boost-react-native-bundle

<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="generator" content="Docutils 0.5: http://docutils.sourceforge.net/" /> <title>Iterator Facade and Adaptor</title> <meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" /> <meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, Zephyr Associates, Inc." /> <meta name="date" content="2006-09-11" /> <link rel="stylesheet" href="../../../rst.css" type="text/css" /> </head> <body> <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@boost-consulting.com">dave@boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>, <a class="last reference external" href="mailto:witt@styleadvisor.com">witt@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://www.open-std.org/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>     <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-></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"><iterator_helper></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://www.open-std.org/JTC1/SC22/WG21/docs/papers/2003/n1550.htm">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://www.open-std.org/JTC1/SC22/WG21/docs/papers/2003/n1550.htm">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://www.open-std.org/JTC1/SC22/WG21/docs/papers/2003/n1550.htm">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://www.open-std.org/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://www.open-std.org/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://www.open-std.org/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>      <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<...></span></tt>, rather than something more descriptive like <tt class="docutils literal"><span class="pre">indirect_iterator<T*></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>  <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>   <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://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#299">issue 299</a> and adopted by proposal <a class="reference external" href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2003/n1550.htm">n1550</a>: the result of <tt class="docutils literal"><span class="pre">p[n]</span></tt> is an object 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-></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-></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-></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-></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>      <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 "Base" 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"><iterator_helper></span></tt> synopsis [lib.iterator.helper.synopsis]</a></h2> <pre class="literal-block"> struct use_default; struct iterator_core_access { /* implementation detail */ }; template < class Derived , class Value , class CategoryOrTraversal , class Reference = Value& , class Difference = ptrdiff_t > class iterator_facade; template < class Derived , class Base , class Value = use_default , class CategoryOrTraversal = use_default , class Reference = use_default , class Difference = use_default > class iterator_adaptor; template < class Iterator , class Value = use_default , class CategoryOrTraversal = use_default , class Reference = use_default , class Difference = use_default > class indirect_iterator; template <class Dereferenceable> struct pointee; template <class Dereferenceable> struct indirect_reference; template <class Iterator> class reverse_iterator; template < class UnaryFunction , class Iterator , class Reference = use_default , class Value = use_default > class transform_iterator; template <class Predicate, class Iterator> class filter_iterator; template < class Incrementable , class CategoryOrTraversal = use_default , class Difference = use_default > class counting_iterator; template <class UnaryFunction> 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>    <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>      <pre class="literal-block"> template < class Derived , class Value , class CategoryOrTraversal , class Reference = Value& , class Difference = ptrdiff_t > class iterator_facade { public: typedef remove_const<Value>::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->() const; /* see <a class="reference internal" href="#brackets">below</a> */ operator[](difference_type n) const; Derived& operator++(); Derived operator++(int); Derived& operator--(); Derived operator--(int); Derived& operator+=(difference_type n); Derived& operator-=(difference_type n); Derived operator-(difference_type n) const; protected: typedef iterator_facade iterator_facade_; }; // Comparison operators template <class Dr1, class V1, class TC1, class R1, class D1, class Dr2, class V2, class TC2, class R2, class D2> typename enable_if_interoperable<Dr1,Dr2,bool>::type // exposition operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); template <class Dr1, class V1, class TC1, class R1, class D1, class Dr2, class V2, class TC2, class R2, class D2> typename enable_if_interoperable<Dr1,Dr2,bool>::type operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); template <class Dr1, class V1, class TC1, class R1, class D1, class Dr2, class V2, class TC2, class R2, class D2> typename enable_if_interoperable<Dr1,Dr2,bool>::type operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); template <class Dr1, class V1, class TC1, class R1, class D1, class Dr2, class V2, class TC2, class R2, class D2> typename enable_if_interoperable<Dr1,Dr2,bool>::type operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); template <class Dr1, class V1, class TC1, class R1, class D1, class Dr2, class V2, class TC2, class R2, class D2> typename enable_if_interoperable<Dr1,Dr2,bool>::type operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); template <class Dr1, class V1, class TC1, class R1, class D1, class Dr2, class V2, class TC2, class R2, class D2> typename enable_if_interoperable<Dr1,Dr2,bool>::type operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); // Iterator difference template <class Dr1, class V1, class TC1, class R1, class D1, class Dr2, class V2, class TC2, class R2, class D2> /* see <a class="reference internal" href="#minus">below</a> */ operator-(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs, iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs); // Iterator addition template <class Dr, class V, class TC, class R, class D> Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&, typename Derived::difference_type n); template <class Dr, class V, class TC, class R, class D> Derived operator+ (typename Derived::difference_type n, iterator_facade<Dr,V,TC,R,D> const&); </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 && 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 && 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 infor