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1<?xml version="1.0" encoding="utf-8" ?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 4<head> 5<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 6<meta name="generator" content="Docutils 0.5: http://docutils.sourceforge.net/" /> 7<title>Iterator Facade</title> 8<meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" /> 9<meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" /> 10<meta name="date" content="2006-09-11" /> 11<meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." /> 12<link rel="stylesheet" href="../../../rst.css" type="text/css" /> 13</head> 14<body> 15<div class="document" id="iterator-facade"> 16<h1 class="title">Iterator Facade</h1> 17<table class="docinfo" frame="void" rules="none"> 18<col class="docinfo-name" /> 19<col class="docinfo-content" /> 20<tbody valign="top"> 21<tr><th class="docinfo-name">Author:</th> 22<td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr> 23<tr><th class="docinfo-name">Contact:</th> 24<td><a class="first reference external" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>, <a class="last reference external" href="mailto:witt@ive.uni-hannover.de">witt@ive.uni-hannover.de</a></td></tr> 25<tr><th class="docinfo-name">Organization:</th> 26<td><a class="first reference external" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference external" href="http://www.osl.iu.edu">Open Systems 27Lab</a>, University of Hanover <a class="last reference external" href="http://www.ive.uni-hannover.de">Institute for Transport 28Railway Operation and Construction</a></td></tr> 29<tr><th class="docinfo-name">Date:</th> 30<td>2006-09-11</td></tr> 31<tr><th class="docinfo-name">Copyright:</th> 32<td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr> 33</tbody> 34</table> 35<!-- Distributed under the Boost --> 36<!-- Software License, Version 1.0. (See accompanying --> 37<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 38<table class="docutils field-list" frame="void" rules="none"> 39<col class="field-name" /> 40<col class="field-body" /> 41<tbody valign="top"> 42<tr class="field"><th class="field-name">abstract:</th><td class="field-body"><!-- Copyright David Abrahams 2006. Distributed under the Boost --> 43<!-- Software License, Version 1.0. (See accompanying --> 44<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 45<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is a base class template that implements the 46interface of standard iterators in terms of a few core functions 47and associated types, to be supplied by a derived iterator class.</td> 48</tr> 49</tbody> 50</table> 51<div class="contents topic" id="table-of-contents"> 52<p class="topic-title first">Table of Contents</p> 53<ul class="simple"> 54<li><a class="reference internal" href="#overview" id="id23">Overview</a><ul> 55<li><a class="reference internal" href="#usage" id="id24">Usage</a></li> 56<li><a class="reference internal" href="#iterator-core-access" id="id25">Iterator Core Access</a></li> 57<li><a class="reference internal" href="#operator" id="id26"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></li> 58<li><a class="reference internal" href="#id2" id="id27"><tt class="docutils literal"><span class="pre">operator-></span></tt></a></li> 59</ul> 60</li> 61<li><a class="reference internal" href="#reference" id="id28">Reference</a><ul> 62<li><a class="reference internal" href="#iterator-facade-requirements" id="id29"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Requirements</a></li> 63<li><a class="reference internal" href="#iterator-facade-operations" id="id30"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> operations</a></li> 64</ul> 65</li> 66<li><a class="reference internal" href="#tutorial-example" id="id31">Tutorial Example</a><ul> 67<li><a class="reference internal" href="#the-problem" id="id32">The Problem</a></li> 68<li><a class="reference internal" href="#a-basic-iterator-using-iterator-facade" id="id33">A Basic Iterator Using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a><ul> 69<li><a class="reference internal" href="#template-arguments-for-iterator-facade" id="id34">Template Arguments for <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a><ul> 70<li><a class="reference internal" href="#derived" id="id35"><tt class="docutils literal"><span class="pre">Derived</span></tt></a></li> 71<li><a class="reference internal" href="#value" id="id36"><tt class="docutils literal"><span class="pre">Value</span></tt></a></li> 72<li><a class="reference internal" href="#categoryortraversal" id="id37"><tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt></a></li> 73<li><a class="reference internal" href="#id12" id="id38"><tt class="docutils literal"><span class="pre">Reference</span></tt></a></li> 74<li><a class="reference internal" href="#difference" id="id39"><tt class="docutils literal"><span class="pre">Difference</span></tt></a></li> 75</ul> 76</li> 77<li><a class="reference internal" href="#constructors-and-data-members" id="id40">Constructors and Data Members</a></li> 78<li><a class="reference internal" href="#implementing-the-core-operations" id="id41">Implementing the Core Operations</a></li> 79</ul> 80</li> 81<li><a class="reference internal" href="#a-constant-node-iterator" id="id42">A constant <tt class="docutils literal"><span class="pre">node_iterator</span></tt></a></li> 82<li><a class="reference internal" href="#interoperability" id="id43">Interoperability</a></li> 83<li><a class="reference internal" href="#telling-the-truth" id="id44">Telling the Truth</a></li> 84<li><a class="reference internal" href="#wrap-up" id="id45">Wrap Up</a></li> 85</ul> 86</li> 87</ul> 88</div> 89<div class="section" id="overview"> 90<h1><a class="toc-backref" href="#id23">Overview</a></h1> 91<!-- Distributed under the Boost --> 92<!-- Software License, Version 1.0. (See accompanying --> 93<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 94<!-- Version 1.1 of this ReStructuredText document corresponds to 95n1530_, the paper accepted by the LWG for TR1. --> 96<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. --> 97<p>While the iterator interface is rich, there is a core subset of the 98interface that is necessary for all the functionality. We have 99identified the following core behaviors for iterators:</p> 100<ul class="simple"> 101<li>dereferencing</li> 102<li>incrementing</li> 103<li>decrementing</li> 104<li>equality comparison</li> 105<li>random-access motion</li> 106<li>distance measurement</li> 107</ul> 108<p>In addition to the behaviors listed above, the core interface elements 109include the associated types exposed through iterator traits: 110<tt class="docutils literal"><span class="pre">value_type</span></tt>, <tt class="docutils literal"><span class="pre">reference</span></tt>, <tt class="docutils literal"><span class="pre">difference_type</span></tt>, and 111<tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p> 112<p>Iterator facade uses the Curiously Recurring Template 113Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id1">[Cop95]</a> so that the user can specify the behavior 114of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> in a derived class. Former designs used 115policy objects to specify the behavior, but that approach was 116discarded for several reasons:</p> 117<blockquote> 118<ol class="arabic simple"> 119<li>the creation and eventual copying of the policy object may create 120overhead that can be avoided with the current approach.</li> 121<li>The policy object approach does not allow for custom constructors 122on the created iterator types, an essential feature if 123<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> should be used in other library 124implementations.</li> 125<li>Without the use of CRTP, the standard requirement that an 126iterator's <tt class="docutils literal"><span class="pre">operator++</span></tt> returns the iterator type itself 127would mean that all iterators built with the library would 128have to be specializations of <tt class="docutils literal"><span class="pre">iterator_facade<...></span></tt>, rather 129than something more descriptive like 130<tt class="docutils literal"><span class="pre">indirect_iterator<T*></span></tt>. Cumbersome type generator 131metafunctions would be needed to build new parameterized 132iterators, and a separate <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> layer would be 133impossible.</li> 134</ol> 135</blockquote> 136<div class="section" id="usage"> 137<h2><a class="toc-backref" href="#id24">Usage</a></h2> 138<p>The user of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> derives his iterator class from a 139specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and passes the derived 140iterator class as <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s first template parameter. 141The order of the other template parameters have been carefully 142chosen to take advantage of useful defaults. For example, when 143defining a constant lvalue iterator, the user can pass a 144const-qualified version of the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt> as 145<tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s <tt class="docutils literal"><span class="pre">Value</span></tt> parameter and omit the 146<tt class="docutils literal"><span class="pre">Reference</span></tt> parameter which follows.</p> 147<p>The derived iterator class must define member functions implementing 148the iterator's core behaviors. The following table describes 149expressions which are required to be valid depending on the category 150of the derived iterator type. These member functions are described 151briefly below and in more detail in the iterator facade 152requirements.</p> 153<blockquote> 154<table border="1" class="docutils"> 155<colgroup> 156<col width="44%" /> 157<col width="56%" /> 158</colgroup> 159<thead valign="bottom"> 160<tr><th class="head">Expression</th> 161<th class="head">Effects</th> 162</tr> 163</thead> 164<tbody valign="top"> 165<tr><td><tt class="docutils literal"><span class="pre">i.dereference()</span></tt></td> 166<td>Access the value referred to</td> 167</tr> 168<tr><td><tt class="docutils literal"><span class="pre">i.equal(j)</span></tt></td> 169<td>Compare for equality with <tt class="docutils literal"><span class="pre">j</span></tt></td> 170</tr> 171<tr><td><tt class="docutils literal"><span class="pre">i.increment()</span></tt></td> 172<td>Advance by one position</td> 173</tr> 174<tr><td><tt class="docutils literal"><span class="pre">i.decrement()</span></tt></td> 175<td>Retreat by one position</td> 176</tr> 177<tr><td><tt class="docutils literal"><span class="pre">i.advance(n)</span></tt></td> 178<td>Advance by <tt class="docutils literal"><span class="pre">n</span></tt> positions</td> 179</tr> 180<tr><td><tt class="docutils literal"><span class="pre">i.distance_to(j)</span></tt></td> 181<td>Measure the distance to <tt class="docutils literal"><span class="pre">j</span></tt></td> 182</tr> 183</tbody> 184</table> 185</blockquote> 186<!-- Should we add a comment that a zero overhead implementation of iterator_facade 187is possible with proper inlining? --> 188<p>In addition to implementing the core interface functions, an iterator 189derived from <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> typically defines several 190constructors. To model any of the standard iterator concepts, the 191iterator must at least have a copy constructor. Also, if the iterator 192type <tt class="docutils literal"><span class="pre">X</span></tt> is meant to be automatically interoperate with another 193iterator type <tt class="docutils literal"><span class="pre">Y</span></tt> (as with constant and mutable iterators) then 194there must be an implicit conversion from <tt class="docutils literal"><span class="pre">X</span></tt> to <tt class="docutils literal"><span class="pre">Y</span></tt> or from <tt class="docutils literal"><span class="pre">Y</span></tt> 195to <tt class="docutils literal"><span class="pre">X</span></tt> (but not both), typically implemented as a conversion 196constructor. Finally, if the iterator is to model Forward Traversal 197Iterator or a more-refined iterator concept, a default constructor is 198required.</p> 199</div> 200<div class="section" id="iterator-core-access"> 201<h2><a class="toc-backref" href="#id25">Iterator Core Access</a></h2> 202<p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and the operator implementations need to be able 203to access the core member functions in the derived class. Making the 204core member functions public would expose an implementation detail to 205the user. The design used here ensures that implementation details do 206not appear in the public interface of the derived iterator type.</p> 207<p>Preventing direct access to the core member functions has two 208advantages. First, there is no possibility for the user to accidently 209use a member function of the iterator when a member of the value_type 210was intended. This has been an issue with smart pointer 211implementations in the past. The second and main advantage is that 212library implementers can freely exchange a hand-rolled iterator 213implementation for one based on <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> without fear of 214breaking code that was accessing the public core member functions 215directly.</p> 216<p>In a naive implementation, keeping the derived class' core member 217functions private would require it to grant friendship to 218<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and each of the seven operators. In order to 219reduce the burden of limiting access, <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> is 220provided, a class that acts as a gateway to the core member functions 221in the derived iterator class. The author of the derived class only 222needs to grant friendship to <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> to make his core 223member functions available to the library.</p> 224<!-- This is no long uptodate -thw --> 225<!-- Yes it is; I made sure of it! -DWA --> 226<p><tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> will be typically implemented as an empty 227class containing only private static member functions which invoke the 228iterator core member functions. There is, however, no need to 229standardize the gateway protocol. Note that even if 230<tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> used public member functions it would not 231open a safety loophole, as every core member function preserves the 232invariants of the iterator.</p> 233</div> 234<div class="section" id="operator"> 235<h2><a class="toc-backref" href="#id26"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></h2> 236<p>The indexing operator for a generalized iterator presents special 237challenges. A random access iterator's <tt class="docutils literal"><span class="pre">operator[]</span></tt> is only 238required to return something convertible to its <tt class="docutils literal"><span class="pre">value_type</span></tt>. 239Requiring that it return an lvalue would rule out currently-legal 240random-access iterators which hold the referenced value in a data 241member (e.g. <a class="reference external" href="counting_iterator.html"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a>), because <tt class="docutils literal"><span class="pre">*(p+n)</span></tt> is a reference 242into the temporary iterator <tt class="docutils literal"><span class="pre">p+n</span></tt>, which is destroyed when 243<tt class="docutils literal"><span class="pre">operator[]</span></tt> returns.</p> 244<p>Writable iterators built with <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implement the 245semantics required by the preferred resolution to <a class="reference external" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#299">issue 299</a> and 246adopted 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 247convertible to the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt>, and <tt class="docutils literal"><span class="pre">p[n]</span> <span class="pre">=</span> <span class="pre">x</span></tt> is 248equivalent to <tt class="docutils literal"><span class="pre">*(p</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">x</span></tt> (Note: This result object may be 249implemented as a proxy containing a copy of <tt class="docutils literal"><span class="pre">p+n</span></tt>). This approach 250will work properly for any random-access iterator regardless of the 251other details of its implementation. A user who knows more about 252the implementation of her iterator is free to implement an 253<tt class="docutils literal"><span class="pre">operator[]</span></tt> that returns an lvalue in the derived iterator 254class; it will hide the one supplied by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> from 255clients of her iterator.</p> 256</div> 257<div class="section" id="id2"> 258<span id="operator-arrow"></span><h2><a class="toc-backref" href="#id27"><tt class="docutils literal"><span class="pre">operator-></span></tt></a></h2> 259<p>The <tt class="docutils literal"><span class="pre">reference</span></tt> type of a readable iterator (and today's input 260iterator) need not in fact be a reference, so long as it is 261convertible to the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt>. When the <tt class="docutils literal"><span class="pre">value_type</span></tt> 262is a class, however, it must still be possible to access members 263through <tt class="docutils literal"><span class="pre">operator-></span></tt>. Therefore, an iterator whose <tt class="docutils literal"><span class="pre">reference</span></tt> 264type is not in fact a reference must return a proxy containing a copy 265of the referenced value from its <tt class="docutils literal"><span class="pre">operator-></span></tt>.</p> 266<p>The return types for <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s <tt class="docutils literal"><span class="pre">operator-></span></tt> and 267<tt class="docutils literal"><span class="pre">operator[]</span></tt> are not explicitly specified. Instead, those types 268are described in terms of a set of requirements, which must be 269satisfied by the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implementation.</p> 270<table class="docutils citation" frame="void" id="cop95" rules="none"> 271<colgroup><col class="label" /><col /></colgroup> 272<tbody valign="top"> 273<tr><td class="label">[Cop95]</td><td><em>(<a class="fn-backref" href="#id1">1</a>, <a class="fn-backref" href="#id10">2</a>)</em> [Coplien, 1995] Coplien, J., Curiously Recurring Template 274Patterns, C++ Report, February 1995, pp. 24-27.</td></tr> 275</tbody> 276</table> 277</div> 278</div> 279<div class="section" id="reference"> 280<h1><a class="toc-backref" href="#id28">Reference</a></h1> 281<!-- Distributed under the Boost --> 282<!-- Software License, Version 1.0. (See accompanying --> 283<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 284<!-- Version 1.3 of this ReStructuredText document corresponds to 285n1530_, the paper accepted by the LWG for TR1. --> 286<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. --> 287<pre class="literal-block"> 288template < 289 class Derived 290 , class Value 291 , class CategoryOrTraversal 292 , class Reference = Value& 293 , class Difference = ptrdiff_t 294> 295class iterator_facade { 296 public: 297 typedef remove_const<Value>::type value_type; 298 typedef Reference reference; 299 typedef Value* pointer; 300 typedef Difference difference_type; 301 typedef /* see <a class="reference internal" href="#iterator-category">below</a> */ iterator_category; 302 303 reference operator*() const; 304 /* see <a class="reference internal" href="#operator-arrow">below</a> */ operator->() const; 305 /* see <a class="reference internal" href="#brackets">below</a> */ operator[](difference_type n) const; 306 Derived& operator++(); 307 Derived operator++(int); 308 Derived& operator--(); 309 Derived operator--(int); 310 Derived& operator+=(difference_type n); 311 Derived& operator-=(difference_type n); 312 Derived operator-(difference_type n) const; 313 protected: 314 typedef iterator_facade iterator_facade_; 315}; 316 317// Comparison operators 318template <class Dr1, class V1, class TC1, class R1, class D1, 319 class Dr2, class V2, class TC2, class R2, class D2> 320typename enable_if_interoperable<Dr1,Dr2,bool>::type // exposition 321operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 322 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 323 324template <class Dr1, class V1, class TC1, class R1, class D1, 325 class Dr2, class V2, class TC2, class R2, class D2> 326typename enable_if_interoperable<Dr1,Dr2,bool>::type 327operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 328 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 329 330template <class Dr1, class V1, class TC1, class R1, class D1, 331 class Dr2, class V2, class TC2, class R2, class D2> 332typename enable_if_interoperable<Dr1,Dr2,bool>::type 333operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 334 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 335 336template <class Dr1, class V1, class TC1, class R1, class D1, 337 class Dr2, class V2, class TC2, class R2, class D2> 338typename enable_if_interoperable<Dr1,Dr2,bool>::type 339operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 340 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 341 342template <class Dr1, class V1, class TC1, class R1, class D1, 343 class Dr2, class V2, class TC2, class R2, class D2> 344typename enable_if_interoperable<Dr1,Dr2,bool>::type 345operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 346 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 347 348template <class Dr1, class V1, class TC1, class R1, class D1, 349 class Dr2, class V2, class TC2, class R2, class D2> 350typename enable_if_interoperable<Dr1,Dr2,bool>::type 351operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 352 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 353 354// Iterator difference 355template <class Dr1, class V1, class TC1, class R1, class D1, 356 class Dr2, class V2, class TC2, class R2, class D2> 357/* see <a class="reference internal" href="#minus">below</a> */ 358operator-(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 359 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 360 361// Iterator addition 362template <class Dr, class V, class TC, class R, class D> 363Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&, 364 typename Derived::difference_type n); 365 366template <class Dr, class V, class TC, class R, class D> 367Derived operator+ (typename Derived::difference_type n, 368 iterator_facade<Dr,V,TC,R,D> const&); 369</pre> 370<p id="iterator-category">The <tt class="docutils literal"><span class="pre">iterator_category</span></tt> member of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is</p> 371<pre class="literal-block"> 372<em>iterator-category</em>(CategoryOrTraversal, value_type, reference) 373</pre> 374<p>where <em>iterator-category</em> is defined as follows:</p> 375<pre class="literal-block" id="id7"> 376<em>iterator-category</em>(C,R,V) := 377 if (C is convertible to std::input_iterator_tag 378 || C is convertible to std::output_iterator_tag 379 ) 380 return C 381 382 else if (C is not convertible to incrementable_traversal_tag) 383 <em>the program is ill-formed</em> 384 385 else return a type X satisfying the following two constraints: 386 387 1. X is convertible to X1, and not to any more-derived 388 type, where X1 is defined by: 389 390 if (R is a reference type 391 && C is convertible to forward_traversal_tag) 392 { 393 if (C is convertible to random_access_traversal_tag) 394 X1 = random_access_iterator_tag 395 else if (C is convertible to bidirectional_traversal_tag) 396 X1 = bidirectional_iterator_tag 397 else 398 X1 = forward_iterator_tag 399 } 400 else 401 { 402 if (C is convertible to single_pass_traversal_tag 403 && R is convertible to V) 404 X1 = input_iterator_tag 405 else 406 X1 = C 407 } 408 409 2. <a class="reference external" href="new-iter-concepts.html#category-to-traversal"><em>category-to-traversal</em></a>(X) is convertible to the most 410 derived traversal tag type to which X is also 411 convertible, and not to any more-derived traversal tag 412 type. 413</pre> 414<p>[Note: the intention is to allow <tt class="docutils literal"><span class="pre">iterator_category</span></tt> to be one of 415the five original category tags when convertibility to one of the 416traversal tags would add no information]</p> 417<!-- Copyright David Abrahams 2004. Use, modification and distribution is --> 418<!-- subject to the Boost Software License, Version 1.0. (See accompanying --> 419<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 420<p>The <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> template used above is for exposition 421purposes. The member operators should only be in an overload set 422provided the derived types <tt class="docutils literal"><span class="pre">Dr1</span></tt> and <tt class="docutils literal"><span class="pre">Dr2</span></tt> are interoperable, 423meaning that at least one of the types is convertible to the other. The 424<tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> approach uses SFINAE to take the operators 425out of the overload set when the types are not interoperable. 426The operators should behave <em>as-if</em> <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> 427were defined to be:</p> 428<pre class="literal-block"> 429template <bool, typename> enable_if_interoperable_impl 430{}; 431 432template <typename T> enable_if_interoperable_impl<true,T> 433{ typedef T type; }; 434 435template<typename Dr1, typename Dr2, typename T> 436struct enable_if_interoperable 437 : enable_if_interoperable_impl< 438 is_convertible<Dr1,Dr2>::value || is_convertible<Dr2,Dr1>::value 439 , T 440 > 441{}; 442</pre> 443<div class="section" id="iterator-facade-requirements"> 444<h2><a class="toc-backref" href="#id29"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Requirements</a></h2> 445<p>The following table describes the typical valid expressions on 446<tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s <tt class="docutils literal"><span class="pre">Derived</span></tt> parameter, depending on the 447iterator concept(s) it will model. The operations in the first 448column must be made accessible to member functions of class 449<tt class="docutils literal"><span class="pre">iterator_core_access</span></tt>. In addition, 450<tt class="docutils literal"><span class="pre">static_cast<Derived*>(iterator_facade*)</span></tt> shall be well-formed.</p> 451<p>In the table below, <tt class="docutils literal"><span class="pre">F</span></tt> is <tt class="docutils literal"><span class="pre">iterator_facade<X,V,C,R,D></span></tt>, <tt class="docutils literal"><span class="pre">a</span></tt> is an 452object of type <tt class="docutils literal"><span class="pre">X</span></tt>, <tt class="docutils literal"><span class="pre">b</span></tt> and <tt class="docutils literal"><span class="pre">c</span></tt> are objects of type <tt class="docutils literal"><span class="pre">const</span> <span class="pre">X</span></tt>, 453<tt class="docutils literal"><span class="pre">n</span></tt> is an object of <tt class="docutils literal"><span class="pre">F::difference_type</span></tt>, <tt class="docutils literal"><span class="pre">y</span></tt> is a constant 454object of a single pass iterator type interoperable with <tt class="docutils literal"><span class="pre">X</span></tt>, and <tt class="docutils literal"><span class="pre">z</span></tt> 455is a constant object of a random access traversal iterator type 456interoperable with <tt class="docutils literal"><span class="pre">X</span></tt>.</p> 457<div class="topic" id="core-operations"> 458<p class="topic-title first"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Core Operations</p> 459<table border="1" class="docutils"> 460<colgroup> 461<col width="21%" /> 462<col width="23%" /> 463<col width="27%" /> 464<col width="29%" /> 465</colgroup> 466<thead valign="bottom"> 467<tr><th class="head">Expression</th> 468<th class="head">Return Type</th> 469<th class="head">Assertion/Note</th> 470<th class="head">Used to implement Iterator 471Concept(s)</th> 472</tr> 473</thead> 474<tbody valign="top"> 475<tr><td><tt class="docutils literal"><span class="pre">c.dereference()</span></tt></td> 476<td><tt class="docutils literal"><span class="pre">F::reference</span></tt></td> 477<td> </td> 478<td>Readable Iterator, Writable 479Iterator</td> 480</tr> 481<tr><td><tt class="docutils literal"><span class="pre">c.equal(y)</span></tt></td> 482<td>convertible to bool</td> 483<td>true iff <tt class="docutils literal"><span class="pre">c</span></tt> and <tt class="docutils literal"><span class="pre">y</span></tt> 484refer to the same 485position.</td> 486<td>Single Pass Iterator</td> 487</tr> 488<tr><td><tt class="docutils literal"><span class="pre">a.increment()</span></tt></td> 489<td>unused</td> 490<td> </td> 491<td>Incrementable Iterator</td> 492</tr> 493<tr><td><tt class="docutils literal"><span class="pre">a.decrement()</span></tt></td> 494<td>unused</td> 495<td> </td> 496<td>Bidirectional Traversal 497Iterator</td> 498</tr> 499<tr><td><tt class="docutils literal"><span class="pre">a.advance(n)</span></tt></td> 500<td>unused</td> 501<td> </td> 502<td>Random Access Traversal 503Iterator</td> 504</tr> 505<tr><td><tt class="docutils literal"><span class="pre">c.distance_to(z)</span></tt></td> 506<td>convertible to 507<tt class="docutils literal"><span class="pre">F::difference_type</span></tt></td> 508<td>equivalent to 509<tt class="docutils literal"><span class="pre">distance(c,</span> <span class="pre">X(z))</span></tt>.</td> 510<td>Random Access Traversal 511Iterator</td> 512</tr> 513</tbody> 514</table> 515</div> 516</div> 517<div class="section" id="iterator-facade-operations"> 518<h2><a class="toc-backref" href="#id30"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> operations</a></h2> 519<p>The operations in this section are described in terms of operations on 520the core interface of <tt class="docutils literal"><span class="pre">Derived</span></tt> which may be inaccessible 521(i.e. private). The implementation should access these operations 522through member functions of class <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt>.</p> 523<p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p> 524<table class="docutils field-list" frame="void" rules="none"> 525<col class="field-name" /> 526<col class="field-body" /> 527<tbody valign="top"> 528<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">static_cast<Derived</span> <span class="pre">const*>(this)->dereference()</span></tt></td> 529</tr> 530</tbody> 531</table> 532<p><tt class="docutils literal"><span class="pre">operator->()</span> <span class="pre">const;</span></tt> (see <a class="reference internal" href="#operator-arrow">below</a>)</p> 533<table class="docutils field-list" frame="void" rules="none"> 534<col class="field-name" /> 535<col class="field-body" /> 536<tbody valign="top"> 537<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">If <tt class="docutils literal"><span class="pre">reference</span></tt> is a reference type, an object 538of type <tt class="docutils literal"><span class="pre">pointer</span></tt> equal to:</p> 539<pre class="literal-block"> 540&static_cast<Derived const*>(this)->dereference() 541</pre> 542<p class="last">Otherwise returns an object of unspecified type such that, 543<tt class="docutils literal"><span class="pre">(*static_cast<Derived</span> <span class="pre">const*>(this))->m</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">(w</span> <span class="pre">=</span> <span class="pre">**static_cast<Derived</span> <span class="pre">const*>(this),</span> 544<span class="pre">w.m)</span></tt> for some temporary object <tt class="docutils literal"><span class="pre">w</span></tt> of type <tt class="docutils literal"><span class="pre">value_type</span></tt>.</p> 545</td> 546</tr> 547</tbody> 548</table> 549<p id="brackets"><em>unspecified</em> <tt class="docutils literal"><span class="pre">operator[](difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p> 550<table class="docutils field-list" frame="void" rules="none"> 551<col class="field-name" /> 552<col class="field-body" /> 553<tbody valign="top"> 554<tr class="field"><th class="field-name">Returns:</th><td class="field-body">an object convertible to <tt class="docutils literal"><span class="pre">value_type</span></tt>. For constant 555objects <tt class="docutils literal"><span class="pre">v</span></tt> of type <tt class="docutils literal"><span class="pre">value_type</span></tt>, and <tt class="docutils literal"><span class="pre">n</span></tt> of type 556<tt class="docutils literal"><span class="pre">difference_type</span></tt>, <tt class="docutils literal"><span class="pre">(*this)[n]</span> <span class="pre">=</span> <span class="pre">v</span></tt> is equivalent to 557<tt class="docutils literal"><span class="pre">*(*this</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">v</span></tt>, and <tt class="docutils literal"><span class="pre">static_cast<value_type</span> 558<span class="pre">const&>((*this)[n])</span></tt> is equivalent to 559<tt class="docutils literal"><span class="pre">static_cast<value_type</span> <span class="pre">const&>(*(*this</span> <span class="pre">+</span> <span class="pre">n))</span></tt></td> 560</tr> 561</tbody> 562</table> 563<p><tt class="docutils literal"><span class="pre">Derived&</span> <span class="pre">operator++();</span></tt></p> 564<table class="docutils field-list" frame="void" rules="none"> 565<col class="field-name" /> 566<col class="field-body" /> 567<tbody valign="top"> 568<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block"> 569static_cast<Derived*>(this)->increment(); 570return *static_cast<Derived*>(this); 571</pre> 572</td> 573</tr> 574</tbody> 575</table> 576<p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator++(int);</span></tt></p> 577<table class="docutils field-list" frame="void" rules="none"> 578<col class="field-name" /> 579<col class="field-body" /> 580<tbody valign="top"> 581<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block"> 582Derived tmp(static_cast<Derived const*>(this)); 583++*this; 584return tmp; 585</pre> 586</td> 587</tr> 588</tbody> 589</table> 590<p><tt class="docutils literal"><span class="pre">Derived&</span> <span class="pre">operator--();</span></tt></p> 591<table class="docutils field-list" frame="void" rules="none"> 592<col class="field-name" /> 593<col class="field-body" /> 594<tbody valign="top"> 595<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block"> 596static_cast<Derived*>(this)->decrement(); 597return *static_cast<Derived*>(this); 598</pre> 599</td> 600</tr> 601</tbody> 602</table> 603<p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator--(int);</span></tt></p> 604<table class="docutils field-list" frame="void" rules="none"> 605<col class="field-name" /> 606<col class="field-body" /> 607<tbody valign="top"> 608<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block"> 609Derived tmp(static_cast<Derived const*>(this)); 610--*this; 611return tmp; 612</pre> 613</td> 614</tr> 615</tbody> 616</table> 617<p><tt class="docutils literal"><span class="pre">Derived&</span> <span class="pre">operator+=(difference_type</span> <span class="pre">n);</span></tt></p> 618<table class="docutils field-list" frame="void" rules="none"> 619<col class="field-name" /> 620<col class="field-body" /> 621<tbody valign="top"> 622<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block"> 623static_cast<Derived*>(this)->advance(n); 624return *static_cast<Derived*>(this); 625</pre> 626</td> 627</tr> 628</tbody> 629</table> 630<p><tt class="docutils literal"><span class="pre">Derived&</span> <span class="pre">operator-=(difference_type</span> <span class="pre">n);</span></tt></p> 631<table class="docutils field-list" frame="void" rules="none"> 632<col class="field-name" /> 633<col class="field-body" /> 634<tbody valign="top"> 635<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block"> 636static_cast<Derived*>(this)->advance(-n); 637return *static_cast<Derived*>(this); 638</pre> 639</td> 640</tr> 641</tbody> 642</table> 643<p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator-(difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p> 644<table class="docutils field-list" frame="void" rules="none"> 645<col class="field-name" /> 646<col class="field-body" /> 647<tbody valign="top"> 648<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block"> 649Derived tmp(static_cast<Derived const*>(this)); 650return tmp -= n; 651</pre> 652</td> 653</tr> 654</tbody> 655</table> 656<pre class="literal-block"> 657template <class Dr, class V, class TC, class R, class D> 658Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&, 659 typename Derived::difference_type n); 660 661template <class Dr, class V, class TC, class R, class D> 662Derived operator+ (typename Derived::difference_type n, 663 iterator_facade<Dr,V,TC,R,D> const&); 664</pre> 665<table class="docutils field-list" frame="void" rules="none"> 666<col class="field-name" /> 667<col class="field-body" /> 668<tbody valign="top"> 669<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block"> 670Derived tmp(static_cast<Derived const*>(this)); 671return tmp += n; 672</pre> 673</td> 674</tr> 675</tbody> 676</table> 677<pre class="literal-block"> 678template <class Dr1, class V1, class TC1, class R1, class D1, 679 class Dr2, class V2, class TC2, class R2, class D2> 680typename enable_if_interoperable<Dr1,Dr2,bool>::type 681operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 682 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 683</pre> 684<table class="docutils field-list" frame="void" rules="none"> 685<col class="field-name" /> 686<col class="field-body" /> 687<tbody valign="top"> 688<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p> 689<dl class="last docutils"> 690<dt>then</dt> 691<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).equal((Dr2</span> <span class="pre">const&)rhs)</span></tt>.</p> 692</dd> 693<dt>Otherwise,</dt> 694<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).equal((Dr1</span> <span class="pre">const&)lhs)</span></tt>.</p> 695</dd> 696</dl> 697</td> 698</tr> 699</tbody> 700</table> 701<pre class="literal-block"> 702template <class Dr1, class V1, class TC1, class R1, class D1, 703 class Dr2, class V2, class TC2, class R2, class D2> 704typename enable_if_interoperable<Dr1,Dr2,bool>::type 705operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 706 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 707</pre> 708<table class="docutils field-list" frame="void" rules="none"> 709<col class="field-name" /> 710<col class="field-body" /> 711<tbody valign="top"> 712<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p> 713<dl class="last docutils"> 714<dt>then</dt> 715<dd><p class="first last"><tt class="docutils literal"><span class="pre">!((Dr1</span> <span class="pre">const&)lhs).equal((Dr2</span> <span class="pre">const&)rhs)</span></tt>.</p> 716</dd> 717<dt>Otherwise,</dt> 718<dd><p class="first last"><tt class="docutils literal"><span class="pre">!((Dr2</span> <span class="pre">const&)rhs).equal((Dr1</span> <span class="pre">const&)lhs)</span></tt>.</p> 719</dd> 720</dl> 721</td> 722</tr> 723</tbody> 724</table> 725<pre class="literal-block"> 726template <class Dr1, class V1, class TC1, class R1, class D1, 727 class Dr2, class V2, class TC2, class R2, class D2> 728typename enable_if_interoperable<Dr1,Dr2,bool>::type 729operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 730 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 731</pre> 732<table class="docutils field-list" frame="void" rules="none"> 733<col class="field-name" /> 734<col class="field-body" /> 735<tbody valign="top"> 736<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p> 737<dl class="last docutils"> 738<dt>then</dt> 739<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre"><</span> <span class="pre">0</span></tt>.</p> 740</dd> 741<dt>Otherwise,</dt> 742<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre">></span> <span class="pre">0</span></tt>.</p> 743</dd> 744</dl> 745</td> 746</tr> 747</tbody> 748</table> 749<pre class="literal-block"> 750template <class Dr1, class V1, class TC1, class R1, class D1, 751 class Dr2, class V2, class TC2, class R2, class D2> 752typename enable_if_interoperable<Dr1,Dr2,bool>::type 753operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 754 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 755</pre> 756<table class="docutils field-list" frame="void" rules="none"> 757<col class="field-name" /> 758<col class="field-body" /> 759<tbody valign="top"> 760<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p> 761<dl class="last docutils"> 762<dt>then</dt> 763<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre"><=</span> <span class="pre">0</span></tt>.</p> 764</dd> 765<dt>Otherwise,</dt> 766<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre">>=</span> <span class="pre">0</span></tt>.</p> 767</dd> 768</dl> 769</td> 770</tr> 771</tbody> 772</table> 773<pre class="literal-block"> 774template <class Dr1, class V1, class TC1, class R1, class D1, 775 class Dr2, class V2, class TC2, class R2, class D2> 776typename enable_if_interoperable<Dr1,Dr2,bool>::type 777operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 778 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 779</pre> 780<table class="docutils field-list" frame="void" rules="none"> 781<col class="field-name" /> 782<col class="field-body" /> 783<tbody valign="top"> 784<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p> 785<dl class="last docutils"> 786<dt>then</dt> 787<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre">></span> <span class="pre">0</span></tt>.</p> 788</dd> 789<dt>Otherwise,</dt> 790<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre"><</span> <span class="pre">0</span></tt>.</p> 791</dd> 792</dl> 793</td> 794</tr> 795</tbody> 796</table> 797<pre class="literal-block"> 798template <class Dr1, class V1, class TC1, class R1, class D1, 799 class Dr2, class V2, class TC2, class R2, class D2> 800typename enable_if_interoperable<Dr1,Dr2,bool>::type 801operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 802 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 803</pre> 804<table class="docutils field-list" frame="void" rules="none"> 805<col class="field-name" /> 806<col class="field-body" /> 807<tbody valign="top"> 808<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p> 809<dl class="last docutils"> 810<dt>then</dt> 811<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre">>=</span> <span class="pre">0</span></tt>.</p> 812</dd> 813<dt>Otherwise,</dt> 814<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre"><=</span> <span class="pre">0</span></tt>.</p> 815</dd> 816</dl> 817</td> 818</tr> 819</tbody> 820</table> 821<pre class="literal-block" id="minus"> 822template <class Dr1, class V1, class TC1, class R1, class D1, 823 class Dr2, class V2, class TC2, class R2, class D2> 824typename enable_if_interoperable<Dr1,Dr2,difference>::type 825operator -(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, 826 iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); 827</pre> 828<table class="docutils field-list" frame="void" rules="none"> 829<col class="field-name" /> 830<col class="field-body" /> 831<tbody valign="top"> 832<tr class="field"><th class="field-name">Return Type:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p> 833<blockquote> 834<dl class="docutils"> 835<dt>then</dt> 836<dd><p class="first last"><tt class="docutils literal"><span class="pre">difference</span></tt> shall be 837<tt class="docutils literal"><span class="pre">iterator_traits<Dr1>::difference_type</span></tt>.</p> 838</dd> 839<dt>Otherwise</dt> 840<dd><p class="first last"><tt class="docutils literal"><span class="pre">difference</span></tt> shall be <tt class="docutils literal"><span class="pre">iterator_traits<Dr2>::difference_type</span></tt></p> 841</dd> 842</dl> 843</blockquote> 844</td> 845</tr> 846<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p> 847<dl class="last docutils"> 848<dt>then</dt> 849<dd><p class="first last"><tt class="docutils literal"><span class="pre">-((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span></tt>.</p> 850</dd> 851<dt>Otherwise,</dt> 852<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span></tt>.</p> 853</dd> 854</dl> 855</td> 856</tr> 857</tbody> 858</table> 859</div> 860</div> 861<div class="section" id="tutorial-example"> 862<h1><a class="toc-backref" href="#id31">Tutorial Example</a></h1> 863<!-- Copyright David Abrahams 2004. Use, modification and distribution is --> 864<!-- subject to the Boost Software License, Version 1.0. (See accompanying --> 865<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 866<p>In this section we'll walk through the implementation of a few 867iterators using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, based around the simple 868example of a linked list of polymorphic objects. This example was 869inspired by a <a class="reference external" href="http://thread.gmane.org/gmane.comp.lib.boost.user/5100">posting</a> by Keith Macdonald on the <a class="reference external" href="http://www.boost.org/more/mailing_lis…
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