">
1073 <dt>then</dt>
1074 <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>
1075 </dd>
1076 <dt>Otherwise,</dt>
1077 <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>
1078 </dd>
1079 </dl>
1080 </td>
1081 </tr>
1082 </tbody>
1083 </table>
1084 <pre class="literal-block">
1085 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
1086           class Dr2, class V2, class TC2, class R2, class D2&gt;
1087 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
1088 operator &lt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
1089             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
1090 </pre>
1091 <table class="docutils field-list" frame="void" rules="none">
1092 <col class="field-name" />
1093 <col class="field-body" />
1094 <tbody valign="top">
1095 <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>
1096 <dl class="last docutils">
1097 <dt>then</dt>
1098 <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>
1099 </dd>
1100 <dt>Otherwise,</dt>
1101 <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>
1102 </dd>
1103 </dl>
1104 </td>
1105 </tr>
1106 </tbody>
1107 </table>
1108 <pre class="literal-block">
1109 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
1110           class Dr2, class V2, class TC2, class R2, class D2&gt;
1111 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
1112 operator &gt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
1113            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
1114 </pre>
1115 <table class="docutils field-list" frame="void" rules="none">
1116 <col class="field-name" />
1117 <col class="field-body" />
1118 <tbody valign="top">
1119 <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>
1120 <dl class="last docutils">
1121 <dt>then</dt>
1122 <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>
1123 </dd>
1124 <dt>Otherwise,</dt>
1125 <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>
1126 </dd>
1127 </dl>
1128 </td>
1129 </tr>
1130 </tbody>
1131 </table>
1132 <pre class="literal-block">
1133 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
1134           class Dr2, class V2, class TC2, class R2, class D2&gt;
1135 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
1136 operator &gt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
1137             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
1138 </pre>
1139 <table class="docutils field-list" frame="void" rules="none">
1140 <col class="field-name" />
1141 <col class="field-body" />
1142 <tbody valign="top">
1143 <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>
1144 <dl class="last docutils">
1145 <dt>then</dt>
1146 <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>
1147 </dd>
1148 <dt>Otherwise,</dt>
1149 <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>
1150 </dd>
1151 </dl>
1152 </td>
1153 </tr>
1154 </tbody>
1155 </table>
1156 <pre class="literal-block" id="minus">
1157 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
1158           class Dr2, class V2, class TC2, class R2, class D2&gt;
1159 typename enable_if_interoperable&lt;Dr1,Dr2,difference&gt;::type
1160 operator -(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
1161            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
1162 </pre>
1163 <table class="docutils field-list" frame="void" rules="none">
1164 <col class="field-name" />
1165 <col class="field-body" />
1166 <tbody valign="top">
1167 <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>
1168 <blockquote>
1169 <dl class="docutils">
1170 <dt>then</dt>
1171 <dd><p class="first last"><tt class="docutils literal"><span class="pre">difference</span></tt> shall be
1172 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dr1&gt;::difference_type</span></tt>.</p>
1173 </dd>
1174 <dt>Otherwise</dt>
1175 <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>
1176 </dd>
1177 </dl>
1178 </blockquote>
1179 </td>
1180 </tr>
1181 <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>
1182 <dl class="last docutils">
1183 <dt>then</dt>
1184 <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>
1185 </dd>
1186 <dt>Otherwise,</dt>
1187 <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>
1188 </dd>
1189 </dl>
1190 </td>
1191 </tr>
1192 </tbody>
1193 </table>
1194 </div>
1195 </div>
1196 <div class="section" id="iterator-adaptor-lib-iterator-adaptor">
1197 <h2><a class="toc-backref" href="#id33">Iterator adaptor [lib.iterator.adaptor]</a></h2>
1198 <!-- Distributed under the Boost -->
1199 <!-- Software License, Version 1.0. (See accompanying -->
1200 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
1201 <!-- Version 1.1 of this ReStructuredText document corresponds to
1202 n1530_, the paper accepted by the LWG. -->
1203 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
1204 <p>Each specialization of the <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template is derived from
1205 a specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>. The core interface functions
1206 expected by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> are implemented in terms of the
1207 <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="docutils literal"><span class="pre">Base</span></tt> template parameter. A class derived
1208 from <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> typically redefines some of the core
1209 interface functions to adapt the behavior of the <tt class="docutils literal"><span class="pre">Base</span></tt> type.
1210 Whether the derived class models any of the standard iterator concepts
1211 depends on the operations supported by the <tt class="docutils literal"><span class="pre">Base</span></tt> type and which
1212 core interface functions of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> are redefined in the
1213 <tt class="docutils literal"><span class="pre">Derived</span></tt> class.</p>
1214 <div class="section" id="class-template-iterator-adaptor">
1215 <h3><a class="toc-backref" href="#id34">Class template <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt></a></h3>
1216 <!-- Distributed under the Boost -->
1217 <!-- Software License, Version 1.0. (See accompanying -->
1218 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
1219 <!-- Version 1.4 of this ReStructuredText document corresponds to
1220 n1530_, the paper accepted by the LWG for TR1. -->
1221 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
1222 <pre class="literal-block">
1223 template &lt;
1224     class Derived
1225   , class Base
1226   , class Value               = use_default
1227   , class CategoryOrTraversal = use_default
1228   , class Reference           = use_default
1229   , class Difference = use_default
1230 &gt;
1231 class iterator_adaptor
1232   : public iterator_facade&lt;Derived, <em>V'</em>, <em>C'</em>, <em>R'</em>, <em>D'</em>&gt; // see <a class="reference internal" href="#base-parameters">details</a>
1233 {
1234     friend class iterator_core_access;
1235  public:
1236     iterator_adaptor();
1237     explicit iterator_adaptor(Base const&amp; iter);
1238     typedef Base base_type;
1239     Base const&amp; base() const;
1240  protected:
1241     typedef iterator_adaptor iterator_adaptor_;
1242     Base const&amp; base_reference() const;
1243     Base&amp; base_reference();
1244  private: // Core iterator interface for iterator_facade.
1245     typename iterator_adaptor::reference dereference() const;
1246
1247     template &lt;
1248     class OtherDerived, class OtherIterator, class V, class C, class R, class D
1249     &gt;
1250     bool equal(iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; x) const;
1251
1252     void advance(typename iterator_adaptor::difference_type n);
1253     void increment();
1254     void decrement();
1255
1256     template &lt;
1257         class OtherDerived, class OtherIterator, class V, class C, class R, class D
1258     &gt;
1259     typename iterator_adaptor::difference_type distance_to(
1260         iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; y) const;
1261
1262  private:
1263     Base m_iterator; // exposition only
1264 };
1265 </pre>
1266 </div>
1267 <div class="section" id="iterator-adaptor-requirements">
1268 <span id="requirements"></span><h3><a class="toc-backref" href="#id35"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></h3>
1269 <p><tt class="docutils literal"><span class="pre">static_cast&lt;Derived*&gt;(iterator_adaptor*)</span></tt> shall be well-formed.
1270 The <tt class="docutils literal"><span class="pre">Base</span></tt> argument shall be Assignable and Copy Constructible.</p>
1271 </div>
1272 <div class="section" id="iterator-adaptor-base-class-parameters">
1273 <span id="base-parameters"></span><h3><a class="toc-backref" href="#id36"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></h3>
1274 <p>The <em>V'</em>, <em>C'</em>, <em>R'</em>, and <em>D'</em> parameters of the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>
1275 used as a base class in the summary of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>
1276 above are defined as follows:</p>
1277 <pre class="literal-block">
1278 <em>V'</em> = if (Value is use_default)
1279           return iterator_traits&lt;Base&gt;::value_type
1280       else
1281           return Value
1282
1283 <em>C'</em> = if (CategoryOrTraversal is use_default)
1284           return iterator_traversal&lt;Base&gt;::type
1285       else
1286           return CategoryOrTraversal
1287
1288 <em>R'</em> = if (Reference is use_default)
1289           if (Value is use_default)
1290               return iterator_traits&lt;Base&gt;::reference
1291           else
1292               return Value&amp;
1293       else
1294           return Reference
1295
1296 <em>D'</em> = if (Difference is use_default)
1297           return iterator_traits&lt;Base&gt;::difference_type
1298       else
1299           return Difference
1300 </pre>
1301 <!-- ``iterator_adaptor`` models
1302 - - - - - - - - - - - - - - - - - - - - - - - - - - -
1303
1304 In order for ``Derived`` to model the iterator concepts corresponding
1305 to ``iterator_traits<Derived>::iterator_category``, the expressions
1306 involving ``m_iterator`` in the specifications of those private member
1307 functions of ``iterator_adaptor`` that may be called by
1308 ``iterator_facade<Derived, V, C, R, D>`` in evaluating any valid
1309 expression involving ``Derived`` in those concepts' requirements. -->
1310 <!-- The above is confusing and needs a rewrite. -JGS -->
1311 <!-- That's why it's removed.  We're embracing inheritance, remember? -->
1312 </div>
1313 <div class="section" id="iterator-adaptor-public-operations">
1314 <h3><a class="toc-backref" href="#id37"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></h3>
1315 <p><tt class="docutils literal"><span class="pre">iterator_adaptor();</span></tt></p>
1316 <table class="docutils field-list" frame="void" rules="none">
1317 <col class="field-name" />
1318 <col class="field-body" />
1319 <tbody valign="top">
1320 <tr class="field"><th class="field-name">Requires:</th><td class="field-body">The <tt class="docutils literal"><span class="pre">Base</span></tt> type must be Default Constructible.</td>
1321 </tr>
1322 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> with
1323 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> default constructed.</td>
1324 </tr>
1325 </tbody>
1326 </table>
1327 <p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">iterator_adaptor(Base</span> <span class="pre">const&amp;</span> <span class="pre">iter);</span></tt></p>
1328 <table class="docutils field-list" frame="void" rules="none">
1329 <col class="field-name" />
1330 <col class="field-body" />
1331 <tbody valign="top">
1332 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> with
1333 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">iter</span></tt>.</td>
1334 </tr>
1335 </tbody>
1336 </table>
1337 <p><tt class="docutils literal"><span class="pre">Base</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
1338 <table class="docutils field-list" frame="void" rules="none">
1339 <col class="field-name" />
1340 <col class="field-body" />
1341 <tbody valign="top">
1342 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
1343 </tr>
1344 </tbody>
1345 </table>
1346 </div>
1347 <div class="section" id="iterator-adaptor-protected-member-functions">
1348 <h3><a class="toc-backref" href="#id38"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></h3>
1349 <p><tt class="docutils literal"><span class="pre">Base</span> <span class="pre">const&amp;</span> <span class="pre">base_reference()</span> <span class="pre">const;</span></tt></p>
1350 <table class="docutils field-list" frame="void" rules="none">
1351 <col class="field-name" />
1352 <col class="field-body" />
1353 <tbody valign="top">
1354 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">A const reference to <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td>
1355 </tr>
1356 </tbody>
1357 </table>
1358 <p><tt class="docutils literal"><span class="pre">Base&amp;</span> <span class="pre">base_reference();</span></tt></p>
1359 <table class="docutils field-list" frame="void" rules="none">
1360 <col class="field-name" />
1361 <col class="field-body" />
1362 <tbody valign="top">
1363 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">A non-const reference to <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td>
1364 </tr>
1365 </tbody>
1366 </table>
1367 </div>
1368 <div class="section" id="iterator-adaptor-private-member-functions">
1369 <h3><a class="toc-backref" href="#id39"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></h3>
1370 <p><tt class="docutils literal"><span class="pre">typename</span> <span class="pre">iterator_adaptor::reference</span> <span class="pre">dereference()</span> <span class="pre">const;</span></tt></p>
1371 <table class="docutils field-list" frame="void" rules="none">
1372 <col class="field-name" />
1373 <col class="field-body" />
1374 <tbody valign="top">
1375 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*m_iterator</span></tt></td>
1376 </tr>
1377 </tbody>
1378 </table>
1379 <pre class="literal-block">
1380 template &lt;
1381 class OtherDerived, class OtherIterator, class V, class C, class R, class D
1382 &gt;
1383 bool equal(iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; x) const;
1384 </pre>
1385 <table class="docutils field-list" frame="void" rules="none">
1386 <col class="field-name" />
1387 <col class="field-body" />
1388 <tbody valign="top">
1389 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span> <span class="pre">==</span> <span class="pre">x.base()</span></tt></td>
1390 </tr>
1391 </tbody>
1392 </table>
1393 <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">advance(typename</span> <span class="pre">iterator_adaptor::difference_type</span> <span class="pre">n);</span></tt></p>
1394 <table class="docutils field-list" frame="void" rules="none">
1395 <col class="field-name" />
1396 <col class="field-body" />
1397 <tbody valign="top">
1398 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span> <span class="pre">+=</span> <span class="pre">n;</span></tt></td>
1399 </tr>
1400 </tbody>
1401 </table>
1402 <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">increment();</span></tt></p>
1403 <table class="docutils field-list" frame="void" rules="none">
1404 <col class="field-name" />
1405 <col class="field-body" />
1406 <tbody valign="top">
1407 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator;</span></tt></td>
1408 </tr>
1409 </tbody>
1410 </table>
1411 <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">decrement();</span></tt></p>
1412 <table class="docutils field-list" frame="void" rules="none">
1413 <col class="field-name" />
1414 <col class="field-body" />
1415 <tbody valign="top">
1416 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator;</span></tt></td>
1417 </tr>
1418 </tbody>
1419 </table>
1420 <pre class="literal-block">
1421 template &lt;
1422     class OtherDerived, class OtherIterator, class V, class C, class R, class D
1423 &gt;
1424 typename iterator_adaptor::difference_type distance_to(
1425     iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; y) const;
1426 </pre>
1427 <table class="docutils field-list" frame="void" rules="none">
1428 <col class="field-name" />
1429 <col class="field-body" />
1430 <tbody valign="top">
1431 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">y.base()</span> <span class="pre">-</span> <span class="pre">m_iterator</span></tt></td>
1432 </tr>
1433 </tbody>
1434 </table>
1435 </div>
1436 </div>
1437 <div class="section" id="specialized-adaptors-lib-iterator-special-adaptors">
1438 <h2><a class="toc-backref" href="#id40">Specialized adaptors [lib.iterator.special.adaptors]</a></h2>
1439 <p>The <tt class="docutils literal"><span class="pre">enable_if_convertible&lt;X,Y&gt;::type</span></tt> expression used in
1440 this section is for exposition purposes. The converting constructors
1441 for specialized adaptors should be only be in an overload set provided
1442 that an object of type <tt class="docutils literal"><span class="pre">X</span></tt> is implicitly convertible to an object of
1443 type <tt class="docutils literal"><span class="pre">Y</span></tt>.
1444 The signatures involving <tt class="docutils literal"><span class="pre">enable_if_convertible</span></tt> should behave
1445 <em>as-if</em> <tt class="docutils literal"><span class="pre">enable_if_convertible</span></tt> were defined to be:</p>
1446 <pre class="literal-block">
1447 template &lt;bool&gt; enable_if_convertible_impl
1448 {};
1449
1450 template &lt;&gt; enable_if_convertible_impl&lt;true&gt;
1451 { struct type; };
1452
1453 template&lt;typename From, typename To&gt;
1454 struct enable_if_convertible
1455   : enable_if_convertible_impl&lt;is_convertible&lt;From,To&gt;::value&gt;
1456 {};
1457 </pre>
1458 <p>If an expression other than the default argument is used to supply
1459 the value of a function parameter whose type is written in terms
1460 of <tt class="docutils literal"><span class="pre">enable_if_convertible</span></tt>, the program is ill-formed, no
1461 diagnostic required.</p>
1462 <p>[<em>Note:</em> The <tt class="docutils literal"><span class="pre">enable_if_convertible</span></tt> approach uses SFINAE to
1463 take the constructor out of the overload set when the types are not
1464 implicitly convertible.
1465 ]</p>
1466 <div class="section" id="indirect-iterator">
1467 <h3><a class="toc-backref" href="#id41">Indirect iterator</a></h3>
1468 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
1469 <!-- Software License, Version 1.0. (See accompanying -->
1470 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
1471 <p><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> adapts an iterator by applying an
1472 <em>extra</em> dereference inside of <tt class="docutils literal"><span class="pre">operator*()</span></tt>. For example, this
1473 iterator adaptor makes it possible to view a container of pointers
1474 (e.g. <tt class="docutils literal"><span class="pre">list&lt;foo*&gt;</span></tt>) as if it were a container of the pointed-to type
1475 (e.g. <tt class="docutils literal"><span class="pre">list&lt;foo&gt;</span></tt>).  <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> depends on two
1476 auxiliary traits, <tt class="docutils literal"><span class="pre">pointee</span></tt> and <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>, to
1477 provide support for underlying iterators whose <tt class="docutils literal"><span class="pre">value_type</span></tt> is
1478 not an iterator.</p>
1479 <div class="section" id="class-template-pointee">
1480 <h4><a class="toc-backref" href="#id42">Class template <tt class="docutils literal"><span class="pre">pointee</span></tt></a></h4>
1481 <!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
1482 <!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
1483 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
1484 <pre class="literal-block">
1485 template &lt;class Dereferenceable&gt;
1486 struct pointee
1487 {
1488     typedef /* see below */ type;
1489 };
1490 </pre>
1491 <table class="docutils field-list" frame="void" rules="none">
1492 <col class="field-name" />
1493 <col class="field-body" />
1494 <tbody valign="top">
1495 <tr class="field"><th class="field-name">Requires:</th><td class="field-body">For an object <tt class="docutils literal"><span class="pre">x</span></tt> of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>, <tt class="docutils literal"><span class="pre">*x</span></tt>
1496 is well-formed.  If <tt class="docutils literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
1497 ambiguous nor shall it violate access control, and
1498 <tt class="docutils literal"><span class="pre">Dereferenceable::element_type</span></tt> shall be an accessible type.
1499 Otherwise <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dereferenceable&gt;::value_type</span></tt> shall
1500 be well formed.  [Note: These requirements need not apply to
1501 explicit or partial specializations of <tt class="docutils literal"><span class="pre">pointee</span></tt>]</td>
1502 </tr>
1503 </tbody>
1504 </table>
1505 <p><tt class="docutils literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
1506 <tt class="docutils literal"><span class="pre">x</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>:</p>
1507 <pre class="literal-block">
1508 if ( ++x is ill-formed )
1509 {
1510     return ``Dereferenceable::element_type``
1511 }
1512 else if (``*x`` is a mutable reference to
1513          std::iterator_traits&lt;Dereferenceable&gt;::value_type)
1514 {
1515     return iterator_traits&lt;Dereferenceable&gt;::value_type
1516 }
1517 else
1518 {
1519     return iterator_traits&lt;Dereferenceable&gt;::value_type const
1520 }
1521 </pre>
1522 </div>
1523 <div class="section" id="class-template-indirect-reference">
1524 <h4><a class="toc-backref" href="#id43">Class template <tt class="docutils literal"><span class="pre">indirect_reference</span></tt></a></h4>
1525 <!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
1526 <!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
1527 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
1528 <pre class="literal-block">
1529 template &lt;class Dereferenceable&gt;
1530 struct indirect_reference
1531 {
1532     typedef /* see below */ type;
1533 };
1534 </pre>
1535 <table class="docutils field-list" frame="void" rules="none">
1536 <col class="field-name" />
1537 <col class="field-body" />
1538 <tbody valign="top">
1539 <tr class="field"><th class="field-name">Requires:</th><td class="field-body">For an object <tt class="docutils literal"><span class="pre">x</span></tt> of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>, <tt class="docutils literal"><span class="pre">*x</span></tt>
1540 is well-formed.  If <tt class="docutils literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
1541 ambiguous nor shall it violate access control, and
1542 <tt class="docutils literal"><span class="pre">pointee&lt;Dereferenceable&gt;::type&amp;</span></tt> shall be well-formed.
1543 Otherwise <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dereferenceable&gt;::reference</span></tt> shall
1544 be well formed.  [Note: These requirements need not apply to
1545 explicit or partial specializations of <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>]</td>
1546 </tr>
1547 </tbody>
1548 </table>
1549 <p><tt class="docutils literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
1550 <tt class="docutils literal"><span class="pre">x</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>:</p>
1551 <pre class="literal-block">
1552 if ( ++x is ill-formed )
1553     return ``pointee&lt;Dereferenceable&gt;::type&amp;``
1554 else
1555     std::iterator_traits&lt;Dereferenceable&gt;::reference
1556 </pre>
1557 </div>
1558 <div class="section" id="class-template-indirect-iterator">
1559 <h4><a class="toc-backref" href="#id44">Class template <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt></a></h4>
1560 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
1561 <!-- Software License, Version 1.0. (See accompanying -->
1562 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
1563 <pre class="literal-block">
1564 template &lt;
1565     class Iterator
1566   , class Value = use_default
1567   , class CategoryOrTraversal = use_default
1568   , class Reference = use_default
1569   , class Difference = use_default
1570 &gt;
1571 class indirect_iterator
1572 {
1573  public:
1574     typedef /* see below */ value_type;
1575     typedef /* see below */ reference;
1576     typedef /* see below */ pointer;
1577     typedef /* see below */ difference_type;
1578     typedef /* see below */ iterator_category;
1579
1580     indirect_iterator();
1581     indirect_iterator(Iterator x);
1582
1583     template &lt;
1584         class Iterator2, class Value2, class Category2
1585       , class Reference2, class Difference2
1586     &gt;
1587     indirect_iterator(
1588         indirect_iterator&lt;
1589              Iterator2, Value2, Category2, Reference2, Difference2
1590         &gt; const&amp; y
1591       , typename enable_if_convertible&lt;Iterator2, Iterator&gt;::type* = 0 // exposition
1592     );
1593
1594     Iterator const&amp; base() const;
1595     reference operator*() const;
1596     indirect_iterator&amp; operator++();
1597     indirect_iterator&amp; operator--();
1598 private:
1599    Iterator m_iterator; // exposition
1600 };
1601 </pre>
1602 <p>The member types of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> are defined according to
1603 the following pseudo-code, where <tt class="docutils literal"><span class="pre">V</span></tt> is
1604 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt></p>
1605 <pre class="literal-block">
1606 if (Value is use_default) then
1607     typedef remove_const&lt;pointee&lt;V&gt;::type&gt;::type value_type;
1608 else
1609     typedef remove_const&lt;Value&gt;::type value_type;
1610
1611 if (Reference is use_default) then
1612     if (Value is use_default) then
1613         typedef indirect_reference&lt;V&gt;::type reference;
1614     else
1615         typedef Value&amp; reference;
1616 else
1617     typedef Reference reference;
1618
1619 if (Value is use_default) then
1620     typedef pointee&lt;V&gt;::type* pointer;
1621 else
1622     typedef Value* pointer;
1623
1624 if (Difference is use_default)
1625     typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
1626 else
1627     typedef Difference difference_type;
1628
1629 if (CategoryOrTraversal is use_default)
1630     typedef <em>iterator-category</em> (
1631         iterator_traversal&lt;Iterator&gt;::type,``reference``,``value_type``
1632     ) iterator_category;
1633 else
1634     typedef <em>iterator-category</em> (
1635         CategoryOrTraversal,``reference``,``value_type``
1636     ) iterator_category;
1637 </pre>
1638 </div>
1639 <div class="section" id="indirect-iterator-requirements">
1640 <h4><a class="toc-backref" href="#id45"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> requirements</a></h4>
1641 <p>The expression <tt class="docutils literal"><span class="pre">*v</span></tt>, where <tt class="docutils literal"><span class="pre">v</span></tt> is an object of
1642 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>, shall be valid
1643 expression and convertible to <tt class="docutils literal"><span class="pre">reference</span></tt>.  <tt class="docutils literal"><span class="pre">Iterator</span></tt> shall
1644 model the traversal concept indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.
1645 <tt class="docutils literal"><span class="pre">Value</span></tt>, <tt class="docutils literal"><span class="pre">Reference</span></tt>, and <tt class="docutils literal"><span class="pre">Difference</span></tt> shall be chosen so
1646 that <tt class="docutils literal"><span class="pre">value_type</span></tt>, <tt class="docutils literal"><span class="pre">reference</span></tt>, and <tt class="docutils literal"><span class="pre">difference_type</span></tt> meet
1647 the requirements indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p>
1648 <p>[Note: there are further requirements on the
1649 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt> if the <tt class="docutils literal"><span class="pre">Value</span></tt>
1650 parameter is not <tt class="docutils literal"><span class="pre">use_default</span></tt>, as implied by the algorithm for
1651 deducing the default for the <tt class="docutils literal"><span class="pre">value_type</span></tt> member.]</p>
1652 </div>
1653 <div class="section" id="indirect-iterator-models">
1654 <h4><a class="toc-backref" href="#id46"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models</a></h4>
1655 <p>In addition to the concepts indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>
1656 and by <tt class="docutils literal"><span class="pre">iterator_traversal&lt;indirect_iterator&gt;::type</span></tt>, a
1657 specialization of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models the following
1658 concepts, Where <tt class="docutils literal"><span class="pre">v</span></tt> is an object of
1659 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>:</p>
1660 <blockquote>
1661 <ul class="simple">
1662 <li>Readable Iterator if <tt class="docutils literal"><span class="pre">reference(*v)</span></tt> is convertible to
1663 <tt class="docutils literal"><span class="pre">value_type</span></tt>.</li>
1664 <li>Writable Iterator if <tt class="docutils literal"><span class="pre">reference(*v)</span> <span class="pre">=</span> <span class="pre">t</span></tt> is a valid
1665 expression (where <tt class="docutils literal"><span class="pre">t</span></tt> is an object of type
1666 <tt class="docutils literal"><span class="pre">indirect_iterator::value_type</span></tt>)</li>
1667 <li>Lvalue Iterator if <tt class="docutils literal"><span class="pre">reference</span></tt> is a reference type.</li>
1668 </ul>
1669 </blockquote>
1670 <p><tt class="docutils literal"><span class="pre">indirect_iterator&lt;X,V1,C1,R1,D1&gt;</span></tt> is interoperable with
1671 <tt class="docutils literal"><span class="pre">indirect_iterator&lt;Y,V2,C2,R2,D2&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is
1672 interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
1673 </div>
1674 <div class="section" id="indirect-iterator-operations">
1675 <h4><a class="toc-backref" href="#id47"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> operations</a></h4>
1676 <p>In addition to the operations required by the concepts described
1677 above, specializations of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> provide the
1678 following operations.</p>
1679 <p><tt class="docutils literal"><span class="pre">indirect_iterator();</span></tt></p>
1680 <table class="docutils field-list" frame="void" rules="none">
1681 <col class="field-name" />
1682 <col class="field-body" />
1683 <tbody valign="top">
1684 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
1685 </tr>
1686 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> with
1687 a default-constructed <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td>
1688 </tr>
1689 </tbody>
1690 </table>
1691 <p><tt class="docutils literal"><span class="pre">indirect_iterator(Iterator</span> <span class="pre">x);</span></tt></p>
1692 <table class="docutils field-list" frame="void" rules="none">
1693 <col class="field-name" />
1694 <col class="field-body" />
1695 <tbody valign="top">
1696 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> with
1697 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
1698 </tr>
1699 </tbody>
1700 </table>
1701 <pre class="literal-block">
1702 template &lt;
1703     class Iterator2, class Value2, unsigned Access, class Traversal
1704   , class Reference2, class Difference2
1705 &gt;
1706 indirect_iterator(
1707     indirect_iterator&lt;
1708          Iterator2, Value2, Access, Traversal, Reference2, Difference2
1709     &gt; const&amp; y
1710   , typename enable_if_convertible&lt;Iterator2, Iterator&gt;::type* = 0 // exposition
1711 );
1712 </pre>
1713 <table class="docutils field-list" frame="void" rules="none">
1714 <col class="field-name" />
1715 <col class="field-body" />
1716 <tbody valign="top">
1717 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator2</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
1718 </tr>
1719 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> whose
1720 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> subobject is constructed from <tt class="docutils literal"><span class="pre">y.base()</span></tt>.</td>
1721 </tr>
1722 </tbody>
1723 </table>
1724 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
1725 <table class="docutils field-list" frame="void" rules="none">
1726 <col class="field-name" />
1727 <col class="field-body" />
1728 <tbody valign="top">
1729 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
1730 </tr>
1731 </tbody>
1732 </table>
1733 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
1734 <table class="docutils field-list" frame="void" rules="none">
1735 <col class="field-name" />
1736 <col class="field-body" />
1737 <tbody valign="top">
1738 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">**m_iterator</span></tt></td>
1739 </tr>
1740 </tbody>
1741 </table>
1742 <p><tt class="docutils literal"><span class="pre">indirect_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
1743 <table class="docutils field-list" frame="void" rules="none">
1744 <col class="field-name" />
1745 <col class="field-body" />
1746 <tbody valign="top">
1747 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator</span></tt></td>
1748 </tr>
1749 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
1750 </tr>
1751 </tbody>
1752 </table>
1753 <p><tt class="docutils literal"><span class="pre">indirect_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
1754 <table class="docutils field-list" frame="void" rules="none">
1755 <col class="field-name" />
1756 <col class="field-body" />
1757 <tbody valign="top">
1758 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator</span></tt></td>
1759 </tr>
1760 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
1761 </tr>
1762 </tbody>
1763 </table>
1764 </div>
1765 </div>
1766 <div class="section" id="reverse-iterator">
1767 <h3><a class="toc-backref" href="#id48">Reverse iterator</a></h3>
1768 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
1769 <!-- Software License, Version 1.0. (See accompanying -->
1770 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
1771 <p>The reverse iterator adaptor iterates through the adapted iterator
1772 range in the opposite direction.</p>
1773 <div class="section" id="class-template-reverse-iterator">
1774 <h4><a class="toc-backref" href="#id49">Class template <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt></a></h4>
1775 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
1776 <!-- Software License, Version 1.0. (See accompanying -->
1777 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
1778 <pre class="literal-block">
1779 template &lt;class Iterator&gt;
1780 class reverse_iterator
1781 {
1782 public:
1783   typedef iterator_traits&lt;Iterator&gt;::value_type value_type;
1784   typedef iterator_traits&lt;Iterator&gt;::reference reference;
1785   typedef iterator_traits&lt;Iterator&gt;::pointer pointer;
1786   typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
1787   typedef /* see below */ iterator_category;
1788
1789   reverse_iterator() {}
1790   explicit reverse_iterator(Iterator x) ;
1791
1792   template&lt;class OtherIterator&gt;
1793   reverse_iterator(
1794       reverse_iterator&lt;OtherIterator&gt; const&amp; r
1795     , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
1796   );
1797   Iterator const&amp; base() const;
1798   reference operator*() const;
1799   reverse_iterator&amp; operator++();
1800   reverse_iterator&amp; operator--();
1801 private:
1802   Iterator m_iterator; // exposition
1803 };
1804 </pre>
1805 <p>If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Random Access Traversal Iterator and Readable
1806 Lvalue Iterator, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
1807 <tt class="docutils literal"><span class="pre">random_access_iterator_tag</span></tt>. Otherwise, if
1808 <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Bidirectional Traversal Iterator and Readable
1809 Lvalue Iterator, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
1810 <tt class="docutils literal"><span class="pre">bidirectional_iterator_tag</span></tt>. Otherwise, <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
1811 convertible to <tt class="docutils literal"><span class="pre">input_iterator_tag</span></tt>.</p>
1812 </div>
1813 <div class="section" id="reverse-iterator-requirements">
1814 <h4><a class="toc-backref" href="#id50"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> requirements</a></h4>
1815 <p><tt class="docutils literal"><span class="pre">Iterator</span></tt> must be a model of Bidirectional Traversal Iterator.  The
1816 type <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::reference</span></tt> must be the type of
1817 <tt class="docutils literal"><span class="pre">*i</span></tt>, where <tt class="docutils literal"><span class="pre">i</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</p>
1818 </div>
1819 <div class="section" id="reverse-iterator-models">
1820 <h4><a class="toc-backref" href="#id51"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> models</a></h4>
1821 <p>A specialization of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> models the same iterator
1822 traversal and iterator access concepts modeled by its <tt class="docutils literal"><span class="pre">Iterator</span></tt>
1823 argument.  In addition, it may model old iterator concepts
1824 specified in the following table:</p>
1825 <table border="1" class="docutils">
1826 <colgroup>
1827 <col width="53%" />
1828 <col width="47%" />
1829 </colgroup>
1830 <thead valign="bottom">
1831 <tr><th class="head">If <tt class="docutils literal"><span class="pre">I</span></tt> models</th>
1832 <th class="head">then <tt class="docutils literal"><span class="pre">reverse_iterator&lt;I&gt;</span></tt> models</th>
1833 </tr>
1834 </thead>
1835 <tbody valign="top">
1836 <tr><td>Readable Lvalue Iterator,
1837 Bidirectional Traversal Iterator</td>
1838 <td>Bidirectional Iterator</td>
1839 </tr>
1840 <tr><td>Writable Lvalue Iterator,
1841 Bidirectional Traversal Iterator</td>
1842 <td>Mutable Bidirectional Iterator</td>
1843 </tr>
1844 <tr><td>Readable Lvalue Iterator,
1845 Random Access Traversal Iterator</td>
1846 <td>Random Access Iterator</td>
1847 </tr>
1848 <tr><td>Writable Lvalue Iterator,
1849 Random Access Traversal Iterator</td>
1850 <td>Mutable Random Access Iterator</td>
1851 </tr>
1852 </tbody>
1853 </table>
1854 <p><tt class="docutils literal"><span class="pre">reverse_iterator&lt;X&gt;</span></tt> is interoperable with
1855 <tt class="docutils literal"><span class="pre">reverse_iterator&lt;Y&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is interoperable with
1856 <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
1857 </div>
1858 <div class="section" id="reverse-iterator-operations">
1859 <h4><a class="toc-backref" href="#id52"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> operations</a></h4>
1860 <p>In addition to the operations required by the concepts modeled by
1861 <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt>, <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> provides the following
1862 operations.</p>
1863 <p><tt class="docutils literal"><span class="pre">reverse_iterator();</span></tt></p>
1864 <table class="docutils field-list" frame="void" rules="none">
1865 <col class="field-name" />
1866 <col class="field-body" />
1867 <tbody valign="top">
1868 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
1869 </tr>
1870 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_iterator</span></tt>
1871 default constructed.</td>
1872 </tr>
1873 </tbody>
1874 </table>
1875 <p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">reverse_iterator(Iterator</span> <span class="pre">x);</span></tt></p>
1876 <table class="docutils field-list" frame="void" rules="none">
1877 <col class="field-name" />
1878 <col class="field-body" />
1879 <tbody valign="top">
1880 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> with
1881 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
1882 </tr>
1883 </tbody>
1884 </table>
1885 <pre class="literal-block">
1886 template&lt;class OtherIterator&gt;
1887 reverse_iterator(
1888     reverse_iterator&lt;OtherIterator&gt; const&amp; r
1889   , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
1890 );
1891 </pre>
1892 <table class="docutils field-list" frame="void" rules="none">
1893 <col class="field-name" />
1894 <col class="field-body" />
1895 <tbody valign="top">
1896 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
1897 </tr>
1898 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs instance of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> whose
1899 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> subobject is constructed from <tt class="docutils literal"><span class="pre">y.base()</span></tt>.</td>
1900 </tr>
1901 </tbody>
1902 </table>
1903 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
1904 <table class="docutils field-list" frame="void" rules="none">
1905 <col class="field-name" />
1906 <col class="field-body" />
1907 <tbody valign="top">
1908 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
1909 </tr>
1910 </tbody>
1911 </table>
1912 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
1913 <table class="docutils field-list" frame="void" rules="none">
1914 <col class="field-name" />
1915 <col class="field-body" />
1916 <tbody valign="top">
1917 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"></td>
1918 </tr>
1919 </tbody>
1920 </table>
1921 <pre class="literal-block">
1922 Iterator tmp = m_iterator;
1923 return *--tmp;
1924 </pre>
1925 <p><tt class="docutils literal"><span class="pre">reverse_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
1926 <table class="docutils field-list" frame="void" rules="none">
1927 <col class="field-name" />
1928 <col class="field-body" />
1929 <tbody valign="top">
1930 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator</span></tt></td>
1931 </tr>
1932 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
1933 </tr>
1934 </tbody>
1935 </table>
1936 <p><tt class="docutils literal"><span class="pre">reverse_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
1937 <table class="docutils field-list" frame="void" rules="none">
1938 <col class="field-name" />
1939 <col class="field-body" />
1940 <tbody valign="top">
1941 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator</span></tt></td>
1942 </tr>
1943 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
1944 </tr>
1945 </tbody>
1946 </table>
1947 </div>
1948 </div>
1949 <div class="section" id="transform-iterator">
1950 <h3><a class="toc-backref" href="#id53">Transform iterator</a></h3>
1951 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
1952 <!-- Software License, Version 1.0. (See accompanying -->
1953 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
1954 <p>The transform iterator adapts an iterator by modifying the
1955 <tt class="docutils literal"><span class="pre">operator*</span></tt> to apply a function object to the result of
1956 dereferencing the iterator and returning the result.</p>
1957 <div class="section" id="class-template-transform-iterator">
1958 <h4><a class="toc-backref" href="#id54">Class template <tt class="docutils literal"><span class="pre">transform_iterator</span></tt></a></h4>
1959 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
1960 <!-- Software License, Version 1.0. (See accompanying -->
1961 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
1962 <!-- Version 1.3 of this document was accepted for TR1 -->
1963 <pre class="literal-block">
1964 template &lt;class UnaryFunction,
1965           class Iterator,
1966           class Reference = use_default,
1967           class Value = use_default&gt;
1968 class transform_iterator
1969 {
1970 public:
1971   typedef /* see below */ value_type;
1972   typedef /* see below */ reference;
1973   typedef /* see below */ pointer;
1974   typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
1975   typedef /* see below */ iterator_category;
1976
1977   transform_iterator();
1978   transform_iterator(Iterator const&amp; x, UnaryFunction f);
1979
1980   template&lt;class F2, class I2, class R2, class V2&gt;
1981   transform_iterator(
1982         transform_iterator&lt;F2, I2, R2, V2&gt; const&amp; t
1983       , typename enable_if_convertible&lt;I2, Iterator&gt;::type* = 0      // exposition only
1984       , typename enable_if_convertible&lt;F2, UnaryFunction&gt;::type* = 0 // exposition only
1985   );
1986   UnaryFunction functor() const;
1987   Iterator const&amp; base() const;
1988   reference operator*() const;
1989   transform_iterator&amp; operator++();
1990   transform_iterator&amp; operator--();
1991 private:
1992   Iterator m_iterator; // exposition only
1993   UnaryFunction m_f;   // exposition only
1994 };
1995 </pre>
1996 <p>If <tt class="docutils literal"><span class="pre">Reference</span></tt> is <tt class="docutils literal"><span class="pre">use_default</span></tt> then the <tt class="docutils literal"><span class="pre">reference</span></tt> member of
1997 <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> is
1998 <tt class="docutils literal"><span class="pre">result_of&lt;UnaryFunction(iterator_traits&lt;Iterator&gt;::reference)&gt;::type</span></tt>.
1999 Otherwise, <tt class="docutils literal"><span class="pre">reference</span></tt> is <tt class="docutils literal"><span class="pre">Reference</span></tt>.</p>
2000 <p>If <tt class="docutils literal"><span class="pre">Value</span></tt> is <tt class="docutils literal"><span class="pre">use_default</span></tt> then the <tt class="docutils literal"><span class="pre">value_type</span></tt> member is
2001 <tt class="docutils literal"><span class="pre">remove_cv&lt;remove_reference&lt;reference&gt;</span> <span class="pre">&gt;::type</span></tt>.  Otherwise,
2002 <tt class="docutils literal"><span class="pre">value_type</span></tt> is <tt class="docutils literal"><span class="pre">Value</span></tt>.</p>
2003 <p>If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Readable Lvalue Iterator and if <tt class="docutils literal"><span class="pre">Iterator</span></tt>
2004 models Random Access Traversal Iterator, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
2005 convertible to <tt class="docutils literal"><span class="pre">random_access_iterator_tag</span></tt>. Otherwise, if
2006 <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Bidirectional Traversal Iterator, then
2007 <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
2008 <tt class="docutils literal"><span class="pre">bidirectional_iterator_tag</span></tt>.  Otherwise <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
2009 convertible to <tt class="docutils literal"><span class="pre">forward_iterator_tag</span></tt>. If <tt class="docutils literal"><span class="pre">Iterator</span></tt> does not
2010 model Readable Lvalue Iterator then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
2011 convertible to <tt class="docutils literal"><span class="pre">input_iterator_tag</span></tt>.</p>
2012 </div>
2013 <div class="section" id="transform-iterator-requirements">
2014 <h4><a class="toc-backref" href="#id55"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> requirements</a></h4>
2015 <p>The type <tt class="docutils literal"><span class="pre">UnaryFunction</span></tt> must be Assignable, Copy Constructible, and
2016 the expression <tt class="docutils literal"><span class="pre">f(*i)</span></tt> must be valid where <tt class="docutils literal"><span class="pre">f</span></tt> is an object of
2017 type <tt class="docutils literal"><span class="pre">UnaryFunction</span></tt>, <tt class="docutils literal"><span class="pre">i</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Iterator</span></tt>, and
2018 where the type of <tt class="docutils literal"><span class="pre">f(*i)</span></tt> must be
2019 <tt class="docutils literal"><span class="pre">result_of&lt;UnaryFunction(iterator_traits&lt;Iterator&gt;::reference)&gt;::type</span></tt>.</p>
2020 <p>The argument <tt class="docutils literal"><span class="pre">Iterator</span></tt> shall model Readable Iterator.</p>
2021 </div>
2022 <div class="section" id="transform-iterator-models">
2023 <h4><a class="toc-backref" href="#id56"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models</a></h4>
2024 <p>The resulting <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models the most refined of the
2025 following that is also modeled by <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</p>
2026 <blockquote>
2027 <ul class="simple">
2028 <li>Writable Lvalue Iterator if <tt class="docutils literal"><span class="pre">transform_iterator::reference</span></tt> is a non-const reference.</li>
2029 <li>Readable Lvalue Iterator if <tt class="docutils literal"><span class="pre">transform_iterator::reference</span></tt> is a const reference.</li>
2030 <li>Readable Iterator otherwise.</li>
2031 </ul>
2032 </blockquote>
2033 <p>The <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models the most refined standard traversal
2034 concept that is modeled by the <tt class="docutils literal"><span class="pre">Iterator</span></tt> argument.</p>
2035 <p>If <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> is a model of Readable Lvalue Iterator then
2036 it models the following original iterator concepts depending on what
2037 the <tt class="docutils literal"><span class="pre">Iterator</span></tt> argument models.</p>
2038 <table border="1" class="docutils">
2039 <colgroup>
2040 <col width="47%" />
2041 <col width="53%" />
2042 </colgroup>
2043 <thead valign="bottom">
2044 <tr><th class="head">If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models</th>
2045 <th class="head">then <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models</th>
2046 </tr>
2047 </thead>
2048 <tbody valign="top">
2049 <tr><td>Single Pass Iterator</td>
2050 <td>Input Iterator</td>
2051 </tr>
2052 <tr><td>Forward Traversal Iterator</td>
2053 <td>Forward Iterator</td>
2054 </tr>
2055 <tr><td>Bidirectional Traversal Iterator</td>
2056 <td>Bidirectional Iterator</td>
2057 </tr>
2058 <tr><td>Random Access Traversal Iterator</td>
2059 <td>Random Access Iterator</td>
2060 </tr>
2061 </tbody>
2062 </table>
2063 <p>If <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models Writable Lvalue Iterator then it is a
2064 mutable iterator (as defined in the old iterator requirements).</p>
2065 <p><tt class="docutils literal"><span class="pre">transform_iterator&lt;F1,</span> <span class="pre">X,</span> <span class="pre">R1,</span> <span class="pre">V1&gt;</span></tt> is interoperable with
2066 <tt class="docutils literal"><span class="pre">transform_iterator&lt;F2,</span> <span class="pre">Y,</span> <span class="pre">R2,</span> <span class="pre">V2&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is
2067 interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
2068 </div>
2069 <div class="section" id="transform-iterator-operations">
2070 <h4><a class="toc-backref" href="#id57"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> operations</a></h4>
2071 <p>In addition to the operations required by the concepts modeled by
2072 <tt class="docutils literal"><span class="pre">transform_iterator</span></tt>, <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> provides the following
2073 operations.</p>
2074 <p><tt class="docutils literal"><span class="pre">transform_iterator();</span></tt></p>
2075 <table class="docutils field-list" frame="void" rules="none">
2076 <col class="field-name" />
2077 <col class="field-body" />
2078 <tbody valign="top">
2079 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
2080 and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> default constructed.</td>
2081 </tr>
2082 </tbody>
2083 </table>
2084 <p><tt class="docutils literal"><span class="pre">transform_iterator(Iterator</span> <span class="pre">const&amp;</span> <span class="pre">x,</span> <span class="pre">UnaryFunction</span> <span class="pre">f);</span></tt></p>
2085 <table class="docutils field-list" frame="void" rules="none">
2086 <col class="field-name" />
2087 <col class="field-body" />
2088 <tbody valign="top">
2089 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
2090 initialized to <tt class="docutils literal"><span class="pre">f</span></tt> and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> initialized to <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
2091 </tr>
2092 </tbody>
2093 </table>
2094 <pre class="literal-block">
2095 template&lt;class F2, class I2, class R2, class V2&gt;
2096 transform_iterator(
2097       transform_iterator&lt;F2, I2, R2, V2&gt; const&amp; t
2098     , typename enable_if_convertible&lt;I2, Iterator&gt;::type* = 0      // exposition only
2099     , typename enable_if_convertible&lt;F2, UnaryFunction&gt;::type* = 0 // exposition only
2100 );
2101 </pre>
2102 <table class="docutils field-list" frame="void" rules="none">
2103 <col class="field-name" />
2104 <col class="field-body" />
2105 <tbody valign="top">
2106 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
2107 initialized to <tt class="docutils literal"><span class="pre">t.functor()</span></tt> and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> initialized to
2108 <tt class="docutils literal"><span class="pre">t.base()</span></tt>.</td>
2109 </tr>
2110 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
2111 </tr>
2112 </tbody>
2113 </table>
2114 <p><tt class="docutils literal"><span class="pre">UnaryFunction</span> <span class="pre">functor()</span> <span class="pre">const;</span></tt></p>
2115 <table class="docutils field-list" frame="void" rules="none">
2116 <col class="field-name" />
2117 <col class="field-body" />
2118 <tbody valign="top">
2119 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_f</span></tt></td>
2120 </tr>
2121 </tbody>
2122 </table>
2123 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
2124 <table class="docutils field-list" frame="void" rules="none">
2125 <col class="field-name" />
2126 <col class="field-body" />
2127 <tbody valign="top">
2128 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
2129 </tr>
2130 </tbody>
2131 </table>
2132 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
2133 <table class="docutils field-list" frame="void" rules="none">
2134 <col class="field-name" />
2135 <col class="field-body" />
2136 <tbody valign="top">
2137 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_f(*m_iterator)</span></tt></td>
2138 </tr>
2139 </tbody>
2140 </table>
2141 <p><tt class="docutils literal"><span class="pre">transform_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
2142 <table class="docutils field-list" frame="void" rules="none">
2143 <col class="field-name" />
2144 <col class="field-body" />
2145 <tbody valign="top">
2146 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator</span></tt></td>
2147 </tr>
2148 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
2149 </tr>
2150 </tbody>
2151 </table>
2152 <p><tt class="docutils literal"><span class="pre">transform_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
2153 <table class="docutils field-list" frame="void" rules="none">
2154 <col class="field-name" />
2155 <col class="field-body" />
2156 <tbody valign="top">
2157 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator</span></tt></td>
2158 </tr>
2159 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
2160 </tr>
2161 </tbody>
2162 </table>
2163 </div>
2164 </div>
2165 <div class="section" id="filter-iterator">
2166 <h3><a class="toc-backref" href="#id58">Filter iterator</a></h3>
2167 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
2168 <!-- Software License, Version 1.0. (See accompanying -->
2169 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
2170 <p>The filter iterator adaptor creates a view of an iterator range in
2171 which some elements of the range are skipped. A predicate function
2172 object controls which elements are skipped. When the predicate is
2173 applied to an element, if it returns <tt class="docutils literal"><span class="pre">true</span></tt> then the element is
2174 retained and if it returns <tt class="docutils literal"><span class="pre">false</span></tt> then the element is skipped
2175 over. When skipping over elements, it is necessary for the filter
2176 adaptor to know when to stop so as to avoid going past the end of the
2177 underlying range. A filter iterator is therefore constructed with pair
2178 of iterators indicating the range of elements in the unfiltered
2179 sequence to be traversed.</p>
2180 <div class="section" id="class-template-filter-iterator">
2181 <h4><a class="toc-backref" href="#id59">Class template <tt class="docutils literal"><span class="pre">filter_iterator</span></tt></a></h4>
2182 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt -->
2183 <!-- 2004. Use, modification and distribution is subject to the Boost -->
2184 <!-- Software License, Version 1.0. (See accompanying  file -->
2185 <!-- LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
2186 <pre class="literal-block">
2187 template &lt;class Predicate, class Iterator&gt;
2188 class filter_iterator
2189 {
2190  public:
2191     typedef iterator_traits&lt;Iterator&gt;::value_type value_type;
2192     typedef iterator_traits&lt;Iterator&gt;::reference reference;
2193     typedef iterator_traits&lt;Iterator&gt;::pointer pointer;
2194     typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
2195     typedef /* see below */ iterator_category;
2196
2197     filter_iterator();
2198     filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());
2199     filter_iterator(Iterator x, Iterator end = Iterator());
2200     template&lt;class OtherIterator&gt;
2201     filter_iterator(
2202         filter_iterator&lt;Predicate, OtherIterator&gt; const&amp; t
2203         , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
2204         );
2205     Predicate predicate() const;
2206     Iterator end() const;
2207     Iterator const&amp; base() const;
2208     reference operator*() const;
2209     filter_iterator&amp; operator++();
2210 private:
2211     Predicate m_pred; // exposition only
2212     Iterator m_iter;  // exposition only
2213     Iterator m_end;   // exposition only
2214 };
2215 </pre>
2216 <p>If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Readable Lvalue Iterator and Bidirectional Traversal
2217 Iterator then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
2218 <tt class="docutils literal"><span class="pre">std::bidirectional_iterator_tag</span></tt>.
2219 Otherwise, if <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Readable Lvalue Iterator and Forward Traversal
2220 Iterator then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
2221 <tt class="docutils literal"><span class="pre">std::forward_iterator_tag</span></tt>.
2222 Otherwise <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
2223 convertible to <tt class="docutils literal"><span class="pre">std::input_iterator_tag</span></tt>.</p>
2224 </div>
2225 <div class="section" id="filter-iterator-requirements">
2226 <h4><a class="toc-backref" href="#id60"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt> requirements</a></h4>
2227 <p>The <tt class="docutils literal"><span class="pre">Iterator</span></tt> argument shall meet the requirements of Readable
2228 Iterator and Single Pass Iterator or it shall meet the requirements of
2229 Input Iterator.</p>
2230 <p>The <tt class="docutils literal"><span class="pre">Predicate</span></tt> argument must be Assignable, Copy Constructible, and
2231 the expression <tt class="docutils literal"><span class="pre">p(x)</span></tt> must be valid where <tt class="docutils literal"><span class="pre">p</span></tt> is an object of type
2232 <tt class="docutils literal"><span class="pre">Predicate</span></tt>, <tt class="docutils literal"><span class="pre">x</span></tt> is an object of type
2233 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>, and where the type of
2234 <tt class="docutils literal"><span class="pre">p(x)</span></tt> must be convertible to <tt class="docutils literal"><span class="pre">bool</span></tt>.</p>
2235 </div>
2236 <div class="section" id="filter-iterator-models">
2237 <h4><a class="toc-backref" href="#id61"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models</a></h4>
2238 <p>The concepts that <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models are dependent on which
2239 concepts the <tt class="docutils literal"><span class="pre">Iterator</span></tt> argument models, as specified in the
2240 following tables.</p>
2241 <table border="1" class="docutils">
2242 <colgroup>
2243 <col width="44%" />
2244 <col width="56%" />
2245 </colgroup>
2246 <thead valign="bottom">
2247 <tr><th class="head">If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models</th>
2248 <th class="head">then <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models</th>
2249 </tr>
2250 </thead>
2251 <tbody valign="top">
2252 <tr><td>Single Pass Iterator</td>
2253 <td>Single Pass Iterator</td>
2254 </tr>
2255 <tr><td>Forward Traversal Iterator</td>
2256 <td>Forward Traversal Iterator</td>
2257 </tr>
2258 <tr><td>Bidirectional Traversal Iterator</td>
2259 <td>Bidirectional Traversal Iterator</td>
2260 </tr>
2261 </tbody>
2262 </table>
2263 <table border="1" class="docutils">
2264 <colgroup>
2265 <col width="41%" />
2266 <col width="59%" />
2267 </colgroup>
2268 <thead valign="bottom">
2269 <tr><th class="head">If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models</th>
2270 <th class="head">then <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models</th>
2271 </tr>
2272 </thead>
2273 <tbody valign="top">
2274 <tr><td>Readable Iterator</td>
2275 <td>Readable Iterator</td>
2276 </tr>
2277 <tr><td>Writable Iterator</td>
2278 <td>Writable Iterator</td>
2279 </tr>
2280 <tr><td>Lvalue Iterator</td>
2281 <td>Lvalue Iterator</td>
2282 </tr>
2283 </tbody>
2284 </table>
2285 <table border="1" class="docutils">
2286 <colgroup>
2287 <col width="63%" />
2288 <col width="38%" />
2289 </colgroup>
2290 <thead valign="bottom">
2291 <tr><th class="head">If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models</th>
2292 <th class="head">then <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models</th>
2293 </tr>
2294 </thead>
2295 <tbody valign="top">
2296 <tr><td>Readable Iterator, Single Pass Iterator</td>
2297 <td>Input Iterator</td>
2298 </tr>
2299 <tr><td>Readable Lvalue Iterator, Forward Traversal Iterator</td>
2300 <td>Forward Iterator</td>
2301 </tr>
2302 <tr><td>Writable Lvalue Iterator, Forward Traversal Iterator</td>
2303 <td>Mutable Forward Iterator</td>
2304 </tr>
2305 <tr><td>Writable Lvalue Iterator, Bidirectional Iterator</td>
2306 <td>Mutable Bidirectional Iterator</td>
2307 </tr>
2308 </tbody>
2309 </table>
2310 <p><tt class="docutils literal"><span class="pre">filter_iterator&lt;P1,</span> <span class="pre">X&gt;</span></tt> is interoperable with <tt class="docutils literal"><span class="pre">filter_iterator&lt;P2,</span> <span class="pre">Y&gt;</span></tt>
2311 if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
2312 </div>
2313 <div class="section" id="filter-iterator-operations">
2314 <h4><a class="toc-backref" href="#id62"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt> operations</a></h4>
2315 <p>In addition to those operations required by the concepts that
2316 <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models, <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> provides the following
2317 operations.</p>
2318 <p><tt class="docutils literal"><span class="pre">filter_iterator();</span></tt></p>
2319 <table class="docutils field-list" frame="void" rules="none">
2320 <col class="field-name" />
2321 <col class="field-body" />
2322 <tbody valign="top">
2323 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Predicate</span></tt> and <tt class="docutils literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
2324 </tr>
2325 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> whose``m_pred``,  <tt class="docutils literal"><span class="pre">m_iter</span></tt>, and <tt class="docutils literal"><span class="pre">m_end</span></tt>
2326 members are a default constructed.</td>
2327 </tr>
2328 </tbody>
2329 </table>
2330 <p><tt class="docutils literal"><span class="pre">filter_iterator(Predicate</span> <span class="pre">f,</span> <span class="pre">Iterator</span> <span class="pre">x,</span> <span class="pre">Iterator</span> <span class="pre">end</span> <span class="pre">=</span> <span class="pre">Iterator());</span></tt></p>
2331 <table class="docutils field-list" frame="void" rules="none">
2332 <col class="field-name" />
2333 <col class="field-body" />
2334 <tbody valign="top">
2335 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> where <tt class="docutils literal"><span class="pre">m_iter</span></tt> is either
2336 the first position in the range <tt class="docutils literal"><span class="pre">[x,end)</span></tt> such that <tt class="docutils literal"><span class="pre">f(*m_iter)</span> <span class="pre">==</span> <span class="pre">true</span></tt>
2337 or else``m_iter == end``. The member <tt class="docutils literal"><span class="pre">m_pred</span></tt> is constructed from
2338 <tt class="docutils literal"><span class="pre">f</span></tt> and <tt class="docutils literal"><span class="pre">m_end</span></tt> from <tt class="docutils literal"><span class="pre">end</span></tt>.</td>
2339 </tr>
2340 </tbody>
2341 </table>
2342 <p><tt class="docutils literal"><span class="pre">filter_iterator(Iterator</span> <span class="pre">x,</span> <span class="pre">Iterator</span> <span class="pre">end</span> <span class="pre">=</span> <span class="pre">Iterator());</span></tt></p>
2343 <table class="docutils field-list" frame="void" rules="none">
2344 <col class="field-name" />
2345 <col class="field-body" />
2346 <tbody valign="top">
2347 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Predicate</span></tt> must be Default Constructible and
2348 <tt class="docutils literal"><span class="pre">Predicate</span></tt> is a class type (not a function pointer).</td>
2349 </tr>
2350 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> where <tt class="docutils literal"><span class="pre">m_iter</span></tt> is either
2351 the first position in the range <tt class="docutils literal"><span class="pre">[x,end)</span></tt> such that <tt class="docutils literal"><span class="pre">m_pred(*m_iter)</span> <span class="pre">==</span> <span class="pre">true</span></tt>
2352 or else``m_iter == end``. The member <tt class="docutils literal"><span class="pre">m_pred</span></tt> is default constructed.</td>
2353 </tr>
2354 </tbody>
2355 </table>
2356 <pre class="literal-block">
2357 template &lt;class OtherIterator&gt;
2358 filter_iterator(
2359     filter_iterator&lt;Predicate, OtherIterator&gt; const&amp; t
2360     , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
2361     );``
2362 </pre>
2363 <table class="docutils field-list" frame="void" rules="none">
2364 <col class="field-name" />
2365 <col class="field-body" />
2366 <tbody valign="top">
2367 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
2368 </tr>
2369 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a filter iterator whose members are copied from <tt class="docutils literal"><span class="pre">t</span></tt>.</td>
2370 </tr>
2371 </tbody>
2372 </table>
2373 <p><tt class="docutils literal"><span class="pre">Predicate</span> <span class="pre">predicate()</span> <span class="pre">const;</span></tt></p>
2374 <table class="docutils field-list" frame="void" rules="none">
2375 <col class="field-name" />
2376 <col class="field-body" />
2377 <tbody valign="top">
2378 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_pred</span></tt></td>
2379 </tr>
2380 </tbody>
2381 </table>
2382 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">end()</span> <span class="pre">const;</span></tt></p>
2383 <table class="docutils field-list" frame="void" rules="none">
2384 <col class="field-name" />
2385 <col class="field-body" />
2386 <tbody valign="top">
2387 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_end</span></tt></td>
2388 </tr>
2389 </tbody>
2390 </table>
2391 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
2392 <table class="docutils field-list" frame="void" rules="none">
2393 <col class="field-name" />
2394 <col class="field-body" />
2395 <tbody valign="top">
2396 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
2397 </tr>
2398 </tbody>
2399 </table>
2400 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
2401 <table class="docutils field-list" frame="void" rules="none">
2402 <col class="field-name" />
2403 <col class="field-body" />
2404 <tbody valign="top">
2405 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*m_iter</span></tt></td>
2406 </tr>
2407 </tbody>
2408 </table>
2409 <p><tt class="docutils literal"><span class="pre">filter_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
2410 <table class="docutils field-list" frame="void" rules="none">
2411 <col class="field-name" />
2412 <col class="field-body" />
2413 <tbody valign="top">
2414 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Increments <tt class="docutils literal"><span class="pre">m_iter</span></tt> and then continues to
2415 increment <tt class="docutils literal"><span class="pre">m_iter</span></tt> until either <tt class="docutils literal"><span class="pre">m_iter</span> <span class="pre">==</span> <span class="pre">m_end</span></tt>
2416 or <tt class="docutils literal"><span class="pre">m_pred(*m_iter)</span> <span class="pre">==</span> <span class="pre">true</span></tt>.</td>
2417 </tr>
2418 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
2419 </tr>
2420 </tbody>
2421 </table>
2422 </div>
2423 </div>
2424 <div class="section" id="counting-iterator">
2425 <h3><a class="toc-backref" href="#id63">Counting iterator</a></h3>
2426 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
2427 <!-- Software License, Version 1.0. (See accompanying -->
2428 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
2429 <p><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> adapts an object by adding an <tt class="docutils literal"><span class="pre">operator*</span></tt> that
2430 returns the current value of the object. All other iterator operations
2431 are forwarded to the adapted object.</p>
2432 <div class="section" id="class-template-counting-iterator">
2433 <h4><a class="toc-backref" href="#id64">Class template <tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a></h4>
2434 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
2435 <!-- Software License, Version 1.0. (See accompanying -->
2436 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
2437 <pre class="literal-block">
2438 template &lt;
2439     class Incrementable
2440   , class CategoryOrTraversal = use_default
2441   , class Difference = use_default
2442 &gt;
2443 class counting_iterator
2444 {
2445 public:
2446     typedef Incrementable value_type;
2447     typedef const Incrementable&amp; reference;
2448     typedef const Incrementable* pointer;
2449     typedef /* see below */ difference_type;
2450     typedef /* see below */ iterator_category;
2451
2452     counting_iterator();
2453     counting_iterator(counting_iterator const&amp; rhs);
2454     explicit counting_iterator(Incrementable x);
2455     Incrementable const&amp; base() const;
2456     reference operator*() const;
2457     counting_iterator&amp; operator++();
2458     counting_iterator&amp; operator--();
2459 private:
2460     Incrementable m_inc; // exposition
2461 };
2462 </pre>
2463 <p>If the <tt class="docutils literal"><span class="pre">Difference</span></tt> argument is <tt class="docutils literal"><span class="pre">use_default</span></tt> then
2464 <tt class="docutils literal"><span class="pre">difference_type</span></tt> is an unspecified signed integral
2465 type. Otherwise <tt class="docutils literal"><span class="pre">difference_type</span></tt> is <tt class="docutils literal"><span class="pre">Difference</span></tt>.</p>
2466 <p><tt class="docutils literal"><span class="pre">iterator_category</span></tt> is determined according to the following
2467 algorithm:</p>
2468 <pre class="literal-block">
2469 if (CategoryOrTraversal is not use_default)
2470     return CategoryOrTraversal
2471 else if (numeric_limits&lt;Incrementable&gt;::is_specialized)
2472     return <a class="reference internal" href="#id12"><em>iterator-category</em></a>(
2473         random_access_traversal_tag, Incrementable, const Incrementable&amp;)
2474 else
2475     return <a class="reference internal" href="#id12"><em>iterator-category</em></a>(
2476          iterator_traversal&lt;Incrementable&gt;::type,
2477          Incrementable, const Incrementable&amp;)
2478 </pre>
2479 <dl class="docutils">
2480 <dt>[<em>Note:</em> implementers are encouraged to provide an implementation of</dt>
2481 <dd><tt class="docutils literal"><span class="pre">operator-</span></tt> and a <tt class="docutils literal"><span class="pre">difference_type</span></tt> that avoids overflows in
2482 the cases where <tt class="docutils literal"><span class="pre">std::numeric_limits&lt;Incrementable&gt;::is_specialized</span></tt>
2483 is true.]</dd>
2484 </dl>
2485 </div>
2486 <div class="section" id="counting-iterator-requirements">
2487 <h4><a class="toc-backref" href="#id65"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> requirements</a></h4>
2488 <p>The <tt class="docutils literal"><span class="pre">Incrementable</span></tt> argument shall be Copy Constructible and Assignable.</p>
2489 <p>If <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to <tt class="docutils literal"><span class="pre">forward_iterator_tag</span></tt>
2490 or <tt class="docutils literal"><span class="pre">forward_traversal_tag</span></tt>, the following must be well-formed:</p>
2491 <pre class="literal-block">
2492 Incrementable i, j;
2493 ++i;         // pre-increment
2494 i == j;      // operator equal
2495 </pre>
2496 <p>If <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
2497 <tt class="docutils literal"><span class="pre">bidirectional_iterator_tag</span></tt> or <tt class="docutils literal"><span class="pre">bidirectional_traversal_tag</span></tt>,
2498 the following expression must also be well-formed:</p>
2499 <pre class="literal-block">
2500 --i
2501 </pre>
2502 <p>If <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
2503 <tt class="docutils literal"><span class="pre">random_access_iterator_tag</span></tt> or <tt class="docutils literal"><span class="pre">random_access_traversal_tag</span></tt>,
2504 the following must must also be valid:</p>
2505 <pre class="literal-block">
2506 counting_iterator::difference_type n;
2507 i += n;
2508 n = i - j;
2509 i &lt; j;
2510 </pre>
2511 </div>
2512 <div class="section" id="counting-iterator-models">
2513 <h4><a class="toc-backref" href="#id66"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models</a></h4>
2514 <p>Specializations of <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> model Readable Lvalue
2515 Iterator. In addition, they model the concepts corresponding to the
2516 iterator tags to which their <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible.
2517 Also, if <tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt> is not <tt class="docutils literal"><span class="pre">use_default</span></tt> then
2518 <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models the concept corresponding to the iterator
2519 tag <tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt>.  Otherwise, if
2520 <tt class="docutils literal"><span class="pre">numeric_limits&lt;Incrementable&gt;::is_specialized</span></tt>, then
2521 <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models Random Access Traversal Iterator.
2522 Otherwise, <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models the same iterator traversal
2523 concepts modeled by <tt class="docutils literal"><span class="pre">Incrementable</span></tt>.</p>
2524 <p><tt class="docutils literal"><span class="pre">counting_iterator&lt;X,C1,D1&gt;</span></tt> is interoperable with
2525 <tt class="docutils literal"><span class="pre">counting_iterator&lt;Y,C2,D2&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is
2526 interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
2527 </div>
2528 <div class="section" id="counting-iterator-operations">
2529 <h4><a class="toc-backref" href="#id67"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> operations</a></h4>
2530 <p>In addition to the operations required by the concepts modeled by
2531 <tt class="docutils literal"><span class="pre">counting_iterator</span></tt>, <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> provides the following
2532 operations.</p>
2533 <p><tt class="docutils literal"><span class="pre">counting_iterator();</span></tt></p>
2534 <table class="docutils field-list" frame="void" rules="none">
2535 <col class="field-name" />
2536 <col class="field-body" />
2537 <tbody valign="top">
2538 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Incrementable</span></tt> is Default Constructible.</td>
2539 </tr>
2540 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Default construct the member <tt class="docutils literal"><span class="pre">m_inc</span></tt>.</td>
2541 </tr>
2542 </tbody>
2543 </table>
2544 <p><tt class="docutils literal"><span class="pre">counting_iterator(counting_iterator</span> <span class="pre">const&amp;</span> <span class="pre">rhs);</span></tt></p>
2545 <table class="docutils field-list" frame="void" rules="none">
2546 <col class="field-name" />
2547 <col class="field-body" />
2548 <tbody valign="top">
2549 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Construct member <tt class="docutils literal"><span class="pre">m_inc</span></tt> from <tt class="docutils literal"><span class="pre">rhs.m_inc</span></tt>.</td>
2550 </tr>
2551 </tbody>
2552 </table>
2553 <p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">counting_iterator(Incrementable</span> <span class="pre">x);</span></tt></p>
2554 <table class="docutils field-list" frame="void" rules="none">
2555 <col class="field-name" />
2556 <col class="field-body" />
2557 <tbody valign="top">
2558 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Construct member <tt class="docutils literal"><span class="pre">m_inc</span></tt> from <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
2559 </tr>
2560 </tbody>
2561 </table>
2562 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
2563 <table class="docutils field-list" frame="void" rules="none">
2564 <col class="field-name" />
2565 <col class="field-body" />
2566 <tbody valign="top">
2567 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_inc</span></tt></td>
2568 </tr>
2569 </tbody>
2570 </table>
2571 <p><tt class="docutils literal"><span class="pre">counting_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
2572 <table class="docutils field-list" frame="void" rules="none">
2573 <col class="field-name" />
2574 <col class="field-body" />
2575 <tbody valign="top">
2576 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_inc</span></tt></td>
2577 </tr>
2578 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
2579 </tr>
2580 </tbody>
2581 </table>
2582 <p><tt class="docutils literal"><span class="pre">counting_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
2583 <table class="docutils field-list" frame="void" rules="none">
2584 <col class="field-name" />
2585 <col class="field-body" />
2586 <tbody valign="top">
2587 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_inc</span></tt></td>
2588 </tr>
2589 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
2590 </tr>
2591 </tbody>
2592 </table>
2593 <p><tt class="docutils literal"><span class="pre">Incrementable</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
2594 <table class="docutils field-list" frame="void" rules="none">
2595 <col class="field-name" />
2596 <col class="field-body" />
2597 <tbody valign="top">
2598 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_inc</span></tt></td>
2599 </tr>
2600 </tbody>
2601 </table>
2602 </div>
2603 </div>
2604 <div class="section" id="function-output-iterator">
2605 <h3><a class="toc-backref" href="#id68">Function output iterator</a></h3>
2606 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
2607 <!-- Software License, Version 1.0. (See accompanying -->
2608 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
2609 <p>The function output iterator adaptor makes it easier to create custom
2610 output iterators. The adaptor takes a unary function and creates a
2611 model of Output Iterator. Each item assigned to the output iterator is
2612 passed as an argument to the unary function.  The motivation for this
2613 iterator is that creating a conforming output iterator is non-trivial,
2614 particularly because the proper implementation usually requires a
2615 proxy object.</p>
2616 <div class="section" id="class-template-function-output-iterator">
2617 <h4><a class="toc-backref" href="#id69">Class template <tt class="docutils literal"><span class="pre">function_output_iterator</span></tt></a></h4>
2618 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
2619 <!-- Software License, Version 1.0. (See accompanying -->
2620 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
2621 </div>
2622 <div class="section" id="header">
2623 <h4><a class="toc-backref" href="#id70">Header</a></h4>
2624 <pre class="literal-block">
2625 #include &lt;boost/function_output_iterator.hpp&gt;
2626 </pre>
2627 <pre class="literal-block">
2628 template &lt;class UnaryFunction&gt;
2629 class function_output_iterator {
2630 public:
2631   typedef std::output_iterator_tag iterator_category;
2632   typedef void                     value_type;
2633   typedef void                     difference_type;
2634   typedef void                     pointer;
2635   typedef void                     reference;
2636
2637   explicit function_output_iterator();
2638
2639   explicit function_output_iterator(const UnaryFunction&amp; f);
2640
2641   /* see below */ operator*();
2642   function_output_iterator&amp; operator++();
2643   function_output_iterator&amp; operator++(int);
2644 private:
2645   UnaryFunction m_f;     // exposition only
2646 };
2647 </pre>
2648 </div>
2649 <div class="section" id="function-output-iterator-requirements">
2650 <h4><a class="toc-backref" href="#id71"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> requirements</a></h4>
2651 <p><tt class="docutils literal"><span class="pre">UnaryFunction</span></tt> must be Assignable and Copy Constructible.</p>
2652 </div>
2653 <div class="section" id="function-output-iterator-models">
2654 <h4><a class="toc-backref" href="#id72"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> models</a></h4>
2655 <p><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> is a model of the Writable and
2656 Incrementable Iterator concepts.</p>
2657 </div>
2658 <div class="section" id="function-output-iterator-operations">
2659 <h4><a class="toc-backref" href="#id73"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> operations</a></h4>
2660 <p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">function_output_iterator(const</span> <span class="pre">UnaryFunction&amp;</span> <span class="pre">f</span> <span class="pre">=</span> <span class="pre">UnaryFunction());</span></tt></p>
2661 <table class="docutils field-list" frame="void" rules="none">
2662 <col class="field-name" />
2663 <col class="field-body" />
2664 <tbody valign="top">
2665 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">function_output_iterator</span></tt>
2666 with <tt class="docutils literal"><span class="pre">m_f</span></tt> constructed from <tt class="docutils literal"><span class="pre">f</span></tt>.</td>
2667 </tr>
2668 </tbody>
2669 </table>
2670 <p><tt class="docutils literal"><span class="pre">operator*();</span></tt></p>
2671 <table class="docutils field-list" frame="void" rules="none">
2672 <col class="field-name" />
2673 <col class="field-body" />
2674 <tbody valign="top">
2675 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An object <tt class="docutils literal"><span class="pre">r</span></tt> of unspecified type such that <tt class="docutils literal"><span class="pre">r</span> <span class="pre">=</span> <span class="pre">t</span></tt>
2676 is equivalent to <tt class="docutils literal"><span class="pre">m_f(t)</span></tt> for all <tt class="docutils literal"><span class="pre">t</span></tt>.</td>
2677 </tr>
2678 </tbody>
2679 </table>
2680 <p><tt class="docutils literal"><span class="pre">function_output_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
2681 <table class="docutils field-list" frame="void" rules="none">
2682 <col class="field-name" />
2683 <col class="field-body" />
2684 <tbody valign="top">
2685 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
2686 </tr>
2687 </tbody>
2688 </table>
2689 <p><tt class="docutils literal"><span class="pre">function_output_iterator&amp;</span> <span class="pre">operator++(int);</span></tt></p>
2690 <table class="docutils field-list" frame="void" rules="none">
2691 <col class="field-name" />
2692 <col class="field-body" />
2693 <tbody valign="top">
2694 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
2695 </tr>
2696 </tbody>
2697 </table>
2698 <!-- LocalWords:  Abrahams Siek Witt istream ostream iter MTL strided interoperate
2699 LocalWords:  CRTP metafunctions inlining lvalue JGS incrementable BGL LEDA cv
2700 LocalWords:  GraphBase struct ptrdiff UnaryFunction const int typename bool pp
2701 LocalWords:  lhs rhs SFINAE markup iff tmp OtherDerived OtherIterator DWA foo
2702 LocalWords:  dereferenceable subobject AdaptableUnaryFunction impl pre ifdef'd
2703 LocalWords:  OtherIncrementable Coplien -->
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