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  7. <title>Iterator Facade and Adaptor</title>
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  8. <meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
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  9. <meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, Zephyr Associates, Inc." />
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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. 

  15. <h1 class="title">Iterator Facade and Adaptor</h1>
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  16. <table class="docinfo" frame="void" rules="none">
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  17. <col class="docinfo-name" />
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  18. <col class="docinfo-content" />
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  19. <tbody valign="top">
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  20. <tr><th class="docinfo-name">Author:</th>
    21. 

  21. <td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr>
    22. 

  22. <tr><th class="docinfo-name">Contact:</th>
    23. 

  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. 

  24. <tr><th class="docinfo-name">Organization:</th>
    25. 

  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. 

  26. Lab</a>, <a class="last reference external" href="http://www.styleadvisor.com">Zephyr Associates, Inc.</a></td></tr>
    27. 

  27. <tr><th class="docinfo-name">Date:</th>
    28. 

  28. <td>2006-09-11</td></tr>
    29. 

  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://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1530.html">N1530</a>=03-0113, which was
    30. 

  30. accepted for Technical Report 1 by the C++ standard
    31. 

  31. committee's library working group.</td>
    32. 

  32. </tr>
    33. 

  33. </tbody>
    34. 

  34. </table>
    35. 

  35. <!-- Distributed under the Boost -->
    36. 

  36. <!-- Software License, Version 1.0. (See accompanying -->
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  37. <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
    38. 

  38. <!-- Version 1.9 of this ReStructuredText document corresponds to
    39. 

  39. n1530_, the paper accepted by the LWG. -->
    40. 

  40. <table class="docutils field-list" frame="void" rules="none">
    41. 

  41. <col class="field-name" />
    42. 

  42. <col class="field-body" />
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  43. <tbody valign="top">
    44. 

  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. 

  45. </tr>
    46. 

  46. </tbody>
    47. 

  47. </table>
    48. 

  48. <table class="docutils field-list" frame="void" rules="none">
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  49. <col class="field-name" />
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  50. <col class="field-body" />
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  51. <tbody valign="top">
    52. 

  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. 

  53. build standard-conforming iterators, both from scratch and
    54. 

  54. by adapting other iterators.</td>
    55. 

  55. </tr>
    56. 

  56. </tbody>
    57. 

  57. </table>
    58. 

  58. <div class="contents topic" id="table-of-contents">
    59. 

  59. <p class="topic-title first">Table of Contents</p>
    60. 

  60. <ul class="simple">
    61. 

  61. <li><a class="reference internal" href="#motivation" id="id15">Motivation</a></li>
    62. 

  62. <li><a class="reference internal" href="#impact-on-the-standard" id="id16">Impact on the Standard</a></li>
    63. 

  63. <li><a class="reference internal" href="#design" id="id17">Design</a><ul>
    64. 

  64. <li><a class="reference internal" href="#iterator-concepts" id="id18">Iterator Concepts</a></li>
    65. 

  65. <li><a class="reference internal" href="#interoperability" id="id19">Interoperability</a></li>
    66. 

  66. <li><a class="reference internal" href="#iterator-facade" id="id20">Iterator Facade</a><ul>
    67. 

  67. <li><a class="reference internal" href="#usage" id="id21">Usage</a></li>
    68. 

  68. <li><a class="reference internal" href="#iterator-core-access" id="id22">Iterator Core Access</a></li>
    69. 

  69. <li><a class="reference internal" href="#operator" id="id23"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></li>
    70. 

  70. <li><a class="reference internal" href="#id6" id="id24"><tt class="docutils literal"><span class="pre">operator-&gt;</span></tt></a></li>
    71. 

  71. </ul>
    72. 

  72. </li>
    73. 

  73. <li><a class="reference internal" href="#iterator-adaptor" id="id25">Iterator Adaptor</a></li>
    74. 

  74. <li><a class="reference internal" href="#specialized-adaptors" id="id26">Specialized Adaptors</a></li>
    75. 

  75. </ul>
    76. 

  76. </li>
    77. 

  77. <li><a class="reference internal" href="#proposed-text" id="id27">Proposed Text</a><ul>
    78. 

  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. 

  79. <li><a class="reference internal" href="#iterator-facade-lib-iterator-facade" id="id29">Iterator facade [lib.iterator.facade]</a><ul>
    80. 

  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. 

  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. 

  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. 

  83. </ul>
    84. 

  84. </li>
    85. 

  85. <li><a class="reference internal" href="#iterator-adaptor-lib-iterator-adaptor" id="id33">Iterator adaptor [lib.iterator.adaptor]</a><ul>
    86. 

  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. 

  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. 

  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. 

  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. 

  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. 

  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. 

  92. </ul>
    93. 

  93. </li>
    94. 

  94. <li><a class="reference internal" href="#specialized-adaptors-lib-iterator-special-adaptors" id="id40">Specialized adaptors [lib.iterator.special.adaptors]</a><ul>
    95. 

  95. <li><a class="reference internal" href="#indirect-iterator" id="id41">Indirect iterator</a><ul>
    96. 

  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. 

  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. 

  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. 

  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. 

  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. 

  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. 

  102. </ul>
    103. 

  103. </li>
    104. 

  104. <li><a class="reference internal" href="#reverse-iterator" id="id48">Reverse iterator</a><ul>
    105. 

  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. 

  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. 

  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. 

  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. 

  109. </ul>
    110. 

  110. </li>
    111. 

  111. <li><a class="reference internal" href="#transform-iterator" id="id53">Transform iterator</a><ul>
    112. 

  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. 

  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. 

  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. 

  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. 

  116. </ul>
    117. 

  117. </li>
    118. 

  118. <li><a class="reference internal" href="#filter-iterator" id="id58">Filter iterator</a><ul>
    119. 

  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. 

  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. 

  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. 

  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. 

  123. </ul>
    124. 

  124. </li>
    125. 

  125. <li><a class="reference internal" href="#counting-iterator" id="id63">Counting iterator</a><ul>
    126. 

  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. 

  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. 

  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. 

  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. 

  130. </ul>
    131. 

  131. </li>
    132. 

  132. <li><a class="reference internal" href="#function-output-iterator" id="id68">Function output iterator</a><ul>
    133. 

  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. 

  134. <li><a class="reference internal" href="#header" id="id70">Header</a></li>
    135. 

  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. 

  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. 

  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. 

  138. </ul>
    139. 

  139. </li>
    140. 

  140. </ul>
    141. 

  141. </li>
    142. 

  142. </ul>
    143. 

  143. </li>
    144. 

  144. </ul>
    145. 

  145. </div>
    146. 

  146. <div class="section" id="motivation">
    147. 

  147. <h1><a class="toc-backref" href="#id15">Motivation</a></h1>
    148. 

  148. <p>Iterators play an important role in modern C++ programming. The
    149. 

  149. iterator is the central abstraction of the algorithms of the Standard
    150. 

  150. Library, allowing algorithms to be re-used in in a wide variety of
    151. 

  151. contexts.  The C++ Standard Library contains a wide variety of useful
    152. 

  152. iterators. Every one of the standard containers comes with constant
    153. 

  153. and mutable iterators<a class="footnote-reference" href="#mutable" id="id1"><sup>2</sup></a>, and also reverse versions of those
    154. 

  154. same iterators which traverse the container in the opposite direction.
    155. 

  155. The Standard also supplies <tt class="docutils literal"><span class="pre">istream_iterator</span></tt> and
    156. 

  156. <tt class="docutils literal"><span class="pre">ostream_iterator</span></tt> for reading from and writing to streams,
    157. 

  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. 

  158. <tt class="docutils literal"><span class="pre">back_insert_iterator</span></tt> for inserting elements into containers, and
    159. 

  159. <tt class="docutils literal"><span class="pre">raw_storage_iterator</span></tt> for initializing raw memory [7].</p>
    160. 

  160. <p>Despite the many iterators supplied by the Standard Library, obvious
    161. 

  161. and useful iterators are missing, and creating new iterator types is
    162. 

  162. still a common task for C++ programmers.  The literature documents
    163. 

  163. several of these, for example line_iterator [3] and Constant_iterator
    164. 

  164. [9].  The iterator abstraction is so powerful that we expect
    165. 

  165. programmers will always need to invent new iterator types.</p>
    166. 

  166. <p>Although it is easy to create iterators that <em>almost</em> conform to the
    167. 

  167. standard, the iterator requirements contain subtleties which can make
    168. 

  168. creating an iterator which <em>actually</em> conforms quite difficult.
    169. 

  169. Further, the iterator interface is rich, containing many operators
    170. 

  170. that are technically redundant and tedious to implement.  To automate
    171. 

  171. the repetitive work of constructing iterators, we propose
    172. 

  172. <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, an iterator base class template which provides
    173. 

  173. the rich interface of standard iterators and delegates its
    174. 

  174. implementation to member functions of the derived class.  In addition
    175. 

  175. to reducing the amount of code necessary to create an iterator, the
    176. 

  176. <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> also provides compile-time error detection.
    177. 

  177. Iterator implementation mistakes that often go unnoticed are turned
    178. 

  178. into compile-time errors because the derived class implementation must
    179. 

  179. match the expectations of the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>.</p>
    180. 

  180. <p>A common pattern of iterator construction is the adaptation of one
    181. 

  181. iterator to form a new one.  The functionality of an iterator is
    182. 

  182. composed of four orthogonal aspects: traversal, indirection, equality
    183. 

  183. comparison and distance measurement.  Adapting an old iterator to
    184. 

  184. create a new one often saves work because one can reuse one aspect of
    185. 

  185. functionality while redefining the other.  For example, the Standard
    186. 

  186. provides <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt>, which adapts any Bidirectional Iterator
    187. 

  187. by inverting its direction of traversal.  As with plain iterators,
    188. 

  188. iterator adaptors defined outside the Standard have become commonplace
    189. 

  189. in the literature:</p>
    190. 

  190. <ul class="simple">
    191. 

  191. <li>Checked iter[13] adds bounds-checking to an existing iterator.</li>
    192. 

  192. <li>The iterators of the View Template Library[14], which adapts
    193. 

  193. containers, are themselves adaptors over the underlying iterators.</li>
    194. 

  194. <li>Smart iterators [5] adapt an iterator's dereferencing behavior by
    195. 

  195. applying a function object to the object being referenced and
    196. 

  196. returning the result.</li>
    197. 

  197. <li>Custom iterators [4], in which a variety of adaptor types are enumerated.</li>
    198. 

  198. <li>Compound iterators [1], which access a slice out of a container of containers.</li>
    199. 

  199. <li>Several iterator adaptors from the MTL [12].  The MTL contains a
    200. 

  200. strided iterator, where each call to <tt class="docutils literal"><span class="pre">operator++()</span></tt> moves the
    201. 

  201. iterator ahead by some constant factor, and a scaled iterator, which
    202. 

  202. multiplies the dereferenced value by some constant.</li>
    203. 

  203. </ul>
    204. 

  204. <table class="docutils footnote" frame="void" id="concept" rules="none">
    205. 

  205. <colgroup><col class="label" /><col /></colgroup>
    206. 

  206. <tbody valign="top">
    207. 

  207. <tr><td class="label">[1]</td><td>We use the term concept to mean a set of requirements
    208. 

  208. that a type must satisfy to be used with a particular template
    209. 

  209. parameter.</td></tr>
    210. 

  210. </tbody>
    211. 

  211. </table>
    212. 

  212. <table class="docutils footnote" frame="void" id="mutable" rules="none">
    213. 

  213. <colgroup><col class="label" /><col /></colgroup>
    214. 

  214. <tbody valign="top">
    215. 

  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. 

  216. can be changed by assigning to the dereferenced iterator, while
    217. 

  217. constant iterator refers to iterators over objects that cannot be
    218. 

  218. modified.</td></tr>
    219. 

  219. </tbody>
    220. 

  220. </table>
    221. 

  221. <p>To fulfill the need for constructing adaptors, we propose the
    222. 

  222. <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template.  Instantiations of
    223. 

  223. <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> serve as a base classes for new iterators,
    224. 

  224. providing the default behavior of forwarding all operations to the
    225. 

  225. underlying iterator.  The user can selectively replace these features
    226. 

  226. in the derived iterator class.  This proposal also includes a number
    227. 

  227. of more specialized adaptors, such as the <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> that
    228. 

  228. applies some user-specified function during the dereference of the
    229. 

  229. iterator.</p>
    230. 

  230. </div>
    231. 

  231. <div class="section" id="impact-on-the-standard">
    232. 

  232. <h1><a class="toc-backref" href="#id16">Impact on the Standard</a></h1>
    233. 

  233. <p>This proposal is purely an addition to the C++ standard library.
    234. 

  234. However, note that this proposal relies on the proposal for New
    235. 

  235. Iterator Concepts.</p>
    236. 

  236. </div>
    237. 

  237. <div class="section" id="design">
    238. 

  238. <h1><a class="toc-backref" href="#id17">Design</a></h1>
    239. 

  239. <div class="section" id="iterator-concepts">
    240. 

  240. <h2><a class="toc-backref" href="#id18">Iterator Concepts</a></h2>
    241. 

  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. 

  242. as proposed in <a class="reference external" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html">n1550</a>, since user-defined and especially adapted
    243. 

  243. iterators suffer from the well known categorization problems that are
    244. 

  244. inherent to the current iterator categories.</p>
    245. 

  245. <p>This proposal does not strictly depend on proposal <a class="reference external" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html">n1550</a>, as there
    246. 

  246. is a direct mapping between new and old categories. This proposal
    247. 

  247. could be reformulated using this mapping if <a class="reference external" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html">n1550</a> was not accepted.</p>
    248. 

  248. </div>
    249. 

  249. <div class="section" id="interoperability">
    250. 

  250. <h2><a class="toc-backref" href="#id19">Interoperability</a></h2>
    251. 

  251. <p>The question of iterator interoperability is poorly addressed in the
    252. 

  252. current standard.  There are currently two defect reports that are
    253. 

  253. concerned with interoperability issues.</p>
    254. 

  254. <p>Issue <a class="reference external" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#179">179</a> concerns the fact that mutable container iterator types
    255. 

  255. are only required to be convertible to the corresponding constant
    256. 

  256. iterator types, but objects of these types are not required to
    257. 

  257. interoperate in comparison or subtraction expressions.  This situation
    258. 

  258. is tedious in practice and out of line with the way built in types
    259. 

  259. work.  This proposal implements the proposed resolution to issue
    260. 

  260. <a class="reference external" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#179">179</a>, as most standard library implementations do nowadays. In other
    261. 

  261. words, if an iterator type A has an implicit or user defined
    262. 

  262. conversion to an iterator type B, the iterator types are interoperable
    263. 

  263. and the usual set of operators are available.</p>
    264. 

  264. <p>Issue <a class="reference external" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#280">280</a> concerns the current lack of interoperability between
    265. 

  265. reverse iterator types. The proposed new reverse_iterator template
    266. 

  266. fixes the issues raised in 280. It provides the desired
    267. 

  267. interoperability without introducing unwanted overloads.</p>
    268. 

  268. </div>
    269. 

  269. <div class="section" id="iterator-facade">
    270. 

  270. <h2><a class="toc-backref" href="#id20">Iterator Facade</a></h2>
    271. 

  271. <!-- Distributed under the Boost -->
    272. 

  272. <!-- Software License, Version 1.0. (See accompanying -->
    273. 

  273. <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
    274. 

  274. <!-- Version 1.1 of this ReStructuredText document corresponds to
    275. 

  275. n1530_, the paper accepted by the LWG for TR1. -->
    276. 

  276. <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
    277. 

  277. <p>While the iterator interface is rich, there is a core subset of the
    278. 

  278. interface that is necessary for all the functionality.  We have
    279. 

  279. identified the following core behaviors for iterators:</p>
    280. 

  280. <ul class="simple">
    281. 

  281. <li>dereferencing</li>
    282. 

  282. <li>incrementing</li>
    283. 

  283. <li>decrementing</li>
    284. 

  284. <li>equality comparison</li>
    285. 

  285. <li>random-access motion</li>
    286. 

  286. <li>distance measurement</li>
    287. 

  287. </ul>
    288. 

  288. <p>In addition to the behaviors listed above, the core interface elements
    289. 

  289. include the associated types exposed through iterator traits:
    290. 

  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. 

  291. <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p>
    292. 

  292. <p>Iterator facade uses the Curiously Recurring Template
    293. 

  293. Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id4">[Cop95]</a> so that the user can specify the behavior
    294. 

  294. of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> in a derived class.  Former designs used
    295. 

  295. policy objects to specify the behavior, but that approach was
    296. 

  296. discarded for several reasons:</p>
    297. 

  297. <blockquote>
    298. 

  298. <ol class="arabic simple">
    299. 

  299. <li>the creation and eventual copying of the policy object may create
    300. 

  300. overhead that can be avoided with the current approach.</li>
    301. 

  301. <li>The policy object approach does not allow for custom constructors
    302. 

  302. on the created iterator types, an essential feature if
    303. 

  303. <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> should be used in other library
    304. 

  304. implementations.</li>
    305. 

  305. <li>Without the use of CRTP, the standard requirement that an
    306. 

  306. iterator's <tt class="docutils literal"><span class="pre">operator++</span></tt> returns the iterator type itself
    307. 

  307. would mean that all iterators built with the library would
    308. 

  308. have to be specializations of <tt class="docutils literal"><span class="pre">iterator_facade&lt;...&gt;</span></tt>, rather
    309. 

  309. than something more descriptive like
    310. 

  310. <tt class="docutils literal"><span class="pre">indirect_iterator&lt;T*&gt;</span></tt>.  Cumbersome type generator
    311. 

  311. metafunctions would be needed to build new parameterized
    312. 

  312. iterators, and a separate <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> layer would be
    313. 

  313. impossible.</li>
    314. 

  314. </ol>
    315. 

  315. </blockquote>
    316. 

  316. <div class="section" id="usage">
    317. 

  317. <h3><a class="toc-backref" href="#id21">Usage</a></h3>
    318. 

  318. <p>The user of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> derives his iterator class from a
    319. 

  319. specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and passes the derived
    320. 

  320. iterator class as <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s first template parameter.
    321. 

  321. The order of the other template parameters have been carefully
    322. 

  322. chosen to take advantage of useful defaults.  For example, when
    323. 

  323. defining a constant lvalue iterator, the user can pass a
    324. 

  324. const-qualified version of the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt> as
    325. 

  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. 

  326. <tt class="docutils literal"><span class="pre">Reference</span></tt> parameter which follows.</p>
    327. 

  327. <p>The derived iterator class must define member functions implementing
    328. 

  328. the iterator's core behaviors.  The following table describes
    329. 

  329. expressions which are required to be valid depending on the category
    330. 

  330. of the derived iterator type.  These member functions are described
    331. 

  331. briefly below and in more detail in the iterator facade
    332. 

  332. requirements.</p>
    333. 

  333. <blockquote>
    334. 

  334. <table border="1" class="docutils">
    335. 

  335. <colgroup>
    336. 

  336. <col width="44%" />
    337. 

  337. <col width="56%" />
    338. 

  338. </colgroup>
    339. 

  339. <thead valign="bottom">
    340. 

  340. <tr><th class="head">Expression</th>
    341. 

  341. <th class="head">Effects</th>
    342. 

  342. </tr>
    343. 

  343. </thead>
    344. 

  344. <tbody valign="top">
    345. 

  345. <tr><td><tt class="docutils literal"><span class="pre">i.dereference()</span></tt></td>
    346. 

  346. <td>Access the value referred to</td>
    347. 

  347. </tr>
    348. 

  348. <tr><td><tt class="docutils literal"><span class="pre">i.equal(j)</span></tt></td>
    349. 

  349. <td>Compare for equality with <tt class="docutils literal"><span class="pre">j</span></tt></td>
    350. 

  350. </tr>
    351. 

  351. <tr><td><tt class="docutils literal"><span class="pre">i.increment()</span></tt></td>
    352. 

  352. <td>Advance by one position</td>
    353. 

  353. </tr>
    354. 

  354. <tr><td><tt class="docutils literal"><span class="pre">i.decrement()</span></tt></td>
    355. 

  355. <td>Retreat by one position</td>
    356. 

  356. </tr>
    357. 

  357. <tr><td><tt class="docutils literal"><span class="pre">i.advance(n)</span></tt></td>
    358. 

  358. <td>Advance by <tt class="docutils literal"><span class="pre">n</span></tt> positions</td>
    359. 

  359. </tr>
    360. 

  360. <tr><td><tt class="docutils literal"><span class="pre">i.distance_to(j)</span></tt></td>
    361. 

  361. <td>Measure the distance to <tt class="docutils literal"><span class="pre">j</span></tt></td>
    362. 

  362. </tr>
    363. 

  363. </tbody>
    364. 

  364. </table>
    365. 

  365. </blockquote>
    366. 

  366. <!-- Should we add a comment that a zero overhead implementation of iterator_facade
    367. 

  367. is possible with proper inlining? -->
    368. 

  368. <p>In addition to implementing the core interface functions, an iterator
    369. 

  369. derived from <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> typically defines several
    370. 

  370. constructors. To model any of the standard iterator concepts, the
    371. 

  371. iterator must at least have a copy constructor. Also, if the iterator
    372. 

  372. type <tt class="docutils literal"><span class="pre">X</span></tt> is meant to be automatically interoperate with another
    373. 

  373. iterator type <tt class="docutils literal"><span class="pre">Y</span></tt> (as with constant and mutable iterators) then
    374. 

  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. 

  375. to <tt class="docutils literal"><span class="pre">X</span></tt> (but not both), typically implemented as a conversion
    376. 

  376. constructor. Finally, if the iterator is to model Forward Traversal
    377. 

  377. Iterator or a more-refined iterator concept, a default constructor is
    378. 

  378. required.</p>
    379. 

  379. </div>
    380. 

  380. <div class="section" id="iterator-core-access">
    381. 

  381. <h3><a class="toc-backref" href="#id22">Iterator Core Access</a></h3>
    382. 

  382. <p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and the operator implementations need to be able
    383. 

  383. to access the core member functions in the derived class.  Making the
    384. 

  384. core member functions public would expose an implementation detail to
    385. 

  385. the user.  The design used here ensures that implementation details do
    386. 

  386. not appear in the public interface of the derived iterator type.</p>
    387. 

  387. <p>Preventing direct access to the core member functions has two
    388. 

  388. advantages.  First, there is no possibility for the user to accidently
    389. 

  389. use a member function of the iterator when a member of the value_type
    390. 

  390. was intended.  This has been an issue with smart pointer
    391. 

  391. implementations in the past.  The second and main advantage is that
    392. 

  392. library implementers can freely exchange a hand-rolled iterator
    393. 

  393. implementation for one based on <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> without fear of
    394. 

  394. breaking code that was accessing the public core member functions
    395. 

  395. directly.</p>
    396. 

  396. <p>In a naive implementation, keeping the derived class' core member
    397. 

  397. functions private would require it to grant friendship to
    398. 

  398. <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and each of the seven operators.  In order to
    399. 

  399. reduce the burden of limiting access, <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> is
    400. 

  400. provided, a class that acts as a gateway to the core member functions
    401. 

  401. in the derived iterator class.  The author of the derived class only
    402. 

  402. needs to grant friendship to <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> to make his core
    403. 

  403. member functions available to the library.</p>
    404. 

  404. <!-- This is no long uptodate -thw -->
    405. 

  405. <!-- Yes it is; I made sure of it! -DWA -->
    406. 

  406. <p><tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> will be typically implemented as an empty
    407. 

  407. class containing only private static member functions which invoke the
    408. 

  408. iterator core member functions. There is, however, no need to
    409. 

  409. standardize the gateway protocol.  Note that even if
    410. 

  410. <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> used public member functions it would not
    411. 

  411. open a safety loophole, as every core member function preserves the
    412. 

  412. invariants of the iterator.</p>
    413. 

  413. </div>
    414. 

  414. <div class="section" id="operator">
    415. 

  415. <h3><a class="toc-backref" href="#id23"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></h3>
    416. 

  416. <p>The indexing operator for a generalized iterator presents special
    417. 

  417. challenges.  A random access iterator's <tt class="docutils literal"><span class="pre">operator[]</span></tt> is only
    418. 

  418. required to return something convertible to its <tt class="docutils literal"><span class="pre">value_type</span></tt>.
    419. 

  419. Requiring that it return an lvalue would rule out currently-legal
    420. 

  420. random-access iterators which hold the referenced value in a data
    421. 

  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. 

  422. into the temporary iterator <tt class="docutils literal"><span class="pre">p+n</span></tt>, which is destroyed when
    423. 

  423. <tt class="docutils literal"><span class="pre">operator[]</span></tt> returns.</p>
    424. 

  424. <p>Writable iterators built with <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implement the
    425. 

  425. semantics required by the preferred resolution to <a class="reference external" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#299">issue 299</a> and
    426. 

  426. adopted by proposal <a class="reference external" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html">n1550</a>: the result of <tt class="docutils literal"><span class="pre">p[n]</span></tt> is an object
    427. 

  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. 

  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. 

  429. implemented as a proxy containing a copy of <tt class="docutils literal"><span class="pre">p+n</span></tt>).  This approach
    430. 

  430. will work properly for any random-access iterator regardless of the
    431. 

  431. other details of its implementation.  A user who knows more about
    432. 

  432. the implementation of her iterator is free to implement an
    433. 

  433. <tt class="docutils literal"><span class="pre">operator[]</span></tt> that returns an lvalue in the derived iterator
    434. 

  434. class; it will hide the one supplied by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> from
    435. 

  435. clients of her iterator.</p>
    436. 

  436. </div>
    437. 

  437. <div class="section" id="id6">
    438. 

  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. 

  439. <p>The <tt class="docutils literal"><span class="pre">reference</span></tt> type of a readable iterator (and today's input
    440. 

  440. iterator) need not in fact be a reference, so long as it is
    441. 

  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. 

  442. is a class, however, it must still be possible to access members
    443. 

  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. 

  444. type is not in fact a reference must return a proxy containing a copy
    445. 

  445. of the referenced value from its <tt class="docutils literal"><span class="pre">operator-&gt;</span></tt>.</p>
    446. 

  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. 

  447. <tt class="docutils literal"><span class="pre">operator[]</span></tt> are not explicitly specified. Instead, those types
    448. 

  448. are described in terms of a set of requirements, which must be
    449. 

  449. satisfied by the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implementation.</p>
    450. 

  450. <table class="docutils citation" frame="void" id="cop95" rules="none">
    451. 

  451. <colgroup><col class="label" /><col /></colgroup>
    452. 

  452. <tbody valign="top">
    453. 

  453. <tr><td class="label"><a class="fn-backref" href="#id4">[Cop95]</a></td><td>[Coplien, 1995] Coplien, J., Curiously Recurring Template
    454. 

  454. Patterns, C++ Report, February 1995, pp. 24-27.</td></tr>
    455. 

  455. </tbody>
    456. 

  456. </table>
    457. 

  457. </div>
    458. 

  458. </div>
    459. 

  459. <div class="section" id="iterator-adaptor">
    460. 

  460. <h2><a class="toc-backref" href="#id25">Iterator Adaptor</a></h2>
    461. 

  461. <!-- Distributed under the Boost -->
    462. 

  462. <!-- Software License, Version 1.0. (See accompanying -->
    463. 

  463. <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
    464. 

  464. <!-- Version 1.2 of this ReStructuredText document corresponds to
    465. 

  465. n1530_, the paper accepted by the LWG for TR1. -->
    466. 

  466. <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
    467. 

  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. 

  468. type to create a new iterator.  Instantiations of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>
    469. 

  469. are derived from a corresponding instantiation of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>
    470. 

  470. and implement the core behaviors in terms of the <tt class="docutils literal"><span class="pre">Base</span></tt> type. In
    471. 

  471. essence, <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> merely forwards all operations to an
    472. 

  472. instance of the <tt class="docutils literal"><span class="pre">Base</span></tt> type, which it stores as a member.</p>
    473. 

  473. <table class="docutils footnote" frame="void" id="base" rules="none">
    474. 

  474. <colgroup><col class="label" /><col /></colgroup>
    475. 

  475. <tbody valign="top">
    476. 

  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. 

  477. not meant to imply the use of derivation. We have followed the lead
    478. 

  478. of the standard library, which provides a base() function to access
    479. 

  479. the underlying iterator object of a <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> adaptor.</td></tr>
    480. 

  480. </tbody>
    481. 

  481. </table>
    482. 

  482. <p>The user of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> creates a class derived from an
    483. 

  483. instantiation of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> and then selectively
    484. 

  484. redefines some of the core member functions described in the
    485. 

  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. 

  486. not meet the full requirements for an iterator; it need only
    487. 

  487. support the operations used by the core interface functions of
    488. 

  488. <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> that have not been redefined in the user's
    489. 

  489. derived class.</p>
    490. 

  490. <p>Several of the template parameters of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> default
    491. 

  491. to <tt class="docutils literal"><span class="pre">use_default</span></tt>. This allows the
    492. 

  492. user to make use of a default parameter even when she wants to
    493. 

  493. specify a parameter later in the parameter list.  Also, the
    494. 

  494. defaults for the corresponding associated types are somewhat
    495. 

  495. complicated, so metaprogramming is required to compute them, and
    496. 

  496. <tt class="docutils literal"><span class="pre">use_default</span></tt> can help to simplify the implementation.  Finally,
    497. 

  497. the identity of the <tt class="docutils literal"><span class="pre">use_default</span></tt> type is not left unspecified
    498. 

  498. because specification helps to highlight that the <tt class="docutils literal"><span class="pre">Reference</span></tt>
    499. 

  499. template parameter may not always be identical to the iterator's
    500. 

  500. <tt class="docutils literal"><span class="pre">reference</span></tt> type, and will keep users from making mistakes based on
    501. 

  501. that assumption.</p>
    502. 

  502. </div>
    503. 

  503. <div class="section" id="specialized-adaptors">
    504. 

  504. <h2><a class="toc-backref" href="#id26">Specialized Adaptors</a></h2>
    505. 

  505. <p>This proposal also contains several examples of specialized adaptors
    506. 

  506. which were easily implemented using <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>:</p>
    507. 

  507. <ul class="simple">
    508. 

  508. <li><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt>, which iterates over iterators, pointers,
    509. 

  509. or smart pointers and applies an extra level of dereferencing.</li>
    510. 

  510. <li>A new <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt>, which inverts the direction of a Base
    511. 

  511. iterator's motion, while allowing adapted constant and mutable
    512. 

  512. iterators to interact in the expected ways (unlike those in most
    513. 

  513. implementations of C++98).</li>
    514. 

  514. <li><tt class="docutils literal"><span class="pre">transform_iterator</span></tt>, which applies a user-defined function object
    515. 

  515. to the underlying values when dereferenced.</li>
    516. 

  516. <li><tt class="docutils literal"><span class="pre">filter_iterator</span></tt>, which provides a view of an iterator range in
    517. 

  517. which some elements of the underlying range are skipped.</li>
    518. 

  518. </ul>
    519. 

  519. <ul class="simple" id="counting">
    520. 

  520. <li><tt class="docutils literal"><span class="pre">counting_iterator</span></tt>, which adapts any incrementable type
    521. 

  521. (e.g. integers, iterators) so that incrementing/decrementing the
    522. 

  522. adapted iterator and dereferencing it produces successive values of
    523. 

  523. the Base type.</li>
    524. 

  524. <li><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt>, which makes it easier to create custom
    525. 

  525. output iterators.</li>
    526. 

  526. </ul>
    527. 

  527. <p>Based on examples in the Boost library, users have generated many new
    528. 

  528. adaptors, among them a permutation adaptor which applies some
    529. 

  529. permutation to a random access iterator, and a strided adaptor, which
    530. 

  530. adapts a random access iterator by multiplying its unit of motion by a
    531. 

  531. constant factor.  In addition, the Boost Graph Library (BGL) uses
    532. 

  532. iterator adaptors to adapt other graph libraries, such as LEDA [10]
    533. 

  533. and Stanford GraphBase [8], to the BGL interface (which requires C++
    534. 

  534. Standard compliant iterators).</p>
    535. 

  535. </div>
    536. 

  536. </div>
    537. 

  537. <div class="section" id="proposed-text">
    538. 

  538. <h1><a class="toc-backref" href="#id27">Proposed Text</a></h1>
    539. 

  539. <div class="section" id="header-iterator-helper-synopsis-lib-iterator-helper-synopsis">
    540. 

  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. 

  541. <pre class="literal-block">
    542. 

  542. struct use_default;
    543. 

  543. 

  544. struct iterator_core_access { /* implementation detail */ };
    545. 

  545. 

  546. template &lt;
    547. 

  547.     class Derived
    548. 

  548.   , class Value
    549. 

  549.   , class CategoryOrTraversal
    550. 

  550.   , class Reference  = Value&amp;
    551. 

  551.   , class Difference = ptrdiff_t
    552. 

  552. &gt;
    553. 

  553. class iterator_facade;
    554. 

  554. 

  555. template &lt;
    556. 

  556.     class Derived
    557. 

  557.   , class Base
    558. 

  558.   , class Value      = use_default
    559. 

  559.   , class CategoryOrTraversal  = use_default
    560. 

  560.   , class Reference  = use_default
    561. 

  561.   , class Difference = use_default
    562. 

  562. &gt;
    563. 

  563. class iterator_adaptor;
    564. 

  564. 

  565. template &lt;
    566. 

  566.     class Iterator
    567. 

  567.   , class Value = use_default
    568. 

  568.   , class CategoryOrTraversal = use_default
    569. 

  569.   , class Reference = use_default
    570. 

  570.   , class Difference = use_default
    571. 

  571. &gt;
    572. 

  572. class indirect_iterator;
    573. 

  573. 

  574. template &lt;class Dereferenceable&gt;
    575. 

  575. struct pointee;
    576. 

  576. 

  577. template &lt;class Dereferenceable&gt;
    578. 

  578. struct indirect_reference;
    579. 

  579. 

  580. template &lt;class Iterator&gt;
    581. 

  581. class reverse_iterator;
    582. 

  582. 

  583. template &lt;
    584. 

  584.     class UnaryFunction
    585. 

  585.   , class Iterator
    586. 

  586.   , class Reference = use_default
    587. 

  587.   , class Value = use_default
    588. 

  588. &gt;
    589. 

  589. class transform_iterator;
    590. 

  590. 

  591. template &lt;class Predicate, class Iterator&gt;
    592. 

  592. class filter_iterator;
    593. 

  593. 

  594. template &lt;
    595. 

  595.     class Incrementable
    596. 

  596.   , class CategoryOrTraversal  = use_default
    597. 

  597.   , class Difference = use_default
    598. 

  598. &gt;
    599. 

  599. class counting_iterator;
    600. 

  600. 

  601. template &lt;class UnaryFunction&gt;
    602. 

  602. class function_output_iterator;
    603. 

  603. </pre>
    604. 

  604. </div>
    605. 

  605. <div class="section" id="iterator-facade-lib-iterator-facade">
    606. 

  606. <h2><a class="toc-backref" href="#id29">Iterator facade [lib.iterator.facade]</a></h2>
    607. 

  607. <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
    608. 

  608. <!-- Software License, Version 1.0. (See accompanying -->
    609. 

  609. <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
    610. 

  610. <p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is a base class template that implements the
    611. 

  611. interface of standard iterators in terms of a few core functions
    612. 

  612. and associated types, to be supplied by a derived iterator class.</p>
    613. 

  613. <div class="section" id="class-template-iterator-facade">
    614. 

  614. <h3><a class="toc-backref" href="#id30">Class template <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a></h3>
    615. 

  615. <!-- Distributed under the Boost -->
    616. 

  616. <!-- Software License, Version 1.0. (See accompanying -->
    617. 

  617. <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
    618. 

  618. <!-- Version 1.3 of this ReStructuredText document corresponds to
    619. 

  619. n1530_, the paper accepted by the LWG for TR1. -->
    620. 

  620. <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
    621. 

  621. <pre class="literal-block">
    622. 

  622. template &lt;
    623. 

  623.     class Derived
    624. 

  624.   , class Value
    625. 

  625.   , class CategoryOrTraversal
    626. 

  626.   , class Reference  = Value&amp;
    627. 

  627.   , class Difference = ptrdiff_t
    628. 

  628. &gt;
    629. 

  629. class iterator_facade {
    630. 

  630.  public:
    631. 

  631.     typedef remove_const&lt;Value&gt;::type value_type;
    632. 

  632.     typedef Reference reference;
    633. 

  633.     typedef Value* pointer;
    634. 

  634.     typedef Difference difference_type;
    635. 

  635.     typedef /* see <a class="reference internal" href="#iterator-category">below</a> */ iterator_category;
    636. 

  636. 

  637.     reference operator*() const;
    638. 

  638.     /* see <a class="reference internal" href="#operator-arrow">below</a> */ operator-&gt;() const;
    639. 

  639.     /* see <a class="reference internal" href="#brackets">below</a> */ operator[](difference_type n) const;
    640. 

  640.     Derived&amp; operator++();
    641. 

  641.     Derived operator++(int);
    642. 

  642.     Derived&amp; operator--();
    643. 

  643.     Derived operator--(int);
    644. 

  644.     Derived&amp; operator+=(difference_type n);
    645. 

  645.     Derived&amp; operator-=(difference_type n);
    646. 

  646.     Derived operator-(difference_type n) const;
    647. 

  647.  protected:
    648. 

  648.     typedef iterator_facade iterator_facade_;
    649. 

  649. };
    650. 

  650. 

  651. // Comparison operators
    652. 

  652. template &lt;class Dr1, class V1, class TC1, class R1, class D1,
    653. 

  653.           class Dr2, class V2, class TC2, class R2, class D2&gt;
    654. 

  654. typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type // exposition
    655. 

  655. operator ==(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
    656. 

  656.             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
    657. 

  657. 

  658. template &lt;class Dr1, class V1, class TC1, class R1, class D1,
    659. 

  659.           class Dr2, class V2, class TC2, class R2, class D2&gt;
    660. 

  660. typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
    661. 

  661. operator !=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
    662. 

  662.             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
    663. 

  663. 

  664. template &lt;class Dr1, class V1, class TC1, class R1, class D1,
    665. 

  665.           class Dr2, class V2, class TC2, class R2, class D2&gt;
    666. 

  666. typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
    667. 

  667. operator &lt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
    668. 

  668.            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
    669. 

  669. 

  670. template &lt;class Dr1, class V1, class TC1, class R1, class D1,
    671. 

  671.           class Dr2, class V2, class TC2, class R2, class D2&gt;
    672. 

  672. typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
    673. 

  673. operator &lt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
    674. 

  674.             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
    675. 

  675. 

  676. template &lt;class Dr1, class V1, class TC1, class R1, class D1,
    677. 

  677.           class Dr2, class V2, class TC2, class R2, class D2&gt;
    678. 

  678. typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
    679. 

  679. operator &gt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
    680. 

  680.            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
    681. 

  681. 

  682. template &lt;class Dr1, class V1, class TC1, class R1, class D1,
    683. 

  683.           class Dr2, class V2, class TC2, class R2, class D2&gt;
    684. 

  684. typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
    685. 

  685. operator &gt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
    686. 

  686.             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
    687. 

  687. 

  688. // Iterator difference
    689. 

  689. template &lt;class Dr1, class V1, class TC1, class R1, class D1,
    690. 

  690.           class Dr2, class V2, class TC2, class R2, class D2&gt;
    691. 

  691. /* see <a class="reference internal" href="#minus">below</a> */
    692. 

  692. operator-(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
    693. 

  693.           iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
    694. 

  694. 

  695. // Iterator addition
    696. 

  696. template &lt;class Dr, class V, class TC, class R, class D&gt;
    697. 

  697. Derived operator+ (iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;,
    698. 

  698.                    typename Derived::difference_type n);
    699. 

  699. 

  700. template &lt;class Dr, class V, class TC, class R, class D&gt;
    701. 

  701. Derived operator+ (typename Derived::difference_type n,
    702. 

  702.                    iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;);
    703. 

  703. </pre>
    704. 

  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. 

  705. <pre class="literal-block">
    706. 

  706. <em>iterator-category</em>(CategoryOrTraversal, value_type, reference)
    707. 

  707. </pre>
    708. 

  708. <p>where <em>iterator-category</em> is defined as follows:</p>
    709. 

  709. <pre class="literal-block" id="id12">
    710. 

  710. <em>iterator-category</em>(C,R,V) :=
    711. 

  711.    if (C is convertible to std::input_iterator_tag
    712. 

  712.        || C is convertible to std::output_iterator_tag
    713. 

  713.    )
    714. 

  714.        return C
    715. 

  715. 

  716.    else if (C is not convertible to incrementable_traversal_tag)
    717. 

  717.        <em>the program is ill-formed</em>
    718. 

  718. 

  719.    else return a type X satisfying the following two constraints:
    720. 

  720. 

  721.       1. X is convertible to X1, and not to any more-derived
    722. 

  722.          type, where X1 is defined by:
    723. 

  723. 

  724.            if (R is a reference type
    725. 

  725.                &amp;&amp; C is convertible to forward_traversal_tag)
    726. 

  726.            {
    727. 

  727.                if (C is convertible to random_access_traversal_tag)
    728. 

  728.                    X1 = random_access_iterator_tag
    729. 

  729.                else if (C is convertible to bidirectional_traversal_tag)
    730. 

  730.                    X1 = bidirectional_iterator_tag
    731. 

  731.                else
    732. 

  732.                    X1 = forward_iterator_tag
    733. 

  733.            }
    734. 

  734.            else
    735. 

  735.            {
    736. 

  736.                if (C is convertible to single_pass_traversal_tag
    737. 

  737.                    &amp;&amp; R is convertible to V)
    738. 

  738.                    X1 = input_iterator_tag
    739. 

  739.                else
    740. 

  740.                    X1 = C
    741. 

  741.            }
    742. 

  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. 

  744.          derived traversal tag type to which X is also
    745. 

  745.          convertible, and not to any more-derived traversal tag
    746. 

  746.          type.
    747. 

  747. </pre>
    748. 

  748. <p>[Note: the intention is to allow <tt class="docutils literal"><span class="pre">iterator_category</span></tt> to be one of
    749. 

  749. the five original category tags when convertibility to one of the
    750. 

  750. traversal tags would add no information]</p>
    751. 

  751. <!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
    752. 

  752. <!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
    753. 

  753. <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
    754. 

  754. <p>The <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> template used above is for exposition
    755. 

  755. purposes.  The member operators should only be in an overload set
    756. 

  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. 

  757. meaning that at least one of the types is convertible to the other.  The
    758. 

  758. <tt class="docutils literal"><span class="pre">enable_if_intero…
    759.