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 <div class="document" id="iterator-facade-and-adaptor">
 <h1 class="title">Iterator Facade and Adaptor</h1>
 <table class="docinfo" frame="void" rules="none">
 <col class="docinfo-name" />
 <col class="docinfo-content" />
 <tbody valign="top">
 <tr><th class="docinfo-name">Author:</th>
 <td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr>
 <tr><th class="docinfo-name">Contact:</th>
 <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>
 <tr><th class="docinfo-name">Organization:</th>
 <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
 Lab</a>, <a class="last reference external" href="http://www.styleadvisor.com">Zephyr Associates, Inc.</a></td></tr>
 <tr><th class="docinfo-name">Date:</th>
 <td>2006-09-11</td></tr>
 <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
 accepted for Technical Report 1 by the C++ standard
 committee's library working group.</td>
 </tr>
 </tbody>
 </table>
 <!-- Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <!-- Version 1.9 of this ReStructuredText document corresponds to
 n1530_, the paper accepted by the LWG. -->
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">copyright:</th><td class="field-body">Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td>
 </tr>
 </tbody>
 </table>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">abstract:</th><td class="field-body">We propose a set of class templates that help programmers
 build standard-conforming iterators, both from scratch and
 by adapting other iterators.</td>
 </tr>
 </tbody>
 </table>
 <div class="contents topic" id="table-of-contents">
 <p class="topic-title first">Table of Contents</p>
 <ul class="simple">
 <li><a class="reference internal" href="#motivation" id="id15">Motivation</a></li>
 <li><a class="reference internal" href="#impact-on-the-standard" id="id16">Impact on the Standard</a></li>
 <li><a class="reference internal" href="#design" id="id17">Design</a><ul>
 <li><a class="reference internal" href="#iterator-concepts" id="id18">Iterator Concepts</a></li>
 <li><a class="reference internal" href="#interoperability" id="id19">Interoperability</a></li>
 <li><a class="reference internal" href="#iterator-facade" id="id20">Iterator Facade</a><ul>
 <li><a class="reference internal" href="#usage" id="id21">Usage</a></li>
 <li><a class="reference internal" href="#iterator-core-access" id="id22">Iterator Core Access</a></li>
 <li><a class="reference internal" href="#operator" id="id23"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></li>
 <li><a class="reference internal" href="#id6" id="id24"><tt class="docutils literal"><span class="pre">operator-&gt;</span></tt></a></li>
 </ul>
 </li>
 <li><a class="reference internal" href="#iterator-adaptor" id="id25">Iterator Adaptor</a></li>
 <li><a class="reference internal" href="#specialized-adaptors" id="id26">Specialized Adaptors</a></li>
 </ul>
 </li>
 <li><a class="reference internal" href="#proposed-text" id="id27">Proposed Text</a><ul>
 <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>
 <li><a class="reference internal" href="#iterator-facade-lib-iterator-facade" id="id29">Iterator facade [lib.iterator.facade]</a><ul>
 <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>
 <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>
 <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>
 </ul>
 </li>
 <li><a class="reference internal" href="#iterator-adaptor-lib-iterator-adaptor" id="id33">Iterator adaptor [lib.iterator.adaptor]</a><ul>
 <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>
 <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>
 <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>
 <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>
 <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>
 <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>
 </ul>
 </li>
 <li><a class="reference internal" href="#specialized-adaptors-lib-iterator-special-adaptors" id="id40">Specialized adaptors [lib.iterator.special.adaptors]</a><ul>
 <li><a class="reference internal" href="#indirect-iterator" id="id41">Indirect iterator</a><ul>
 <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>
 <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>
 <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>
 <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>
 <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>
 <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>
 </ul>
 </li>
 <li><a class="reference internal" href="#reverse-iterator" id="id48">Reverse iterator</a><ul>
 <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>
 <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>
 <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>
 <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>
 </ul>
 </li>
 <li><a class="reference internal" href="#transform-iterator" id="id53">Transform iterator</a><ul>
 <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>
 <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>
 <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>
 <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>
 </ul>
 </li>
 <li><a class="reference internal" href="#filter-iterator" id="id58">Filter iterator</a><ul>
 <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>
 <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>
 <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>
 <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>
 </ul>
 </li>
 <li><a class="reference internal" href="#counting-iterator" id="id63">Counting iterator</a><ul>
 <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>
 <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>
 <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>
 <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>
 </ul>
 </li>
 <li><a class="reference internal" href="#function-output-iterator" id="id68">Function output iterator</a><ul>
 <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>
 <li><a class="reference internal" href="#header" id="id70">Header</a></li>
 <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>
 <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>
 <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>
 </ul>
 </li>
 </ul>
 </li>
 </ul>
 </li>
 </ul>
 </div>
 <div class="section" id="motivation">
 <h1><a class="toc-backref" href="#id15">Motivation</a></h1>
 <p>Iterators play an important role in modern C++ programming. The
 iterator is the central abstraction of the algorithms of the Standard
 Library, allowing algorithms to be re-used in in a wide variety of
 contexts.  The C++ Standard Library contains a wide variety of useful
 iterators. Every one of the standard containers comes with constant
 and mutable iterators<a class="footnote-reference" href="#mutable" id="id1"><sup>2</sup></a>, and also reverse versions of those
 same iterators which traverse the container in the opposite direction.
 The Standard also supplies <tt class="docutils literal"><span class="pre">istream_iterator</span></tt> and
 <tt class="docutils literal"><span class="pre">ostream_iterator</span></tt> for reading from and writing to streams,
 <tt class="docutils literal"><span class="pre">insert_iterator</span></tt>, <tt class="docutils literal"><span class="pre">front_insert_iterator</span></tt> and
 <tt class="docutils literal"><span class="pre">back_insert_iterator</span></tt> for inserting elements into containers, and
 <tt class="docutils literal"><span class="pre">raw_storage_iterator</span></tt> for initializing raw memory [7].</p>
 <p>Despite the many iterators supplied by the Standard Library, obvious
 and useful iterators are missing, and creating new iterator types is
 still a common task for C++ programmers.  The literature documents
 several of these, for example line_iterator [3] and Constant_iterator
 [9].  The iterator abstraction is so powerful that we expect
 programmers will always need to invent new iterator types.</p>
 <p>Although it is easy to create iterators that <em>almost</em> conform to the
 standard, the iterator requirements contain subtleties which can make
 creating an iterator which <em>actually</em> conforms quite difficult.
 Further, the iterator interface is rich, containing many operators
 that are technically redundant and tedious to implement.  To automate
 the repetitive work of constructing iterators, we propose
 <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, an iterator base class template which provides
 the rich interface of standard iterators and delegates its
 implementation to member functions of the derived class.  In addition
 to reducing the amount of code necessary to create an iterator, the
 <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> also provides compile-time error detection.
 Iterator implementation mistakes that often go unnoticed are turned
 into compile-time errors because the derived class implementation must
 match the expectations of the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>.</p>
 <p>A common pattern of iterator construction is the adaptation of one
 iterator to form a new one.  The functionality of an iterator is
 composed of four orthogonal aspects: traversal, indirection, equality
 comparison and distance measurement.  Adapting an old iterator to
 create a new one often saves work because one can reuse one aspect of
 functionality while redefining the other.  For example, the Standard
 provides <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt>, which adapts any Bidirectional Iterator
 by inverting its direction of traversal.  As with plain iterators,
 iterator adaptors defined outside the Standard have become commonplace
 in the literature:</p>
 <ul class="simple">
 <li>Checked iter[13] adds bounds-checking to an existing iterator.</li>
 <li>The iterators of the View Template Library[14], which adapts
 containers, are themselves adaptors over the underlying iterators.</li>
 <li>Smart iterators [5] adapt an iterator's dereferencing behavior by
 applying a function object to the object being referenced and
 returning the result.</li>
 <li>Custom iterators [4], in which a variety of adaptor types are enumerated.</li>
 <li>Compound iterators [1], which access a slice out of a container of containers.</li>
 <li>Several iterator adaptors from the MTL [12].  The MTL contains a
 strided iterator, where each call to <tt class="docutils literal"><span class="pre">operator++()</span></tt> moves the
 iterator ahead by some constant factor, and a scaled iterator, which
 multiplies the dereferenced value by some constant.</li>
 </ul>
 <table class="docutils footnote" frame="void" id="concept" rules="none">
 <colgroup><col class="label" /><col /></colgroup>
 <tbody valign="top">
 <tr><td class="label">[1]</td><td>We use the term concept to mean a set of requirements
 that a type must satisfy to be used with a particular template
 parameter.</td></tr>
 </tbody>
 </table>
 <table class="docutils footnote" frame="void" id="mutable" rules="none">
 <colgroup><col class="label" /><col /></colgroup>
 <tbody valign="top">
 <tr><td class="label"><a class="fn-backref" href="#id1">[2]</a></td><td>The term mutable iterator refers to iterators over objects that
 can be changed by assigning to the dereferenced iterator, while
 constant iterator refers to iterators over objects that cannot be
 modified.</td></tr>
 </tbody>
 </table>
 <p>To fulfill the need for constructing adaptors, we propose the
 <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template.  Instantiations of
 <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> serve as a base classes for new iterators,
 providing the default behavior of forwarding all operations to the
 underlying iterator.  The user can selectively replace these features
 in the derived iterator class.  This proposal also includes a number
 of more specialized adaptors, such as the <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> that
 applies some user-specified function during the dereference of the
 iterator.</p>
 </div>
 <div class="section" id="impact-on-the-standard">
 <h1><a class="toc-backref" href="#id16">Impact on the Standard</a></h1>
 <p>This proposal is purely an addition to the C++ standard library.
 However, note that this proposal relies on the proposal for New
 Iterator Concepts.</p>
 </div>
 <div class="section" id="design">
 <h1><a class="toc-backref" href="#id17">Design</a></h1>
 <div class="section" id="iterator-concepts">
 <h2><a class="toc-backref" href="#id18">Iterator Concepts</a></h2>
 <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>
 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
 iterators suffer from the well known categorization problems that are
 inherent to the current iterator categories.</p>
 <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
 is a direct mapping between new and old categories. This proposal
 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>
 </div>
 <div class="section" id="interoperability">
 <h2><a class="toc-backref" href="#id19">Interoperability</a></h2>
 <p>The question of iterator interoperability is poorly addressed in the
 current standard.  There are currently two defect reports that are
 concerned with interoperability issues.</p>
 <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
 are only required to be convertible to the corresponding constant
 iterator types, but objects of these types are not required to
 interoperate in comparison or subtraction expressions.  This situation
 is tedious in practice and out of line with the way built in types
 work.  This proposal implements the proposed resolution to issue
 <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
 words, if an iterator type A has an implicit or user defined
 conversion to an iterator type B, the iterator types are interoperable
 and the usual set of operators are available.</p>
 <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
 reverse iterator types. The proposed new reverse_iterator template
 fixes the issues raised in 280. It provides the desired
 interoperability without introducing unwanted overloads.</p>
 </div>
 <div class="section" id="iterator-facade">
 <h2><a class="toc-backref" href="#id20">Iterator Facade</a></h2>
 <!-- Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <!-- Version 1.1 of this ReStructuredText document corresponds to
 n1530_, the paper accepted by the LWG for TR1. -->
 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 <p>While the iterator interface is rich, there is a core subset of the
 interface that is necessary for all the functionality.  We have
 identified the following core behaviors for iterators:</p>
 <ul class="simple">
 <li>dereferencing</li>
 <li>incrementing</li>
 <li>decrementing</li>
 <li>equality comparison</li>
 <li>random-access motion</li>
 <li>distance measurement</li>
 </ul>
 <p>In addition to the behaviors listed above, the core interface elements
 include the associated types exposed through iterator traits:
 <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
 <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p>
 <p>Iterator facade uses the Curiously Recurring Template
 Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id4">[Cop95]</a> so that the user can specify the behavior
 of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> in a derived class.  Former designs used
 policy objects to specify the behavior, but that approach was
 discarded for several reasons:</p>
 <blockquote>
 <ol class="arabic simple">
 <li>the creation and eventual copying of the policy object may create
 overhead that can be avoided with the current approach.</li>
 <li>The policy object approach does not allow for custom constructors
 on the created iterator types, an essential feature if
 <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> should be used in other library
 implementations.</li>
 <li>Without the use of CRTP, the standard requirement that an
 iterator's <tt class="docutils literal"><span class="pre">operator++</span></tt> returns the iterator type itself
 would mean that all iterators built with the library would
 have to be specializations of <tt class="docutils literal"><span class="pre">iterator_facade&lt;...&gt;</span></tt>, rather
 than something more descriptive like
 <tt class="docutils literal"><span class="pre">indirect_iterator&lt;T*&gt;</span></tt>.  Cumbersome type generator
 metafunctions would be needed to build new parameterized
 iterators, and a separate <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> layer would be
 impossible.</li>
 </ol>
 </blockquote>
 <div class="section" id="usage">
 <h3><a class="toc-backref" href="#id21">Usage</a></h3>
 <p>The user of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> derives his iterator class from a
 specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and passes the derived
 iterator class as <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s first template parameter.
 The order of the other template parameters have been carefully
 chosen to take advantage of useful defaults.  For example, when
 defining a constant lvalue iterator, the user can pass a
 const-qualified version of the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt> as
 <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
 <tt class="docutils literal"><span class="pre">Reference</span></tt> parameter which follows.</p>
 <p>The derived iterator class must define member functions implementing
 the iterator's core behaviors.  The following table describes
 expressions which are required to be valid depending on the category
 of the derived iterator type.  These member functions are described
 briefly below and in more detail in the iterator facade
 requirements.</p>
 <blockquote>
 <table border="1" class="docutils">
 <colgroup>
 <col width="44%" />
 <col width="56%" />
 </colgroup>
 <thead valign="bottom">
 <tr><th class="head">Expression</th>
 <th class="head">Effects</th>
 </tr>
 </thead>
 <tbody valign="top">
 <tr><td><tt class="docutils literal"><span class="pre">i.dereference()</span></tt></td>
 <td>Access the value referred to</td>
 </tr>
 <tr><td><tt class="docutils literal"><span class="pre">i.equal(j)</span></tt></td>
 <td>Compare for equality with <tt class="docutils literal"><span class="pre">j</span></tt></td>
 </tr>
 <tr><td><tt class="docutils literal"><span class="pre">i.increment()</span></tt></td>
 <td>Advance by one position</td>
 </tr>
 <tr><td><tt class="docutils literal"><span class="pre">i.decrement()</span></tt></td>
 <td>Retreat by one position</td>
 </tr>
 <tr><td><tt class="docutils literal"><span class="pre">i.advance(n)</span></tt></td>
 <td>Advance by <tt class="docutils literal"><span class="pre">n</span></tt> positions</td>
 </tr>
 <tr><td><tt class="docutils literal"><span class="pre">i.distance_to(j)</span></tt></td>
 <td>Measure the distance to <tt class="docutils literal"><span class="pre">j</span></tt></td>
 </tr>
 </tbody>
 </table>
 </blockquote>
 <!-- Should we add a comment that a zero overhead implementation of iterator_facade
 is possible with proper inlining? -->
 <p>In addition to implementing the core interface functions, an iterator
 derived from <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> typically defines several
 constructors. To model any of the standard iterator concepts, the
 iterator must at least have a copy constructor. Also, if the iterator
 type <tt class="docutils literal"><span class="pre">X</span></tt> is meant to be automatically interoperate with another
 iterator type <tt class="docutils literal"><span class="pre">Y</span></tt> (as with constant and mutable iterators) then
 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>
 to <tt class="docutils literal"><span class="pre">X</span></tt> (but not both), typically implemented as a conversion
 constructor. Finally, if the iterator is to model Forward Traversal
 Iterator or a more-refined iterator concept, a default constructor is
 required.</p>
 </div>
 <div class="section" id="iterator-core-access">
 <h3><a class="toc-backref" href="#id22">Iterator Core Access</a></h3>
 <p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and the operator implementations need to be able
 to access the core member functions in the derived class.  Making the
 core member functions public would expose an implementation detail to
 the user.  The design used here ensures that implementation details do
 not appear in the public interface of the derived iterator type.</p>
 <p>Preventing direct access to the core member functions has two
 advantages.  First, there is no possibility for the user to accidently
 use a member function of the iterator when a member of the value_type
 was intended.  This has been an issue with smart pointer
 implementations in the past.  The second and main advantage is that
 library implementers can freely exchange a hand-rolled iterator
 implementation for one based on <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> without fear of
 breaking code that was accessing the public core member functions
 directly.</p>
 <p>In a naive implementation, keeping the derived class' core member
 functions private would require it to grant friendship to
 <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and each of the seven operators.  In order to
 reduce the burden of limiting access, <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> is
 provided, a class that acts as a gateway to the core member functions
 in the derived iterator class.  The author of the derived class only
 needs to grant friendship to <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> to make his core
 member functions available to the library.</p>
 <!-- This is no long uptodate -thw -->
 <!-- Yes it is; I made sure of it! -DWA -->
 <p><tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> will be typically implemented as an empty
 class containing only private static member functions which invoke the
 iterator core member functions. There is, however, no need to
 standardize the gateway protocol.  Note that even if
 <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> used public member functions it would not
 open a safety loophole, as every core member function preserves the
 invariants of the iterator.</p>
 </div>
 <div class="section" id="operator">
 <h3><a class="toc-backref" href="#id23"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></h3>
 <p>The indexing operator for a generalized iterator presents special
 challenges.  A random access iterator's <tt class="docutils literal"><span class="pre">operator[]</span></tt> is only
 required to return something convertible to its <tt class="docutils literal"><span class="pre">value_type</span></tt>.
 Requiring that it return an lvalue would rule out currently-legal
 random-access iterators which hold the referenced value in a data
 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
 into the temporary iterator <tt class="docutils literal"><span class="pre">p+n</span></tt>, which is destroyed when
 <tt class="docutils literal"><span class="pre">operator[]</span></tt> returns.</p>
 <p>Writable iterators built with <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implement the
 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
 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
 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
 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
 implemented as a proxy containing a copy of <tt class="docutils literal"><span class="pre">p+n</span></tt>).  This approach
 will work properly for any random-access iterator regardless of the
 other details of its implementation.  A user who knows more about
 the implementation of her iterator is free to implement an
 <tt class="docutils literal"><span class="pre">operator[]</span></tt> that returns an lvalue in the derived iterator
 class; it will hide the one supplied by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> from
 clients of her iterator.</p>
 </div>
 <div class="section" id="id6">
 <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>
 <p>The <tt class="docutils literal"><span class="pre">reference</span></tt> type of a readable iterator (and today's input
 iterator) need not in fact be a reference, so long as it is
 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>
 is a class, however, it must still be possible to access members
 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>
 type is not in fact a reference must return a proxy containing a copy
 of the referenced value from its <tt class="docutils literal"><span class="pre">operator-&gt;</span></tt>.</p>
 <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
 <tt class="docutils literal"><span class="pre">operator[]</span></tt> are not explicitly specified. Instead, those types
 are described in terms of a set of requirements, which must be
 satisfied by the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implementation.</p>
 <table class="docutils citation" frame="void" id="cop95" rules="none">
 <colgroup><col class="label" /><col /></colgroup>
 <tbody valign="top">
 <tr><td class="label"><a class="fn-backref" href="#id4">[Cop95]</a></td><td>[Coplien, 1995] Coplien, J., Curiously Recurring Template
 Patterns, C++ Report, February 1995, pp. 24-27.</td></tr>
 </tbody>
 </table>
 </div>
 </div>
 <div class="section" id="iterator-adaptor">
 <h2><a class="toc-backref" href="#id25">Iterator Adaptor</a></h2>
 <!-- Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <!-- Version 1.2 of this ReStructuredText document corresponds to
 n1530_, the paper accepted by the LWG for TR1. -->
 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 <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>
 type to create a new iterator.  Instantiations of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>
 are derived from a corresponding instantiation of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>
 and implement the core behaviors in terms of the <tt class="docutils literal"><span class="pre">Base</span></tt> type. In
 essence, <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> merely forwards all operations to an
 instance of the <tt class="docutils literal"><span class="pre">Base</span></tt> type, which it stores as a member.</p>
 <table class="docutils footnote" frame="void" id="base" rules="none">
 <colgroup><col class="label" /><col /></colgroup>
 <tbody valign="top">
 <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
 not meant to imply the use of derivation. We have followed the lead
 of the standard library, which provides a base() function to access
 the underlying iterator object of a <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> adaptor.</td></tr>
 </tbody>
 </table>
 <p>The user of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> creates a class derived from an
 instantiation of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> and then selectively
 redefines some of the core member functions described in the
 <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
 not meet the full requirements for an iterator; it need only
 support the operations used by the core interface functions of
 <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> that have not been redefined in the user's
 derived class.</p>
 <p>Several of the template parameters of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> default
 to <tt class="docutils literal"><span class="pre">use_default</span></tt>. This allows the
 user to make use of a default parameter even when she wants to
 specify a parameter later in the parameter list.  Also, the
 defaults for the corresponding associated types are somewhat
 complicated, so metaprogramming is required to compute them, and
 <tt class="docutils literal"><span class="pre">use_default</span></tt> can help to simplify the implementation.  Finally,
 the identity of the <tt class="docutils literal"><span class="pre">use_default</span></tt> type is not left unspecified
 because specification helps to highlight that the <tt class="docutils literal"><span class="pre">Reference</span></tt>
 template parameter may not always be identical to the iterator's
 <tt class="docutils literal"><span class="pre">reference</span></tt> type, and will keep users from making mistakes based on
 that assumption.</p>
 </div>
 <div class="section" id="specialized-adaptors">
 <h2><a class="toc-backref" href="#id26">Specialized Adaptors</a></h2>
 <p>This proposal also contains several examples of specialized adaptors
 which were easily implemented using <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>:</p>
 <ul class="simple">
 <li><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt>, which iterates over iterators, pointers,
 or smart pointers and applies an extra level of dereferencing.</li>
 <li>A new <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt>, which inverts the direction of a Base
 iterator's motion, while allowing adapted constant and mutable
 iterators to interact in the expected ways (unlike those in most
 implementations of C++98).</li>
 <li><tt class="docutils literal"><span class="pre">transform_iterator</span></tt>, which applies a user-defined function object
 to the underlying values when dereferenced.</li>
 <li><tt class="docutils literal"><span class="pre">filter_iterator</span></tt>, which provides a view of an iterator range in
 which some elements of the underlying range are skipped.</li>
 </ul>
 <ul class="simple" id="counting">
 <li><tt class="docutils literal"><span class="pre">counting_iterator</span></tt>, which adapts any incrementable type
 (e.g. integers, iterators) so that incrementing/decrementing the
 adapted iterator and dereferencing it produces successive values of
 the Base type.</li>
 <li><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt>, which makes it easier to create custom
 output iterators.</li>
 </ul>
 <p>Based on examples in the Boost library, users have generated many new
 adaptors, among them a permutation adaptor which applies some
 permutation to a random access iterator, and a strided adaptor, which
 adapts a random access iterator by multiplying its unit of motion by a
 constant factor.  In addition, the Boost Graph Library (BGL) uses
 iterator adaptors to adapt other graph libraries, such as LEDA [10]
 and Stanford GraphBase [8], to the BGL interface (which requires C++
 Standard compliant iterators).</p>
 </div>
 </div>
 <div class="section" id="proposed-text">
 <h1><a class="toc-backref" href="#id27">Proposed Text</a></h1>
 <div class="section" id="header-iterator-helper-synopsis-lib-iterator-helper-synopsis">
 <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>
 <pre class="literal-block">
 struct use_default;

 struct iterator_core_access { /* implementation detail */ };

 template &lt;
     class Derived
   , class Value
   , class CategoryOrTraversal
   , class Reference  = Value&amp;
   , class Difference = ptrdiff_t
 &gt;
 class iterator_facade;

 template &lt;
     class Derived
   , class Base
   , class Value      = use_default
   , class CategoryOrTraversal  = use_default
   , class Reference  = use_default
   , class Difference = use_default
 &gt;
 class iterator_adaptor;

 template &lt;
     class Iterator
   , class Value = use_default
   , class CategoryOrTraversal = use_default
   , class Reference = use_default
   , class Difference = use_default
 &gt;
 class indirect_iterator;

 template &lt;class Dereferenceable&gt;
 struct pointee;

 template &lt;class Dereferenceable&gt;
 struct indirect_reference;

 template &lt;class Iterator&gt;
 class reverse_iterator;

 template &lt;
     class UnaryFunction
   , class Iterator
   , class Reference = use_default
   , class Value = use_default
 &gt;
 class transform_iterator;

 template &lt;class Predicate, class Iterator&gt;
 class filter_iterator;

 template &lt;
     class Incrementable
   , class CategoryOrTraversal  = use_default
   , class Difference = use_default
 &gt;
 class counting_iterator;

 template &lt;class UnaryFunction&gt;
 class function_output_iterator;
 </pre>
 </div>
 <div class="section" id="iterator-facade-lib-iterator-facade">
 <h2><a class="toc-backref" href="#id29">Iterator facade [lib.iterator.facade]</a></h2>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is a base class template that implements the
 interface of standard iterators in terms of a few core functions
 and associated types, to be supplied by a derived iterator class.</p>
 <div class="section" id="class-template-iterator-facade">
 <h3><a class="toc-backref" href="#id30">Class template <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a></h3>
 <!-- Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <!-- Version 1.3 of this ReStructuredText document corresponds to
 n1530_, the paper accepted by the LWG for TR1. -->
 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 <pre class="literal-block">
 template &lt;
     class Derived
   , class Value
   , class CategoryOrTraversal
   , class Reference  = Value&amp;
   , class Difference = ptrdiff_t
 &gt;
 class iterator_facade {
  public:
     typedef remove_const&lt;Value&gt;::type value_type;
     typedef Reference reference;
     typedef Value* pointer;
     typedef Difference difference_type;
     typedef /* see <a class="reference internal" href="#iterator-category">below</a> */ iterator_category;

     reference operator*() const;
     /* see <a class="reference internal" href="#operator-arrow">below</a> */ operator-&gt;() const;
     /* see <a class="reference internal" href="#brackets">below</a> */ operator[](difference_type n) const;
     Derived&amp; operator++();
     Derived operator++(int);
     Derived&amp; operator--();
     Derived operator--(int);
     Derived&amp; operator+=(difference_type n);
     Derived&amp; operator-=(difference_type n);
     Derived operator-(difference_type n) const;
  protected:
     typedef iterator_facade iterator_facade_;
 };

 // Comparison operators
 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type // exposition
 operator ==(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);

 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator !=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);

 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator &lt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);

 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator &lt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);

 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator &gt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);

 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator &gt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);

 // Iterator difference
 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 /* see <a class="reference internal" href="#minus">below</a> */
 operator-(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
           iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);

 // Iterator addition
 template &lt;class Dr, class V, class TC, class R, class D&gt;
 Derived operator+ (iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;,
                    typename Derived::difference_type n);

 template &lt;class Dr, class V, class TC, class R, class D&gt;
 Derived operator+ (typename Derived::difference_type n,
                    iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;);
 </pre>
 <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>
 <pre class="literal-block">
 <em>iterator-category</em>(CategoryOrTraversal, value_type, reference)
 </pre>
 <p>where <em>iterator-category</em> is defined as follows:</p>
 <pre class="literal-block" id="id12">
 <em>iterator-category</em>(C,R,V) :=
    if (C is convertible to std::input_iterator_tag
        || C is convertible to std::output_iterator_tag
    )
        return C

    else if (C is not convertible to incrementable_traversal_tag)
        <em>the program is ill-formed</em>

    else return a type X satisfying the following two constraints:

       1. X is convertible to X1, and not to any more-derived
          type, where X1 is defined by:

            if (R is a reference type
                &amp;&amp; C is convertible to forward_traversal_tag)
            {
                if (C is convertible to random_access_traversal_tag)
                    X1 = random_access_iterator_tag
                else if (C is convertible to bidirectional_traversal_tag)
                    X1 = bidirectional_iterator_tag
                else
                    X1 = forward_iterator_tag
            }
            else
            {
                if (C is convertible to single_pass_traversal_tag
                    &amp;&amp; R is convertible to V)
                    X1 = input_iterator_tag
                else
                    X1 = C
            }

       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
          derived traversal tag type to which X is also
          convertible, and not to any more-derived traversal tag
          type.
 </pre>
 <p>[Note: the intention is to allow <tt class="docutils literal"><span class="pre">iterator_category</span></tt> to be one of
 the five original category tags when convertibility to one of the
 traversal tags would add no information]</p>
 <!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
 <!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <p>The <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> template used above is for exposition
 purposes.  The member operators should only be in an overload set
 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,
 meaning that at least one of the types is convertible to the other.  The
 <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> approach uses SFINAE to take the operators
 out of the overload set when the types are not interoperable.
 The operators should behave <em>as-if</em> <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt>
 were defined to be:</p>
 <pre class="literal-block">
 template &lt;bool, typename&gt; enable_if_interoperable_impl
 {};

 template &lt;typename T&gt; enable_if_interoperable_impl&lt;true,T&gt;
 { typedef T type; };

 template&lt;typename Dr1, typename Dr2, typename T&gt;
 struct enable_if_interoperable
   : enable_if_interoperable_impl&lt;
         is_convertible&lt;Dr1,Dr2&gt;::value || is_convertible&lt;Dr2,Dr1&gt;::value
       , T
     &gt;
 {};
 </pre>
 </div>
 <div class="section" id="iterator-facade-requirements">
 <h3><a class="toc-backref" href="#id31"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Requirements</a></h3>
 <p>The following table describes the typical valid expressions on
 <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
 iterator concept(s) it will model.  The operations in the first
 column must be made accessible to member functions of class
 <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt>.  In addition,
 <tt class="docutils literal"><span class="pre">static_cast&lt;Derived*&gt;(iterator_facade*)</span></tt> shall be well-formed.</p>
 <p>In the table below, <tt class="docutils literal"><span class="pre">F</span></tt> is <tt class="docutils literal"><span class="pre">iterator_facade&lt;X,V,C,R,D&gt;</span></tt>, <tt class="docutils literal"><span class="pre">a</span></tt> is an
 object of type <tt class="docutils literal"><span class="pre">X</span></tt>, <tt class="docutils literal"><span class="pre">b</span></tt> and <tt class="docutils literal"><span class="pre">c</span></tt> are objects of type <tt class="docutils literal"><span class="pre">const</span> <span class="pre">X</span></tt>,
 <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
 object of a single pass iterator type interoperable with <tt class="docutils literal"><span class="pre">X</span></tt>, and <tt class="docutils literal"><span class="pre">z</span></tt>
 is a constant object of a random access traversal iterator type
 interoperable with <tt class="docutils literal"><span class="pre">X</span></tt>.</p>
 <div class="topic" id="core-operations">
 <p class="topic-title first"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Core Operations</p>
 <table border="1" class="docutils">
 <colgroup>
 <col width="21%" />
 <col width="23%" />
 <col width="27%" />
 <col width="29%" />
 </colgroup>
 <thead valign="bottom">
 <tr><th class="head">Expression</th>
 <th class="head">Return Type</th>
 <th class="head">Assertion/Note</th>
 <th class="head">Used to implement Iterator
 Concept(s)</th>
 </tr>
 </thead>
 <tbody valign="top">
 <tr><td><tt class="docutils literal"><span class="pre">c.dereference()</span></tt></td>
 <td><tt class="docutils literal"><span class="pre">F::reference</span></tt></td>
 <td>&nbsp;</td>
 <td>Readable Iterator, Writable
 Iterator</td>
 </tr>
 <tr><td><tt class="docutils literal"><span class="pre">c.equal(y)</span></tt></td>
 <td>convertible to bool</td>
 <td>true iff <tt class="docutils literal"><span class="pre">c</span></tt> and <tt class="docutils literal"><span class="pre">y</span></tt>
 refer to the same
 position.</td>
 <td>Single Pass Iterator</td>
 </tr>
 <tr><td><tt class="docutils literal"><span class="pre">a.increment()</span></tt></td>
 <td>unused</td>
 <td>&nbsp;</td>
 <td>Incrementable Iterator</td>
 </tr>
 <tr><td><tt class="docutils literal"><span class="pre">a.decrement()</span></tt></td>
 <td>unused</td>
 <td>&nbsp;</td>
 <td>Bidirectional Traversal
 Iterator</td>
 </tr>
 <tr><td><tt class="docutils literal"><span class="pre">a.advance(n)</span></tt></td>
 <td>unused</td>
 <td>&nbsp;</td>
 <td>Random Access Traversal
 Iterator</td>
 </tr>
 <tr><td><tt class="docutils literal"><span class="pre">c.distance_to(z)</span></tt></td>
 <td>convertible to
 <tt class="docutils literal"><span class="pre">F::difference_type</span></tt></td>
 <td>equivalent to
 <tt class="docutils literal"><span class="pre">distance(c,</span> <span class="pre">X(z))</span></tt>.</td>
 <td>Random Access Traversal
 Iterator</td>
 </tr>
 </tbody>
 </table>
 </div>
 </div>
 <div class="section" id="iterator-facade-operations">
 <h3><a class="toc-backref" href="#id32"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> operations</a></h3>
 <p>The operations in this section are described in terms of operations on
 the core interface of <tt class="docutils literal"><span class="pre">Derived</span></tt> which may be inaccessible
 (i.e. private).  The implementation should access these operations
 through member functions of class <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt>.</p>
 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">static_cast&lt;Derived</span> <span class="pre">const*&gt;(this)-&gt;dereference()</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">operator-&gt;()</span> <span class="pre">const;</span></tt> (see <a class="reference internal" href="#operator-arrow">below</a>)</p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <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
 of type <tt class="docutils literal"><span class="pre">pointer</span></tt> equal to:</p>
 <pre class="literal-block">
 &amp;static_cast&lt;Derived const*&gt;(this)-&gt;dereference()
 </pre>
 <p class="last">Otherwise returns an object of unspecified type such that,
 <tt class="docutils literal"><span class="pre">(*static_cast&lt;Derived</span> <span class="pre">const*&gt;(this))-&gt;m</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">(w</span> <span class="pre">=</span> <span class="pre">**static_cast&lt;Derived</span> <span class="pre">const*&gt;(this),</span>
 <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>
 </td>
 </tr>
 </tbody>
 </table>
 <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>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <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
 objects <tt class="docutils literal"><span class="pre">v</span></tt> of type <tt class="docutils literal"><span class="pre">value_type</span></tt>, and <tt class="docutils literal"><span class="pre">n</span></tt> of type
 <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
 <tt class="docutils literal"><span class="pre">*(*this</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">v</span></tt>, and <tt class="docutils literal"><span class="pre">static_cast&lt;value_type</span>
 <span class="pre">const&amp;&gt;((*this)[n])</span></tt> is equivalent to
 <tt class="docutils literal"><span class="pre">static_cast&lt;value_type</span> <span class="pre">const&amp;&gt;(*(*this</span> <span class="pre">+</span> <span class="pre">n))</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Derived&amp;</span> <span class="pre">operator++();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 static_cast&lt;Derived*&gt;(this)-&gt;increment();
 return *static_cast&lt;Derived*&gt;(this);
 </pre>
 </td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator++(int);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 ++*this;
 return tmp;
 </pre>
 </td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Derived&amp;</span> <span class="pre">operator--();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 static_cast&lt;Derived*&gt;(this)-&gt;decrement();
 return *static_cast&lt;Derived*&gt;(this);
 </pre>
 </td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator--(int);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 --*this;
 return tmp;
 </pre>
 </td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Derived&amp;</span> <span class="pre">operator+=(difference_type</span> <span class="pre">n);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 static_cast&lt;Derived*&gt;(this)-&gt;advance(n);
 return *static_cast&lt;Derived*&gt;(this);
 </pre>
 </td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Derived&amp;</span> <span class="pre">operator-=(difference_type</span> <span class="pre">n);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 static_cast&lt;Derived*&gt;(this)-&gt;advance(-n);
 return *static_cast&lt;Derived*&gt;(this);
 </pre>
 </td>
 </tr>
 </tbody>
 </table>
 <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>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 return tmp -= n;
 </pre>
 </td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;class Dr, class V, class TC, class R, class D&gt;
 Derived operator+ (iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;,
                    typename Derived::difference_type n);

 template &lt;class Dr, class V, class TC, class R, class D&gt;
 Derived operator+ (typename Derived::difference_type n,
                    iterator_facade&lt;Dr,V,TC,R,D&gt; const&amp;);
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
 Derived tmp(static_cast&lt;Derived const*&gt;(this));
 return tmp += n;
 </pre>
 </td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator ==(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
 <dl class="last docutils">
 <dt>then</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&amp;)lhs).equal((Dr2</span> <span class="pre">const&amp;)rhs)</span></tt>.</p>
 </dd>
 <dt>Otherwise,</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).equal((Dr1</span> <span class="pre">const&amp;)lhs)</span></tt>.</p>
 </dd>
 </dl>
 </td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator !=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
 <dl class="last docutils">
 <dt>then</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">!((Dr1</span> <span class="pre">const&amp;)lhs).equal((Dr2</span> <span class="pre">const&amp;)rhs)</span></tt>.</p>
 </dd>
 <dt>Otherwise,</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">!((Dr2</span> <span class="pre">const&amp;)rhs).equal((Dr1</span> <span class="pre">const&amp;)lhs)</span></tt>.</p>
 </dd>
 </dl>
 </td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator &lt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
 <dl class="last docutils">
 <dt>then</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&amp;)lhs).distance_to((Dr2</span> <span class="pre">const&amp;)rhs)</span> <span class="pre">&lt;</span> <span class="pre">0</span></tt>.</p>
 </dd>
 <dt>Otherwise,</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).distance_to((Dr1</span> <span class="pre">const&amp;)lhs)</span> <span class="pre">&gt;</span> <span class="pre">0</span></tt>.</p>
 </dd>
 </dl>
 </td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator &lt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
 <dl class="last docutils">
 <dt>then</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&amp;)lhs).distance_to((Dr2</span> <span class="pre">const&amp;)rhs)</span> <span class="pre">&lt;=</span> <span class="pre">0</span></tt>.</p>
 </dd>
 <dt>Otherwise,</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).distance_to((Dr1</span> <span class="pre">const&amp;)lhs)</span> <span class="pre">&gt;=</span> <span class="pre">0</span></tt>.</p>
 </dd>
 </dl>
 </td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator &gt;(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
 <dl class="last docutils">
 <dt>then</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&amp;)lhs).distance_to((Dr2</span> <span class="pre">const&amp;)rhs)</span> <span class="pre">&gt;</span> <span class="pre">0</span></tt>.</p>
 </dd>
 <dt>Otherwise,</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).distance_to((Dr1</span> <span class="pre">const&amp;)lhs)</span> <span class="pre">&lt;</span> <span class="pre">0</span></tt>.</p>
 </dd>
 </dl>
 </td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,bool&gt;::type
 operator &gt;=(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
             iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
 <dl class="last docutils">
 <dt>then</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&amp;)lhs).distance_to((Dr2</span> <span class="pre">const&amp;)rhs)</span> <span class="pre">&gt;=</span> <span class="pre">0</span></tt>.</p>
 </dd>
 <dt>Otherwise,</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).distance_to((Dr1</span> <span class="pre">const&amp;)lhs)</span> <span class="pre">&lt;=</span> <span class="pre">0</span></tt>.</p>
 </dd>
 </dl>
 </td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block" id="minus">
 template &lt;class Dr1, class V1, class TC1, class R1, class D1,
           class Dr2, class V2, class TC2, class R2, class D2&gt;
 typename enable_if_interoperable&lt;Dr1,Dr2,difference&gt;::type
 operator -(iterator_facade&lt;Dr1,V1,TC1,R1,D1&gt; const&amp; lhs,
            iterator_facade&lt;Dr2,V2,TC2,R2,D2&gt; const&amp; rhs);
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Return Type:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
 <blockquote>
 <dl class="docutils">
 <dt>then</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">difference</span></tt> shall be
 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dr1&gt;::difference_type</span></tt>.</p>
 </dd>
 <dt>Otherwise</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">difference</span></tt> shall be <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dr2&gt;::difference_type</span></tt></p>
 </dd>
 </dl>
 </blockquote>
 </td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible&lt;Dr2,Dr1&gt;::value</span></tt></p>
 <dl class="last docutils">
 <dt>then</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">-((Dr1</span> <span class="pre">const&amp;)lhs).distance_to((Dr2</span> <span class="pre">const&amp;)rhs)</span></tt>.</p>
 </dd>
 <dt>Otherwise,</dt>
 <dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&amp;)rhs).distance_to((Dr1</span> <span class="pre">const&amp;)lhs)</span></tt>.</p>
 </dd>
 </dl>
 </td>
 </tr>
 </tbody>
 </table>
 </div>
 </div>
 <div class="section" id="iterator-adaptor-lib-iterator-adaptor">
 <h2><a class="toc-backref" href="#id33">Iterator adaptor [lib.iterator.adaptor]</a></h2>
 <!-- Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <!-- Version 1.1 of this ReStructuredText document corresponds to
 n1530_, the paper accepted by the LWG. -->
 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 <p>Each specialization of the <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template is derived from
 a specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>. The core interface functions
 expected by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> are implemented in terms of the
 <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="docutils literal"><span class="pre">Base</span></tt> template parameter. A class derived
 from <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> typically redefines some of the core
 interface functions to adapt the behavior of the <tt class="docutils literal"><span class="pre">Base</span></tt> type.
 Whether the derived class models any of the standard iterator concepts
 depends on the operations supported by the <tt class="docutils literal"><span class="pre">Base</span></tt> type and which
 core interface functions of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> are redefined in the
 <tt class="docutils literal"><span class="pre">Derived</span></tt> class.</p>
 <div class="section" id="class-template-iterator-adaptor">
 <h3><a class="toc-backref" href="#id34">Class template <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt></a></h3>
 <!-- Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <!-- Version 1.4 of this ReStructuredText document corresponds to
 n1530_, the paper accepted by the LWG for TR1. -->
 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
 <pre class="literal-block">
 template &lt;
     class Derived
   , class Base
   , class Value               = use_default
   , class CategoryOrTraversal = use_default
   , class Reference           = use_default
   , class Difference = use_default
 &gt;
 class iterator_adaptor
   : public iterator_facade&lt;Derived, <em>V'</em>, <em>C'</em>, <em>R'</em>, <em>D'</em>&gt; // see <a class="reference internal" href="#base-parameters">details</a>
 {
     friend class iterator_core_access;
  public:
     iterator_adaptor();
     explicit iterator_adaptor(Base const&amp; iter);
     typedef Base base_type;
     Base const&amp; base() const;
  protected:
     typedef iterator_adaptor iterator_adaptor_;
     Base const&amp; base_reference() const;
     Base&amp; base_reference();
  private: // Core iterator interface for iterator_facade.
     typename iterator_adaptor::reference dereference() const;

     template &lt;
     class OtherDerived, class OtherIterator, class V, class C, class R, class D
     &gt;
     bool equal(iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; x) const;

     void advance(typename iterator_adaptor::difference_type n);
     void increment();
     void decrement();

     template &lt;
         class OtherDerived, class OtherIterator, class V, class C, class R, class D
     &gt;
     typename iterator_adaptor::difference_type distance_to(
         iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; y) const;

  private:
     Base m_iterator; // exposition only
 };
 </pre>
 </div>
 <div class="section" id="iterator-adaptor-requirements">
 <span id="requirements"></span><h3><a class="toc-backref" href="#id35"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></h3>
 <p><tt class="docutils literal"><span class="pre">static_cast&lt;Derived*&gt;(iterator_adaptor*)</span></tt> shall be well-formed.
 The <tt class="docutils literal"><span class="pre">Base</span></tt> argument shall be Assignable and Copy Constructible.</p>
 </div>
 <div class="section" id="iterator-adaptor-base-class-parameters">
 <span id="base-parameters"></span><h3><a class="toc-backref" href="#id36"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></h3>
 <p>The <em>V'</em>, <em>C'</em>, <em>R'</em>, and <em>D'</em> parameters of the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>
 used as a base class in the summary of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>
 above are defined as follows:</p>
 <pre class="literal-block">
 <em>V'</em> = if (Value is use_default)
           return iterator_traits&lt;Base&gt;::value_type
       else
           return Value

 <em>C'</em> = if (CategoryOrTraversal is use_default)
           return iterator_traversal&lt;Base&gt;::type
       else
           return CategoryOrTraversal

 <em>R'</em> = if (Reference is use_default)
           if (Value is use_default)
               return iterator_traits&lt;Base&gt;::reference
           else
               return Value&amp;
       else
           return Reference

 <em>D'</em> = if (Difference is use_default)
           return iterator_traits&lt;Base&gt;::difference_type
       else
           return Difference
 </pre>
 <!-- ``iterator_adaptor`` models
 - - - - - - - - - - - - - - - - - - - - - - - - - - -

 In order for ``Derived`` to model the iterator concepts corresponding
 to ``iterator_traits<Derived>::iterator_category``, the expressions
 involving ``m_iterator`` in the specifications of those private member
 functions of ``iterator_adaptor`` that may be called by
 ``iterator_facade<Derived, V, C, R, D>`` in evaluating any valid
 expression involving ``Derived`` in those concepts' requirements. -->
 <!-- The above is confusing and needs a rewrite. -JGS -->
 <!-- That's why it's removed.  We're embracing inheritance, remember? -->
 </div>
 <div class="section" id="iterator-adaptor-public-operations">
 <h3><a class="toc-backref" href="#id37"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></h3>
 <p><tt class="docutils literal"><span class="pre">iterator_adaptor();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body">The <tt class="docutils literal"><span class="pre">Base</span></tt> type must be Default Constructible.</td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> with
 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> default constructed.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">iterator_adaptor(Base</span> <span class="pre">const&amp;</span> <span class="pre">iter);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> with
 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">iter</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Base</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
 </tr>
 </tbody>
 </table>
 </div>
 <div class="section" id="iterator-adaptor-protected-member-functions">
 <h3><a class="toc-backref" href="#id38"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></h3>
 <p><tt class="docutils literal"><span class="pre">Base</span> <span class="pre">const&amp;</span> <span class="pre">base_reference()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">A const reference to <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Base&amp;</span> <span class="pre">base_reference();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">A non-const reference to <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 </div>
 <div class="section" id="iterator-adaptor-private-member-functions">
 <h3><a class="toc-backref" href="#id39"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></h3>
 <p><tt class="docutils literal"><span class="pre">typename</span> <span class="pre">iterator_adaptor::reference</span> <span class="pre">dereference()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*m_iterator</span></tt></td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;
 class OtherDerived, class OtherIterator, class V, class C, class R, class D
 &gt;
 bool equal(iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; x) const;
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span> <span class="pre">==</span> <span class="pre">x.base()</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">advance(typename</span> <span class="pre">iterator_adaptor::difference_type</span> <span class="pre">n);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span> <span class="pre">+=</span> <span class="pre">n;</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">increment();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator;</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">decrement();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator;</span></tt></td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;
     class OtherDerived, class OtherIterator, class V, class C, class R, class D
 &gt;
 typename iterator_adaptor::difference_type distance_to(
     iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; y) const;
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">y.base()</span> <span class="pre">-</span> <span class="pre">m_iterator</span></tt></td>
 </tr>
 </tbody>
 </table>
 </div>
 </div>
 <div class="section" id="specialized-adaptors-lib-iterator-special-adaptors">
 <h2><a class="toc-backref" href="#id40">Specialized adaptors [lib.iterator.special.adaptors]</a></h2>
 <p>The <tt class="docutils literal"><span class="pre">enable_if_convertible&lt;X,Y&gt;::type</span></tt> expression used in
 this section is for exposition purposes. The converting constructors
 for specialized adaptors should be only be in an overload set provided
 that an object of type <tt class="docutils literal"><span class="pre">X</span></tt> is implicitly convertible to an object of
 type <tt class="docutils literal"><span class="pre">Y</span></tt>.
 The signatures involving <tt class="docutils literal"><span class="pre">enable_if_convertible</span></tt> should behave
 <em>as-if</em> <tt class="docutils literal"><span class="pre">enable_if_convertible</span></tt> were defined to be:</p>
 <pre class="literal-block">
 template &lt;bool&gt; enable_if_convertible_impl
 {};

 template &lt;&gt; enable_if_convertible_impl&lt;true&gt;
 { struct type; };

 template&lt;typename From, typename To&gt;
 struct enable_if_convertible
   : enable_if_convertible_impl&lt;is_convertible&lt;From,To&gt;::value&gt;
 {};
 </pre>
 <p>If an expression other than the default argument is used to supply
 the value of a function parameter whose type is written in terms
 of <tt class="docutils literal"><span class="pre">enable_if_convertible</span></tt>, the program is ill-formed, no
 diagnostic required.</p>
 <p>[<em>Note:</em> The <tt class="docutils literal"><span class="pre">enable_if_convertible</span></tt> approach uses SFINAE to
 take the constructor out of the overload set when the types are not
 implicitly convertible.
 ]</p>
 <div class="section" id="indirect-iterator">
 <h3><a class="toc-backref" href="#id41">Indirect iterator</a></h3>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <p><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> adapts an iterator by applying an
 <em>extra</em> dereference inside of <tt class="docutils literal"><span class="pre">operator*()</span></tt>. For example, this
 iterator adaptor makes it possible to view a container of pointers
 (e.g. <tt class="docutils literal"><span class="pre">list&lt;foo*&gt;</span></tt>) as if it were a container of the pointed-to type
 (e.g. <tt class="docutils literal"><span class="pre">list&lt;foo&gt;</span></tt>).  <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> depends on two
 auxiliary traits, <tt class="docutils literal"><span class="pre">pointee</span></tt> and <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>, to
 provide support for underlying iterators whose <tt class="docutils literal"><span class="pre">value_type</span></tt> is
 not an iterator.</p>
 <div class="section" id="class-template-pointee">
 <h4><a class="toc-backref" href="#id42">Class template <tt class="docutils literal"><span class="pre">pointee</span></tt></a></h4>
 <!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
 <!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <pre class="literal-block">
 template &lt;class Dereferenceable&gt;
 struct pointee
 {
     typedef /* see below */ type;
 };
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body">For an object <tt class="docutils literal"><span class="pre">x</span></tt> of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>, <tt class="docutils literal"><span class="pre">*x</span></tt>
 is well-formed.  If <tt class="docutils literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
 ambiguous nor shall it violate access control, and
 <tt class="docutils literal"><span class="pre">Dereferenceable::element_type</span></tt> shall be an accessible type.
 Otherwise <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dereferenceable&gt;::value_type</span></tt> shall
 be well formed.  [Note: These requirements need not apply to
 explicit or partial specializations of <tt class="docutils literal"><span class="pre">pointee</span></tt>]</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
 <tt class="docutils literal"><span class="pre">x</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>:</p>
 <pre class="literal-block">
 if ( ++x is ill-formed )
 {
     return ``Dereferenceable::element_type``
 }
 else if (``*x`` is a mutable reference to
          std::iterator_traits&lt;Dereferenceable&gt;::value_type)
 {
     return iterator_traits&lt;Dereferenceable&gt;::value_type
 }
 else
 {
     return iterator_traits&lt;Dereferenceable&gt;::value_type const
 }
 </pre>
 </div>
 <div class="section" id="class-template-indirect-reference">
 <h4><a class="toc-backref" href="#id43">Class template <tt class="docutils literal"><span class="pre">indirect_reference</span></tt></a></h4>
 <!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
 <!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <pre class="literal-block">
 template &lt;class Dereferenceable&gt;
 struct indirect_reference
 {
     typedef /* see below */ type;
 };
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body">For an object <tt class="docutils literal"><span class="pre">x</span></tt> of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>, <tt class="docutils literal"><span class="pre">*x</span></tt>
 is well-formed.  If <tt class="docutils literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
 ambiguous nor shall it violate access control, and
 <tt class="docutils literal"><span class="pre">pointee&lt;Dereferenceable&gt;::type&amp;</span></tt> shall be well-formed.
 Otherwise <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dereferenceable&gt;::reference</span></tt> shall
 be well formed.  [Note: These requirements need not apply to
 explicit or partial specializations of <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>]</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
 <tt class="docutils literal"><span class="pre">x</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>:</p>
 <pre class="literal-block">
 if ( ++x is ill-formed )
     return ``pointee&lt;Dereferenceable&gt;::type&amp;``
 else
     std::iterator_traits&lt;Dereferenceable&gt;::reference
 </pre>
 </div>
 <div class="section" id="class-template-indirect-iterator">
 <h4><a class="toc-backref" href="#id44">Class template <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt></a></h4>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <pre class="literal-block">
 template &lt;
     class Iterator
   , class Value = use_default
   , class CategoryOrTraversal = use_default
   , class Reference = use_default
   , class Difference = use_default
 &gt;
 class indirect_iterator
 {
  public:
     typedef /* see below */ value_type;
     typedef /* see below */ reference;
     typedef /* see below */ pointer;
     typedef /* see below */ difference_type;
     typedef /* see below */ iterator_category;

     indirect_iterator();
     indirect_iterator(Iterator x);

     template &lt;
         class Iterator2, class Value2, class Category2
       , class Reference2, class Difference2
     &gt;
     indirect_iterator(
         indirect_iterator&lt;
              Iterator2, Value2, Category2, Reference2, Difference2
         &gt; const&amp; y
       , typename enable_if_convertible&lt;Iterator2, Iterator&gt;::type* = 0 // exposition
     );

     Iterator const&amp; base() const;
     reference operator*() const;
     indirect_iterator&amp; operator++();
     indirect_iterator&amp; operator--();
 private:
    Iterator m_iterator; // exposition
 };
 </pre>
 <p>The member types of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> are defined according to
 the following pseudo-code, where <tt class="docutils literal"><span class="pre">V</span></tt> is
 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt></p>
 <pre class="literal-block">
 if (Value is use_default) then
     typedef remove_const&lt;pointee&lt;V&gt;::type&gt;::type value_type;
 else
     typedef remove_const&lt;Value&gt;::type value_type;

 if (Reference is use_default) then
     if (Value is use_default) then
         typedef indirect_reference&lt;V&gt;::type reference;
     else
         typedef Value&amp; reference;
 else
     typedef Reference reference;

 if (Value is use_default) then
     typedef pointee&lt;V&gt;::type* pointer;
 else
     typedef Value* pointer;

 if (Difference is use_default)
     typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
 else
     typedef Difference difference_type;

 if (CategoryOrTraversal is use_default)
     typedef <em>iterator-category</em> (
         iterator_traversal&lt;Iterator&gt;::type,``reference``,``value_type``
     ) iterator_category;
 else
     typedef <em>iterator-category</em> (
         CategoryOrTraversal,``reference``,``value_type``
     ) iterator_category;
 </pre>
 </div>
 <div class="section" id="indirect-iterator-requirements">
 <h4><a class="toc-backref" href="#id45"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> requirements</a></h4>
 <p>The expression <tt class="docutils literal"><span class="pre">*v</span></tt>, where <tt class="docutils literal"><span class="pre">v</span></tt> is an object of
 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>, shall be valid
 expression and convertible to <tt class="docutils literal"><span class="pre">reference</span></tt>.  <tt class="docutils literal"><span class="pre">Iterator</span></tt> shall
 model the traversal concept indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.
 <tt class="docutils literal"><span class="pre">Value</span></tt>, <tt class="docutils literal"><span class="pre">Reference</span></tt>, and <tt class="docutils literal"><span class="pre">Difference</span></tt> shall be chosen so
 that <tt class="docutils literal"><span class="pre">value_type</span></tt>, <tt class="docutils literal"><span class="pre">reference</span></tt>, and <tt class="docutils literal"><span class="pre">difference_type</span></tt> meet
 the requirements indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p>
 <p>[Note: there are further requirements on the
 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt> if the <tt class="docutils literal"><span class="pre">Value</span></tt>
 parameter is not <tt class="docutils literal"><span class="pre">use_default</span></tt>, as implied by the algorithm for
 deducing the default for the <tt class="docutils literal"><span class="pre">value_type</span></tt> member.]</p>
 </div>
 <div class="section" id="indirect-iterator-models">
 <h4><a class="toc-backref" href="#id46"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models</a></h4>
 <p>In addition to the concepts indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>
 and by <tt class="docutils literal"><span class="pre">iterator_traversal&lt;indirect_iterator&gt;::type</span></tt>, a
 specialization of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models the following
 concepts, Where <tt class="docutils literal"><span class="pre">v</span></tt> is an object of
 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>:</p>
 <blockquote>
 <ul class="simple">
 <li>Readable Iterator if <tt class="docutils literal"><span class="pre">reference(*v)</span></tt> is convertible to
 <tt class="docutils literal"><span class="pre">value_type</span></tt>.</li>
 <li>Writable Iterator if <tt class="docutils literal"><span class="pre">reference(*v)</span> <span class="pre">=</span> <span class="pre">t</span></tt> is a valid
 expression (where <tt class="docutils literal"><span class="pre">t</span></tt> is an object of type
 <tt class="docutils literal"><span class="pre">indirect_iterator::value_type</span></tt>)</li>
 <li>Lvalue Iterator if <tt class="docutils literal"><span class="pre">reference</span></tt> is a reference type.</li>
 </ul>
 </blockquote>
 <p><tt class="docutils literal"><span class="pre">indirect_iterator&lt;X,V1,C1,R1,D1&gt;</span></tt> is interoperable with
 <tt class="docutils literal"><span class="pre">indirect_iterator&lt;Y,V2,C2,R2,D2&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is
 interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
 </div>
 <div class="section" id="indirect-iterator-operations">
 <h4><a class="toc-backref" href="#id47"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> operations</a></h4>
 <p>In addition to the operations required by the concepts described
 above, specializations of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> provide the
 following operations.</p>
 <p><tt class="docutils literal"><span class="pre">indirect_iterator();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
 </tr>
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> with
 a default-constructed <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">indirect_iterator(Iterator</span> <span class="pre">x);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> with
 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;
     class Iterator2, class Value2, unsigned Access, class Traversal
   , class Reference2, class Difference2
 &gt;
 indirect_iterator(
     indirect_iterator&lt;
          Iterator2, Value2, Access, Traversal, Reference2, Difference2
     &gt; const&amp; y
   , typename enable_if_convertible&lt;Iterator2, Iterator&gt;::type* = 0 // exposition
 );
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator2</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
 </tr>
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> whose
 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> subobject is constructed from <tt class="docutils literal"><span class="pre">y.base()</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">**m_iterator</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">indirect_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator</span></tt></td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">indirect_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator</span></tt></td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 </div>
 </div>
 <div class="section" id="reverse-iterator">
 <h3><a class="toc-backref" href="#id48">Reverse iterator</a></h3>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <p>The reverse iterator adaptor iterates through the adapted iterator
 range in the opposite direction.</p>
 <div class="section" id="class-template-reverse-iterator">
 <h4><a class="toc-backref" href="#id49">Class template <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt></a></h4>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <pre class="literal-block">
 template &lt;class Iterator&gt;
 class reverse_iterator
 {
 public:
   typedef iterator_traits&lt;Iterator&gt;::value_type value_type;
   typedef iterator_traits&lt;Iterator&gt;::reference reference;
   typedef iterator_traits&lt;Iterator&gt;::pointer pointer;
   typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
   typedef /* see below */ iterator_category;

   reverse_iterator() {}
   explicit reverse_iterator(Iterator x) ;

   template&lt;class OtherIterator&gt;
   reverse_iterator(
       reverse_iterator&lt;OtherIterator&gt; const&amp; r
     , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
   );
   Iterator const&amp; base() const;
   reference operator*() const;
   reverse_iterator&amp; operator++();
   reverse_iterator&amp; operator--();
 private:
   Iterator m_iterator; // exposition
 };
 </pre>
 <p>If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Random Access Traversal Iterator and Readable
 Lvalue Iterator, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
 <tt class="docutils literal"><span class="pre">random_access_iterator_tag</span></tt>. Otherwise, if
 <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Bidirectional Traversal Iterator and Readable
 Lvalue Iterator, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
 <tt class="docutils literal"><span class="pre">bidirectional_iterator_tag</span></tt>. Otherwise, <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
 convertible to <tt class="docutils literal"><span class="pre">input_iterator_tag</span></tt>.</p>
 </div>
 <div class="section" id="reverse-iterator-requirements">
 <h4><a class="toc-backref" href="#id50"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> requirements</a></h4>
 <p><tt class="docutils literal"><span class="pre">Iterator</span></tt> must be a model of Bidirectional Traversal Iterator.  The
 type <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::reference</span></tt> must be the type of
 <tt class="docutils literal"><span class="pre">*i</span></tt>, where <tt class="docutils literal"><span class="pre">i</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</p>
 </div>
 <div class="section" id="reverse-iterator-models">
 <h4><a class="toc-backref" href="#id51"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> models</a></h4>
 <p>A specialization of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> models the same iterator
 traversal and iterator access concepts modeled by its <tt class="docutils literal"><span class="pre">Iterator</span></tt>
 argument.  In addition, it may model old iterator concepts
 specified in the following table:</p>
 <table border="1" class="docutils">
 <colgroup>
 <col width="53%" />
 <col width="47%" />
 </colgroup>
 <thead valign="bottom">
 <tr><th class="head">If <tt class="docutils literal"><span class="pre">I</span></tt> models</th>
 <th class="head">then <tt class="docutils literal"><span class="pre">reverse_iterator&lt;I&gt;</span></tt> models</th>
 </tr>
 </thead>
 <tbody valign="top">
 <tr><td>Readable Lvalue Iterator,
 Bidirectional Traversal Iterator</td>
 <td>Bidirectional Iterator</td>
 </tr>
 <tr><td>Writable Lvalue Iterator,
 Bidirectional Traversal Iterator</td>
 <td>Mutable Bidirectional Iterator</td>
 </tr>
 <tr><td>Readable Lvalue Iterator,
 Random Access Traversal Iterator</td>
 <td>Random Access Iterator</td>
 </tr>
 <tr><td>Writable Lvalue Iterator,
 Random Access Traversal Iterator</td>
 <td>Mutable Random Access Iterator</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">reverse_iterator&lt;X&gt;</span></tt> is interoperable with
 <tt class="docutils literal"><span class="pre">reverse_iterator&lt;Y&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is interoperable with
 <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
 </div>
 <div class="section" id="reverse-iterator-operations">
 <h4><a class="toc-backref" href="#id52"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> operations</a></h4>
 <p>In addition to the operations required by the concepts modeled by
 <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt>, <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> provides the following
 operations.</p>
 <p><tt class="docutils literal"><span class="pre">reverse_iterator();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
 </tr>
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_iterator</span></tt>
 default constructed.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">reverse_iterator(Iterator</span> <span class="pre">x);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> with
 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template&lt;class OtherIterator&gt;
 reverse_iterator(
     reverse_iterator&lt;OtherIterator&gt; const&amp; r
   , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
 );
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
 </tr>
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs instance of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> whose
 <tt class="docutils literal"><span class="pre">m_iterator</span></tt> subobject is constructed from <tt class="docutils literal"><span class="pre">y.base()</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"></td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 Iterator tmp = m_iterator;
 return *--tmp;
 </pre>
 <p><tt class="docutils literal"><span class="pre">reverse_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator</span></tt></td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">reverse_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator</span></tt></td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 </div>
 </div>
 <div class="section" id="transform-iterator">
 <h3><a class="toc-backref" href="#id53">Transform iterator</a></h3>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <p>The transform iterator adapts an iterator by modifying the
 <tt class="docutils literal"><span class="pre">operator*</span></tt> to apply a function object to the result of
 dereferencing the iterator and returning the result.</p>
 <div class="section" id="class-template-transform-iterator">
 <h4><a class="toc-backref" href="#id54">Class template <tt class="docutils literal"><span class="pre">transform_iterator</span></tt></a></h4>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <!-- Version 1.3 of this document was accepted for TR1 -->
 <pre class="literal-block">
 template &lt;class UnaryFunction,
           class Iterator,
           class Reference = use_default,
           class Value = use_default&gt;
 class transform_iterator
 {
 public:
   typedef /* see below */ value_type;
   typedef /* see below */ reference;
   typedef /* see below */ pointer;
   typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
   typedef /* see below */ iterator_category;

   transform_iterator();
   transform_iterator(Iterator const&amp; x, UnaryFunction f);

   template&lt;class F2, class I2, class R2, class V2&gt;
   transform_iterator(
         transform_iterator&lt;F2, I2, R2, V2&gt; const&amp; t
       , typename enable_if_convertible&lt;I2, Iterator&gt;::type* = 0      // exposition only
       , typename enable_if_convertible&lt;F2, UnaryFunction&gt;::type* = 0 // exposition only
   );
   UnaryFunction functor() const;
   Iterator const&amp; base() const;
   reference operator*() const;
   transform_iterator&amp; operator++();
   transform_iterator&amp; operator--();
 private:
   Iterator m_iterator; // exposition only
   UnaryFunction m_f;   // exposition only
 };
 </pre>
 <p>If <tt class="docutils literal"><span class="pre">Reference</span></tt> is <tt class="docutils literal"><span class="pre">use_default</span></tt> then the <tt class="docutils literal"><span class="pre">reference</span></tt> member of
 <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> is
 <tt class="docutils literal"><span class="pre">result_of&lt;UnaryFunction(iterator_traits&lt;Iterator&gt;::reference)&gt;::type</span></tt>.
 Otherwise, <tt class="docutils literal"><span class="pre">reference</span></tt> is <tt class="docutils literal"><span class="pre">Reference</span></tt>.</p>
 <p>If <tt class="docutils literal"><span class="pre">Value</span></tt> is <tt class="docutils literal"><span class="pre">use_default</span></tt> then the <tt class="docutils literal"><span class="pre">value_type</span></tt> member is
 <tt class="docutils literal"><span class="pre">remove_cv&lt;remove_reference&lt;reference&gt;</span> <span class="pre">&gt;::type</span></tt>.  Otherwise,
 <tt class="docutils literal"><span class="pre">value_type</span></tt> is <tt class="docutils literal"><span class="pre">Value</span></tt>.</p>
 <p>If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Readable Lvalue Iterator and if <tt class="docutils literal"><span class="pre">Iterator</span></tt>
 models Random Access Traversal Iterator, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
 convertible to <tt class="docutils literal"><span class="pre">random_access_iterator_tag</span></tt>. Otherwise, if
 <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Bidirectional Traversal Iterator, then
 <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
 <tt class="docutils literal"><span class="pre">bidirectional_iterator_tag</span></tt>.  Otherwise <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
 convertible to <tt class="docutils literal"><span class="pre">forward_iterator_tag</span></tt>. If <tt class="docutils literal"><span class="pre">Iterator</span></tt> does not
 model Readable Lvalue Iterator then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
 convertible to <tt class="docutils literal"><span class="pre">input_iterator_tag</span></tt>.</p>
 </div>
 <div class="section" id="transform-iterator-requirements">
 <h4><a class="toc-backref" href="#id55"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> requirements</a></h4>
 <p>The type <tt class="docutils literal"><span class="pre">UnaryFunction</span></tt> must be Assignable, Copy Constructible, and
 the expression <tt class="docutils literal"><span class="pre">f(*i)</span></tt> must be valid where <tt class="docutils literal"><span class="pre">f</span></tt> is an object of
 type <tt class="docutils literal"><span class="pre">UnaryFunction</span></tt>, <tt class="docutils literal"><span class="pre">i</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Iterator</span></tt>, and
 where the type of <tt class="docutils literal"><span class="pre">f(*i)</span></tt> must be
 <tt class="docutils literal"><span class="pre">result_of&lt;UnaryFunction(iterator_traits&lt;Iterator&gt;::reference)&gt;::type</span></tt>.</p>
 <p>The argument <tt class="docutils literal"><span class="pre">Iterator</span></tt> shall model Readable Iterator.</p>
 </div>
 <div class="section" id="transform-iterator-models">
 <h4><a class="toc-backref" href="#id56"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models</a></h4>
 <p>The resulting <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models the most refined of the
 following that is also modeled by <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</p>
 <blockquote>
 <ul class="simple">
 <li>Writable Lvalue Iterator if <tt class="docutils literal"><span class="pre">transform_iterator::reference</span></tt> is a non-const reference.</li>
 <li>Readable Lvalue Iterator if <tt class="docutils literal"><span class="pre">transform_iterator::reference</span></tt> is a const reference.</li>
 <li>Readable Iterator otherwise.</li>
 </ul>
 </blockquote>
 <p>The <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models the most refined standard traversal
 concept that is modeled by the <tt class="docutils literal"><span class="pre">Iterator</span></tt> argument.</p>
 <p>If <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> is a model of Readable Lvalue Iterator then
 it models the following original iterator concepts depending on what
 the <tt class="docutils literal"><span class="pre">Iterator</span></tt> argument models.</p>
 <table border="1" class="docutils">
 <colgroup>
 <col width="47%" />
 <col width="53%" />
 </colgroup>
 <thead valign="bottom">
 <tr><th class="head">If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models</th>
 <th class="head">then <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models</th>
 </tr>
 </thead>
 <tbody valign="top">
 <tr><td>Single Pass Iterator</td>
 <td>Input Iterator</td>
 </tr>
 <tr><td>Forward Traversal Iterator</td>
 <td>Forward Iterator</td>
 </tr>
 <tr><td>Bidirectional Traversal Iterator</td>
 <td>Bidirectional Iterator</td>
 </tr>
 <tr><td>Random Access Traversal Iterator</td>
 <td>Random Access Iterator</td>
 </tr>
 </tbody>
 </table>
 <p>If <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models Writable Lvalue Iterator then it is a
 mutable iterator (as defined in the old iterator requirements).</p>
 <p><tt class="docutils literal"><span class="pre">transform_iterator&lt;F1,</span> <span class="pre">X,</span> <span class="pre">R1,</span> <span class="pre">V1&gt;</span></tt> is interoperable with
 <tt class="docutils literal"><span class="pre">transform_iterator&lt;F2,</span> <span class="pre">Y,</span> <span class="pre">R2,</span> <span class="pre">V2&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is
 interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
 </div>
 <div class="section" id="transform-iterator-operations">
 <h4><a class="toc-backref" href="#id57"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> operations</a></h4>
 <p>In addition to the operations required by the concepts modeled by
 <tt class="docutils literal"><span class="pre">transform_iterator</span></tt>, <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> provides the following
 operations.</p>
 <p><tt class="docutils literal"><span class="pre">transform_iterator();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
 and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> default constructed.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">transform_iterator(Iterator</span> <span class="pre">const&amp;</span> <span class="pre">x,</span> <span class="pre">UnaryFunction</span> <span class="pre">f);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
 initialized to <tt class="docutils literal"><span class="pre">f</span></tt> and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> initialized to <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template&lt;class F2, class I2, class R2, class V2&gt;
 transform_iterator(
       transform_iterator&lt;F2, I2, R2, V2&gt; const&amp; t
     , typename enable_if_convertible&lt;I2, Iterator&gt;::type* = 0      // exposition only
     , typename enable_if_convertible&lt;F2, UnaryFunction&gt;::type* = 0 // exposition only
 );
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
 initialized to <tt class="docutils literal"><span class="pre">t.functor()</span></tt> and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> initialized to
 <tt class="docutils literal"><span class="pre">t.base()</span></tt>.</td>
 </tr>
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">UnaryFunction</span> <span class="pre">functor()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_f</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_f(*m_iterator)</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">transform_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator</span></tt></td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">transform_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator</span></tt></td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 </div>
 </div>
 <div class="section" id="filter-iterator">
 <h3><a class="toc-backref" href="#id58">Filter iterator</a></h3>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <p>The filter iterator adaptor creates a view of an iterator range in
 which some elements of the range are skipped. A predicate function
 object controls which elements are skipped. When the predicate is
 applied to an element, if it returns <tt class="docutils literal"><span class="pre">true</span></tt> then the element is
 retained and if it returns <tt class="docutils literal"><span class="pre">false</span></tt> then the element is skipped
 over. When skipping over elements, it is necessary for the filter
 adaptor to know when to stop so as to avoid going past the end of the
 underlying range. A filter iterator is therefore constructed with pair
 of iterators indicating the range of elements in the unfiltered
 sequence to be traversed.</p>
 <div class="section" id="class-template-filter-iterator">
 <h4><a class="toc-backref" href="#id59">Class template <tt class="docutils literal"><span class="pre">filter_iterator</span></tt></a></h4>
 <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt -->
 <!-- 2004. Use, modification and distribution is subject to the Boost -->
 <!-- Software License, Version 1.0. (See accompanying  file -->
 <!-- LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <pre class="literal-block">
 template &lt;class Predicate, class Iterator&gt;
 class filter_iterator
 {
  public:
     typedef iterator_traits&lt;Iterator&gt;::value_type value_type;
     typedef iterator_traits&lt;Iterator&gt;::reference reference;
     typedef iterator_traits&lt;Iterator&gt;::pointer pointer;
     typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
     typedef /* see below */ iterator_category;

     filter_iterator();
     filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());
     filter_iterator(Iterator x, Iterator end = Iterator());
     template&lt;class OtherIterator&gt;
     filter_iterator(
         filter_iterator&lt;Predicate, OtherIterator&gt; const&amp; t
         , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
         );
     Predicate predicate() const;
     Iterator end() const;
     Iterator const&amp; base() const;
     reference operator*() const;
     filter_iterator&amp; operator++();
 private:
     Predicate m_pred; // exposition only
     Iterator m_iter;  // exposition only
     Iterator m_end;   // exposition only
 };
 </pre>
 <p>If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Readable Lvalue Iterator and Bidirectional Traversal
 Iterator then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
 <tt class="docutils literal"><span class="pre">std::bidirectional_iterator_tag</span></tt>.
 Otherwise, if <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Readable Lvalue Iterator and Forward Traversal
 Iterator then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
 <tt class="docutils literal"><span class="pre">std::forward_iterator_tag</span></tt>.
 Otherwise <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
 convertible to <tt class="docutils literal"><span class="pre">std::input_iterator_tag</span></tt>.</p>
 </div>
 <div class="section" id="filter-iterator-requirements">
 <h4><a class="toc-backref" href="#id60"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt> requirements</a></h4>
 <p>The <tt class="docutils literal"><span class="pre">Iterator</span></tt> argument shall meet the requirements of Readable
 Iterator and Single Pass Iterator or it shall meet the requirements of
 Input Iterator.</p>
 <p>The <tt class="docutils literal"><span class="pre">Predicate</span></tt> argument must be Assignable, Copy Constructible, and
 the expression <tt class="docutils literal"><span class="pre">p(x)</span></tt> must be valid where <tt class="docutils literal"><span class="pre">p</span></tt> is an object of type
 <tt class="docutils literal"><span class="pre">Predicate</span></tt>, <tt class="docutils literal"><span class="pre">x</span></tt> is an object of type
 <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>, and where the type of
 <tt class="docutils literal"><span class="pre">p(x)</span></tt> must be convertible to <tt class="docutils literal"><span class="pre">bool</span></tt>.</p>
 </div>
 <div class="section" id="filter-iterator-models">
 <h4><a class="toc-backref" href="#id61"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models</a></h4>
 <p>The concepts that <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models are dependent on which
 concepts the <tt class="docutils literal"><span class="pre">Iterator</span></tt> argument models, as specified in the
 following tables.</p>
 <table border="1" class="docutils">
 <colgroup>
 <col width="44%" />
 <col width="56%" />
 </colgroup>
 <thead valign="bottom">
 <tr><th class="head">If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models</th>
 <th class="head">then <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models</th>
 </tr>
 </thead>
 <tbody valign="top">
 <tr><td>Single Pass Iterator</td>
 <td>Single Pass Iterator</td>
 </tr>
 <tr><td>Forward Traversal Iterator</td>
 <td>Forward Traversal Iterator</td>
 </tr>
 <tr><td>Bidirectional Traversal Iterator</td>
 <td>Bidirectional Traversal Iterator</td>
 </tr>
 </tbody>
 </table>
 <table border="1" class="docutils">
 <colgroup>
 <col width="41%" />
 <col width="59%" />
 </colgroup>
 <thead valign="bottom">
 <tr><th class="head">If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models</th>
 <th class="head">then <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models</th>
 </tr>
 </thead>
 <tbody valign="top">
 <tr><td>Readable Iterator</td>
 <td>Readable Iterator</td>
 </tr>
 <tr><td>Writable Iterator</td>
 <td>Writable Iterator</td>
 </tr>
 <tr><td>Lvalue Iterator</td>
 <td>Lvalue Iterator</td>
 </tr>
 </tbody>
 </table>
 <table border="1" class="docutils">
 <colgroup>
 <col width="63%" />
 <col width="38%" />
 </colgroup>
 <thead valign="bottom">
 <tr><th class="head">If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models</th>
 <th class="head">then <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models</th>
 </tr>
 </thead>
 <tbody valign="top">
 <tr><td>Readable Iterator, Single Pass Iterator</td>
 <td>Input Iterator</td>
 </tr>
 <tr><td>Readable Lvalue Iterator, Forward Traversal Iterator</td>
 <td>Forward Iterator</td>
 </tr>
 <tr><td>Writable Lvalue Iterator, Forward Traversal Iterator</td>
 <td>Mutable Forward Iterator</td>
 </tr>
 <tr><td>Writable Lvalue Iterator, Bidirectional Iterator</td>
 <td>Mutable Bidirectional Iterator</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">filter_iterator&lt;P1,</span> <span class="pre">X&gt;</span></tt> is interoperable with <tt class="docutils literal"><span class="pre">filter_iterator&lt;P2,</span> <span class="pre">Y&gt;</span></tt>
 if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
 </div>
 <div class="section" id="filter-iterator-operations">
 <h4><a class="toc-backref" href="#id62"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt> operations</a></h4>
 <p>In addition to those operations required by the concepts that
 <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> models, <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> provides the following
 operations.</p>
 <p><tt class="docutils literal"><span class="pre">filter_iterator();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Predicate</span></tt> and <tt class="docutils literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
 </tr>
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> whose``m_pred``,  <tt class="docutils literal"><span class="pre">m_iter</span></tt>, and <tt class="docutils literal"><span class="pre">m_end</span></tt>
 members are a default constructed.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">filter_iterator(Predicate</span> <span class="pre">f,</span> <span class="pre">Iterator</span> <span class="pre">x,</span> <span class="pre">Iterator</span> <span class="pre">end</span> <span class="pre">=</span> <span class="pre">Iterator());</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> where <tt class="docutils literal"><span class="pre">m_iter</span></tt> is either
 the first position in the range <tt class="docutils literal"><span class="pre">[x,end)</span></tt> such that <tt class="docutils literal"><span class="pre">f(*m_iter)</span> <span class="pre">==</span> <span class="pre">true</span></tt>
 or else``m_iter == end``. The member <tt class="docutils literal"><span class="pre">m_pred</span></tt> is constructed from
 <tt class="docutils literal"><span class="pre">f</span></tt> and <tt class="docutils literal"><span class="pre">m_end</span></tt> from <tt class="docutils literal"><span class="pre">end</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">filter_iterator(Iterator</span> <span class="pre">x,</span> <span class="pre">Iterator</span> <span class="pre">end</span> <span class="pre">=</span> <span class="pre">Iterator());</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Predicate</span></tt> must be Default Constructible and
 <tt class="docutils literal"><span class="pre">Predicate</span></tt> is a class type (not a function pointer).</td>
 </tr>
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a <tt class="docutils literal"><span class="pre">filter_iterator</span></tt> where <tt class="docutils literal"><span class="pre">m_iter</span></tt> is either
 the first position in the range <tt class="docutils literal"><span class="pre">[x,end)</span></tt> such that <tt class="docutils literal"><span class="pre">m_pred(*m_iter)</span> <span class="pre">==</span> <span class="pre">true</span></tt>
 or else``m_iter == end``. The member <tt class="docutils literal"><span class="pre">m_pred</span></tt> is default constructed.</td>
 </tr>
 </tbody>
 </table>
 <pre class="literal-block">
 template &lt;class OtherIterator&gt;
 filter_iterator(
     filter_iterator&lt;Predicate, OtherIterator&gt; const&amp; t
     , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
     );``
 </pre>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
 </tr>
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a filter iterator whose members are copied from <tt class="docutils literal"><span class="pre">t</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Predicate</span> <span class="pre">predicate()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_pred</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">end()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_end</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*m_iter</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">filter_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Increments <tt class="docutils literal"><span class="pre">m_iter</span></tt> and then continues to
 increment <tt class="docutils literal"><span class="pre">m_iter</span></tt> until either <tt class="docutils literal"><span class="pre">m_iter</span> <span class="pre">==</span> <span class="pre">m_end</span></tt>
 or <tt class="docutils literal"><span class="pre">m_pred(*m_iter)</span> <span class="pre">==</span> <span class="pre">true</span></tt>.</td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 </div>
 </div>
 <div class="section" id="counting-iterator">
 <h3><a class="toc-backref" href="#id63">Counting iterator</a></h3>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <p><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> adapts an object by adding an <tt class="docutils literal"><span class="pre">operator*</span></tt> that
 returns the current value of the object. All other iterator operations
 are forwarded to the adapted object.</p>
 <div class="section" id="class-template-counting-iterator">
 <h4><a class="toc-backref" href="#id64">Class template <tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a></h4>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <pre class="literal-block">
 template &lt;
     class Incrementable
   , class CategoryOrTraversal = use_default
   , class Difference = use_default
 &gt;
 class counting_iterator
 {
 public:
     typedef Incrementable value_type;
     typedef const Incrementable&amp; reference;
     typedef const Incrementable* pointer;
     typedef /* see below */ difference_type;
     typedef /* see below */ iterator_category;

     counting_iterator();
     counting_iterator(counting_iterator const&amp; rhs);
     explicit counting_iterator(Incrementable x);
     Incrementable const&amp; base() const;
     reference operator*() const;
     counting_iterator&amp; operator++();
     counting_iterator&amp; operator--();
 private:
     Incrementable m_inc; // exposition
 };
 </pre>
 <p>If the <tt class="docutils literal"><span class="pre">Difference</span></tt> argument is <tt class="docutils literal"><span class="pre">use_default</span></tt> then
 <tt class="docutils literal"><span class="pre">difference_type</span></tt> is an unspecified signed integral
 type. Otherwise <tt class="docutils literal"><span class="pre">difference_type</span></tt> is <tt class="docutils literal"><span class="pre">Difference</span></tt>.</p>
 <p><tt class="docutils literal"><span class="pre">iterator_category</span></tt> is determined according to the following
 algorithm:</p>
 <pre class="literal-block">
 if (CategoryOrTraversal is not use_default)
     return CategoryOrTraversal
 else if (numeric_limits&lt;Incrementable&gt;::is_specialized)
     return <a class="reference internal" href="#id12"><em>iterator-category</em></a>(
         random_access_traversal_tag, Incrementable, const Incrementable&amp;)
 else
     return <a class="reference internal" href="#id12"><em>iterator-category</em></a>(
          iterator_traversal&lt;Incrementable&gt;::type,
          Incrementable, const Incrementable&amp;)
 </pre>
 <dl class="docutils">
 <dt>[<em>Note:</em> implementers are encouraged to provide an implementation of</dt>
 <dd><tt class="docutils literal"><span class="pre">operator-</span></tt> and a <tt class="docutils literal"><span class="pre">difference_type</span></tt> that avoids overflows in
 the cases where <tt class="docutils literal"><span class="pre">std::numeric_limits&lt;Incrementable&gt;::is_specialized</span></tt>
 is true.]</dd>
 </dl>
 </div>
 <div class="section" id="counting-iterator-requirements">
 <h4><a class="toc-backref" href="#id65"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> requirements</a></h4>
 <p>The <tt class="docutils literal"><span class="pre">Incrementable</span></tt> argument shall be Copy Constructible and Assignable.</p>
 <p>If <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to <tt class="docutils literal"><span class="pre">forward_iterator_tag</span></tt>
 or <tt class="docutils literal"><span class="pre">forward_traversal_tag</span></tt>, the following must be well-formed:</p>
 <pre class="literal-block">
 Incrementable i, j;
 ++i;         // pre-increment
 i == j;      // operator equal
 </pre>
 <p>If <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
 <tt class="docutils literal"><span class="pre">bidirectional_iterator_tag</span></tt> or <tt class="docutils literal"><span class="pre">bidirectional_traversal_tag</span></tt>,
 the following expression must also be well-formed:</p>
 <pre class="literal-block">
 --i
 </pre>
 <p>If <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
 <tt class="docutils literal"><span class="pre">random_access_iterator_tag</span></tt> or <tt class="docutils literal"><span class="pre">random_access_traversal_tag</span></tt>,
 the following must must also be valid:</p>
 <pre class="literal-block">
 counting_iterator::difference_type n;
 i += n;
 n = i - j;
 i &lt; j;
 </pre>
 </div>
 <div class="section" id="counting-iterator-models">
 <h4><a class="toc-backref" href="#id66"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models</a></h4>
 <p>Specializations of <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> model Readable Lvalue
 Iterator. In addition, they model the concepts corresponding to the
 iterator tags to which their <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible.
 Also, if <tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt> is not <tt class="docutils literal"><span class="pre">use_default</span></tt> then
 <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models the concept corresponding to the iterator
 tag <tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt>.  Otherwise, if
 <tt class="docutils literal"><span class="pre">numeric_limits&lt;Incrementable&gt;::is_specialized</span></tt>, then
 <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models Random Access Traversal Iterator.
 Otherwise, <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models the same iterator traversal
 concepts modeled by <tt class="docutils literal"><span class="pre">Incrementable</span></tt>.</p>
 <p><tt class="docutils literal"><span class="pre">counting_iterator&lt;X,C1,D1&gt;</span></tt> is interoperable with
 <tt class="docutils literal"><span class="pre">counting_iterator&lt;Y,C2,D2&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is
 interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
 </div>
 <div class="section" id="counting-iterator-operations">
 <h4><a class="toc-backref" href="#id67"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> operations</a></h4>
 <p>In addition to the operations required by the concepts modeled by
 <tt class="docutils literal"><span class="pre">counting_iterator</span></tt>, <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> provides the following
 operations.</p>
 <p><tt class="docutils literal"><span class="pre">counting_iterator();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Incrementable</span></tt> is Default Constructible.</td>
 </tr>
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Default construct the member <tt class="docutils literal"><span class="pre">m_inc</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">counting_iterator(counting_iterator</span> <span class="pre">const&amp;</span> <span class="pre">rhs);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Construct member <tt class="docutils literal"><span class="pre">m_inc</span></tt> from <tt class="docutils literal"><span class="pre">rhs.m_inc</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">counting_iterator(Incrementable</span> <span class="pre">x);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Construct member <tt class="docutils literal"><span class="pre">m_inc</span></tt> from <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_inc</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">counting_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_inc</span></tt></td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">counting_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_inc</span></tt></td>
 </tr>
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">Incrementable</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_inc</span></tt></td>
 </tr>
 </tbody>
 </table>
 </div>
 </div>
 <div class="section" id="function-output-iterator">
 <h3><a class="toc-backref" href="#id68">Function output iterator</a></h3>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <p>The function output iterator adaptor makes it easier to create custom
 output iterators. The adaptor takes a unary function and creates a
 model of Output Iterator. Each item assigned to the output iterator is
 passed as an argument to the unary function.  The motivation for this
 iterator is that creating a conforming output iterator is non-trivial,
 particularly because the proper implementation usually requires a
 proxy object.</p>
 <div class="section" id="class-template-function-output-iterator">
 <h4><a class="toc-backref" href="#id69">Class template <tt class="docutils literal"><span class="pre">function_output_iterator</span></tt></a></h4>
 <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
 <!-- Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 </div>
 <div class="section" id="header">
 <h4><a class="toc-backref" href="#id70">Header</a></h4>
 <pre class="literal-block">
 #include &lt;boost/function_output_iterator.hpp&gt;
 </pre>
 <pre class="literal-block">
 template &lt;class UnaryFunction&gt;
 class function_output_iterator {
 public:
   typedef std::output_iterator_tag iterator_category;
   typedef void                     value_type;
   typedef void                     difference_type;
   typedef void                     pointer;
   typedef void                     reference;

   explicit function_output_iterator();

   explicit function_output_iterator(const UnaryFunction&amp; f);

   /* see below */ operator*();
   function_output_iterator&amp; operator++();
   function_output_iterator&amp; operator++(int);
 private:
   UnaryFunction m_f;     // exposition only
 };
 </pre>
 </div>
 <div class="section" id="function-output-iterator-requirements">
 <h4><a class="toc-backref" href="#id71"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> requirements</a></h4>
 <p><tt class="docutils literal"><span class="pre">UnaryFunction</span></tt> must be Assignable and Copy Constructible.</p>
 </div>
 <div class="section" id="function-output-iterator-models">
 <h4><a class="toc-backref" href="#id72"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> models</a></h4>
 <p><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> is a model of the Writable and
 Incrementable Iterator concepts.</p>
 </div>
 <div class="section" id="function-output-iterator-operations">
 <h4><a class="toc-backref" href="#id73"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt> operations</a></h4>
 <p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">function_output_iterator(const</span> <span class="pre">UnaryFunction&amp;</span> <span class="pre">f</span> <span class="pre">=</span> <span class="pre">UnaryFunction());</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">function_output_iterator</span></tt>
 with <tt class="docutils literal"><span class="pre">m_f</span></tt> constructed from <tt class="docutils literal"><span class="pre">f</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">operator*();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body">An object <tt class="docutils literal"><span class="pre">r</span></tt> of unspecified type such that <tt class="docutils literal"><span class="pre">r</span> <span class="pre">=</span> <span class="pre">t</span></tt>
 is equivalent to <tt class="docutils literal"><span class="pre">m_f(t)</span></tt> for all <tt class="docutils literal"><span class="pre">t</span></tt>.</td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">function_output_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 <p><tt class="docutils literal"><span class="pre">function_output_iterator&amp;</span> <span class="pre">operator++(int);</span></tt></p>
 <table class="docutils field-list" frame="void" rules="none">
 <col class="field-name" />
 <col class="field-body" />
 <tbody valign="top">
 <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
 </tr>
 </tbody>
 </table>
 <!-- LocalWords:  Abrahams Siek Witt istream ostream iter MTL strided interoperate
 LocalWords:  CRTP metafunctions inlining lvalue JGS incrementable BGL LEDA cv
 LocalWords:  GraphBase struct ptrdiff UnaryFunction const int typename bool pp
 LocalWords:  lhs rhs SFINAE markup iff tmp OtherDerived OtherIterator DWA foo
 LocalWords:  dereferenceable subobject AdaptableUnaryFunction impl pre ifdef'd
 LocalWords:  OtherIncrementable Coplien -->
 </div>
 </div>
 </div>
 </div>
 </div>
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