<?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>New Iterator Concepts</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" /> <meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." /> <link rel="stylesheet" href="../../../rst.css" type="text/css" /> </head> <body> <div class="document" id="new-iterator-concepts"> <h1 class="title">New Iterator Concepts</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://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1550.html">n1550</a>=03-0133, which was accepted for Technical Report 1 by the C++ standard committee's library working group. This proposal is a revision of paper <a class="reference external" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2001/n1297.html">n1297</a>, <a class="reference external" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1477.html">n1477</a>, and <a class="reference external" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1531.html">n1531</a>.</td> </tr> <tr><th class="docinfo-name">Copyright:</th> <td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</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 1.25 of this ReStructuredText document is the same as n1550_, 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">Abstract:</th><td class="field-body">We propose a new system of iterator concepts that treat access and positioning independently. This allows the concepts to more closely match the requirements of algorithms and provides better categorizations of iterators that are used in practice.</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="id1">Motivation</a></li> <li><a class="reference internal" href="#impact-on-the-standard" id="id2">Impact on the Standard</a><ul> <li><a class="reference internal" href="#possible-but-not-proposed-changes-to-the-working-paper" id="id3">Possible (but not proposed) Changes to the Working Paper</a><ul> <li><a class="reference internal" href="#changes-to-algorithm-requirements" id="id4">Changes to Algorithm Requirements</a></li> <li><a class="reference internal" href="#deprecations" id="id5">Deprecations</a></li> <li><a class="reference internal" href="#vector-bool" id="id6"><tt class="docutils literal"><span class="pre">vector<bool></span></tt></a></li> </ul> </li> </ul> </li> <li><a class="reference internal" href="#design" id="id7">Design</a></li> <li><a class="reference internal" href="#proposed-text" id="id8">Proposed Text</a><ul> <li><a class="reference internal" href="#addition-to-lib-iterator-requirements" id="id9">Addition to [lib.iterator.requirements]</a><ul> <li><a class="reference internal" href="#iterator-value-access-concepts-lib-iterator-value-access" id="id10">Iterator Value Access Concepts [lib.iterator.value.access]</a><ul> <li><a class="reference internal" href="#readable-iterators-lib-readable-iterators" id="id11">Readable Iterators [lib.readable.iterators]</a></li> <li><a class="reference internal" href="#writable-iterators-lib-writable-iterators" id="id12">Writable Iterators [lib.writable.iterators]</a></li> <li><a class="reference internal" href="#swappable-iterators-lib-swappable-iterators" id="id13">Swappable Iterators [lib.swappable.iterators]</a></li> <li><a class="reference internal" href="#lvalue-iterators-lib-lvalue-iterators" id="id14">Lvalue Iterators [lib.lvalue.iterators]</a></li> </ul> </li> <li><a class="reference internal" href="#iterator-traversal-concepts-lib-iterator-traversal" id="id15">Iterator Traversal Concepts [lib.iterator.traversal]</a><ul> <li><a class="reference internal" href="#incrementable-iterators-lib-incrementable-iterators" id="id16">Incrementable Iterators [lib.incrementable.iterators]</a></li> <li><a class="reference internal" href="#single-pass-iterators-lib-single-pass-iterators" id="id17">Single Pass Iterators [lib.single.pass.iterators]</a></li> <li><a class="reference internal" href="#forward-traversal-iterators-lib-forward-traversal-iterators" id="id18">Forward Traversal Iterators [lib.forward.traversal.iterators]</a></li> <li><a class="reference internal" href="#bidirectional-traversal-iterators-lib-bidirectional-traversal-iterators" id="id19">Bidirectional Traversal Iterators [lib.bidirectional.traversal.iterators]</a></li> <li><a class="reference internal" href="#random-access-traversal-iterators-lib-random-access-traversal-iterators" id="id20">Random Access Traversal Iterators [lib.random.access.traversal.iterators]</a></li> <li><a class="reference internal" href="#interoperable-iterators-lib-interoperable-iterators" id="id21">Interoperable Iterators [lib.interoperable.iterators]</a></li> </ul> </li> </ul> </li> <li><a class="reference internal" href="#addition-to-lib-iterator-synopsis" id="id22">Addition to [lib.iterator.synopsis]</a></li> <li><a class="reference internal" href="#addition-to-lib-iterator-traits" id="id23">Addition to [lib.iterator.traits]</a></li> </ul> </li> <li><a class="reference internal" href="#footnotes" id="id24">Footnotes</a></li> </ul> </div> <div class="section" id="motivation"> <h1><a class="toc-backref" href="#id1">Motivation</a></h1> <p>The standard iterator categories and requirements are flawed because they use a single hierarchy of concepts to address two orthogonal issues: <em>iterator traversal</em> and <em>value access</em>. As a result, many algorithms with requirements expressed in terms of the iterator categories are too strict. Also, many real-world iterators can not be accurately categorized. A proxy-based iterator with random-access traversal, for example, may only legally have a category of "input iterator", so generic algorithms are unable to take advantage of its random-access capabilities. The current iterator concept hierarchy is geared towards iterator traversal (hence the category names), while requirements that address value access sneak in at various places. The following table gives a summary of the current value access requirements in the iterator categories.</p> <table border="1" class="docutils"> <colgroup> <col width="31%" /> <col width="69%" /> </colgroup> <thead valign="bottom"> <tr><th class="head" colspan="2">Value Access Requirements in Existing Iterator Categories</th> </tr> </thead> <tbody valign="top"> <tr><td>Output Iterator</td> <td><tt class="docutils literal"><span class="pre">*i</span> <span class="pre">=</span> <span class="pre">a</span></tt></td> </tr> <tr><td>Input Iterator</td> <td><tt class="docutils literal"><span class="pre">*i</span></tt> is convertible to <tt class="docutils literal"><span class="pre">T</span></tt></td> </tr> <tr><td>Forward Iterator</td> <td><tt class="docutils literal"><span class="pre">*i</span></tt> is <tt class="docutils literal"><span class="pre">T&</span></tt> (or <tt class="docutils literal"><span class="pre">const</span> <span class="pre">T&</span></tt> once <a class="reference external" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/lwg-active.html#200">issue 200</a> is resolved)</td> </tr> <tr><td>Random Access Iterator</td> <td><tt class="docutils literal"><span class="pre">i[n]</span></tt> is convertible to <tt class="docutils literal"><span class="pre">T</span></tt> (also <tt class="docutils literal"><span class="pre">i[n]</span> <span class="pre">=</span> <span class="pre">t</span></tt> is required for mutable iterators once <a class="reference external" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/lwg-active.html#299">issue 299</a> is resolved)</td> </tr> </tbody> </table> <p>Because iterator traversal and value access are mixed together in a single hierarchy, many useful iterators can not be appropriately categorized. For example, <tt class="docutils literal"><span class="pre">vector<bool>::iterator</span></tt> is almost a random access iterator, but the return type is not <tt class="docutils literal"><span class="pre">bool&</span></tt> (see <a class="reference external" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/lwg-active.html#96">issue 96</a> and Herb Sutter's paper J16/99-0008 = WG21 N1185). Therefore, the iterators of <tt class="docutils literal"><span class="pre">vector<bool></span></tt> only meet the requirements of input iterator and output iterator. This is so nonintuitive that the C++ standard contradicts itself on this point. In paragraph 23.2.4/1 it says that a <tt class="docutils literal"><span class="pre">vector</span></tt> is a sequence that supports random access iterators.</p> <p>Another difficult-to-categorize iterator is the transform iterator, an adaptor which applies a unary function object to the dereferenced value of the some underlying iterator (see <a class="reference external" href="http://www.boost.org/libs/utility/transform_iterator.htm">transform_iterator</a>). For unary functions such as <tt class="docutils literal"><span class="pre">times</span></tt>, the return type of <tt class="docutils literal"><span class="pre">operator*</span></tt> clearly needs to be the <tt class="docutils literal"><span class="pre">result_type</span></tt> of the function object, which is typically not a reference. Because random access iterators are required to return lvalues from <tt class="docutils literal"><span class="pre">operator*</span></tt>, if you wrap <tt class="docutils literal"><span class="pre">int*</span></tt> with a transform iterator, you do not get a random access iterator as might be expected, but an input iterator.</p> <p>A third example is found in the vertex and edge iterators of the <a class="reference external" href="http://www.boost.org/libs/graph/doc/table_of_contents.html">Boost Graph Library</a>. These iterators return vertex and edge descriptors, which are lightweight handles created on-the-fly. They must be returned by-value. As a result, their current standard iterator category is <tt class="docutils literal"><span class="pre">input_iterator_tag</span></tt>, which means that, strictly speaking, you could not use these iterators with algorithms like <tt class="docutils literal"><span class="pre">min_element()</span></tt>. As a temporary solution, the concept <a class="reference external" href="http://www.boost.org/libs/utility/MultiPassInputIterator.html">Multi-Pass Input Iterator</a> was introduced to describe the vertex and edge descriptors, but as the design notes for the concept suggest, a better solution is needed.</p> <p>In short, there are many useful iterators that do not fit into the current standard iterator categories. As a result, the following bad things happen:</p> <ul class="simple"> <li>Iterators are often mis-categorized.</li> <li>Algorithm requirements are more strict than necessary, because they cannot separate the need for random access or bidirectional traversal from the need for a true reference return type.</li> </ul> </div> <div class="section" id="impact-on-the-standard"> <h1><a class="toc-backref" href="#id2">Impact on the Standard</a></h1> <p>This proposal for TR1 is a pure extension. Further, the new iterator concepts are backward-compatible with the old iterator requirements, and old iterators are forward-compatible with the new iterator concepts. That is to say, iterators that satisfy the old requirements also satisfy appropriate concepts in the new system, and iterators modeling the new concepts will automatically satisfy the appropriate old requirements.</p> <!-- I think we need to say something about the resolution to allow convertibility to any of the old-style tags as a TR issue (hope it made it). -DWA --> <!-- Hmm, not sure I understand. Are you talking about whether a standards conforming input iterator is allowed to have a tag that is not input_iterator_tag but that is convertible to input_iterator_tag? -JGS --> <div class="section" id="possible-but-not-proposed-changes-to-the-working-paper"> <h2><a class="toc-backref" href="#id3">Possible (but not proposed) Changes to the Working Paper</a></h2> <p>The extensions in this paper suggest several changes we might make to the working paper for the next standard. These changes are not a formal part of this proposal for TR1.</p> <div class="section" id="changes-to-algorithm-requirements"> <h3><a class="toc-backref" href="#id4">Changes to Algorithm Requirements</a></h3> <p>The algorithms in the standard library could benefit from the new iterator concepts because the new concepts provide a more accurate way to express their type requirements. The result is algorithms that are usable in more situations and have fewer type requirements.</p> <p>For the next working paper (but not for TR1), the committee should consider the following changes to the type requirements of algorithms. These changes are phrased as textual substitutions, listing the algorithms to which each textual substitution applies.</p> <p>Forward Iterator -> Forward Traversal Iterator and Readable Iterator</p> <blockquote> <tt class="docutils literal"><span class="pre">find_end,</span> <span class="pre">adjacent_find,</span> <span class="pre">search,</span> <span class="pre">search_n,</span> <span class="pre">rotate_copy,</span> <span class="pre">lower_bound,</span> <span class="pre">upper_bound,</span> <span class="pre">equal_range,</span> <span class="pre">binary_search,</span> <span class="pre">min_element,</span> <span class="pre">max_element</span></tt></blockquote> <p>Forward Iterator (1) -> Single Pass Iterator and Readable Iterator, Forward Iterator (2) -> Forward Traversal Iterator and Readable Iterator</p> <blockquote> <tt class="docutils literal"><span class="pre">find_first_of</span></tt></blockquote> <p>Forward Iterator -> Readable Iterator and Writable Iterator</p> <blockquote> <tt class="docutils literal"><span class="pre">iter_swap</span></tt></blockquote> <p>Forward Iterator -> Single Pass Iterator and Writable Iterator</p> <blockquote> <tt class="docutils literal"><span class="pre">fill,</span> <span class="pre">generate</span></tt></blockquote> <p>Forward Iterator -> Forward Traversal Iterator and Swappable Iterator</p> <blockquote> <tt class="docutils literal"><span class="pre">rotate</span></tt></blockquote> <p>Forward Iterator (1) -> Swappable Iterator and Single Pass Iterator, Forward Iterator (2) -> Swappable Iterator and Incrementable Iterator</p> <blockquote> <tt class="docutils literal"><span class="pre">swap_ranges</span></tt></blockquote> <dl class="docutils"> <dt>Forward Iterator -> Forward Traversal Iterator and Readable Iterator and Writable Iterator</dt> <dd><tt class="docutils literal"><span class="pre">remove,</span> <span class="pre">remove_if,</span> <span class="pre">unique</span></tt></dd> </dl> <p>Forward Iterator -> Single Pass Iterator and Readable Iterator and Writable Iterator</p> <blockquote> <tt class="docutils literal"><span class="pre">replace,</span> <span class="pre">replace_if</span></tt></blockquote> <dl class="docutils"> <dt>Bidirectional Iterator -> Bidirectional Traversal Iterator and Swappable Iterator</dt> <dd><tt class="docutils literal"><span class="pre">reverse</span></tt></dd> <dt>Bidirectional Iterator -> Bidirectional Traversal Iterator and Readable and Swappable Iterator</dt> <dd><tt class="docutils literal"><span class="pre">partition</span></tt></dd> </dl> <p>Bidirectional Iterator (1) -> Bidirectional Traversal Iterator and Readable Iterator, Bidirectional Iterator (2) -> Bidirectional Traversal Iterator and Writable Iterator</p> <blockquote> <tt class="docutils literal"><span class="pre">copy_backwards</span></tt></blockquote> <dl class="docutils"> <dt>Bidirectional Iterator -> Bidirectional Traversal Iterator and Swappable Iterator and Readable Iterator</dt> <dd><tt class="docutils literal"><span class="pre">next_permutation,</span> <span class="pre">prev_permutation</span></tt></dd> <dt>Bidirectional Iterator -> Bidirectional Traversal Iterator and Readable Iterator and Writable Iterator</dt> <dd><tt class="docutils literal"><span class="pre">stable_partition,</span> <span class="pre">inplace_merge</span></tt></dd> <dt>Bidirectional Iterator -> Bidirectional Traversal Iterator and Readable Iterator</dt> <dd><tt class="docutils literal"><span class="pre">reverse_copy</span></tt></dd> <dt>Random Access Iterator -> Random Access Traversal Iterator and Readable and Writable Iterator</dt> <dd><tt class="docutils literal"><span class="pre">random_shuffle,</span> <span class="pre">sort,</span> <span class="pre">stable_sort,</span> <span class="pre">partial_sort,</span> <span class="pre">nth_element,</span> <span class="pre">push_heap,</span> <span class="pre">pop_heap</span> <span class="pre">make_heap,</span> <span class="pre">sort_heap</span></tt></dd> <dt>Input Iterator (2) -> Incrementable Iterator and Readable Iterator</dt> <dd><tt class="docutils literal"><span class="pre">equal,</span> <span class="pre">mismatch</span></tt></dd> <dt>Input Iterator (2) -> Incrementable Iterator and Readable Iterator</dt> <dd><tt class="docutils literal"><span class="pre">transform</span></tt></dd> </dl> </div> <div class="section" id="deprecations"> <h3><a class="toc-backref" href="#id5">Deprecations</a></h3> <p>For the next working paper (but not for TR1), the committee should consider deprecating the old iterator tags, and std::iterator_traits, since it will be superceded by individual traits metafunctions.</p> </div> <div class="section" id="vector-bool"> <h3><a class="toc-backref" href="#id6"><tt class="docutils literal"><span class="pre">vector<bool></span></tt></a></h3> <p>For the next working paper (but not for TR1), the committee should consider reclassifying <tt class="docutils literal"><span class="pre">vector<bool>::iterator</span></tt> as a Random Access Traversal Iterator and Readable Iterator and Writable Iterator.</p> </div> </div> </div> <div class="section" id="design"> <h1><a class="toc-backref" href="#id7">Design</a></h1> <p>The iterator requirements are to be separated into two groups. One set of concepts handles the syntax and semantics of value access:</p> <ul class="simple"> <li>Readable Iterator</li> <li>Writable Iterator</li> <li>Swappable Iterator</li> <li>Lvalue Iterator</li> </ul> <p>The access concepts describe requirements related to <tt class="docutils literal"><span class="pre">operator*</span></tt> and <tt class="docutils literal"><span class="pre">operator-></span></tt>, including the <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">pointer</span></tt> associated types.</p> <p>The other set of concepts handles traversal:</p> <ul class="simple"> <li>Incrementable Iterator</li> <li>Single Pass Iterator</li> <li>Forward Traversal Iterator</li> <li>Bidirectional Traversal Iterator</li> <li>Random Access Traversal Iterator</li> </ul> <p>The refinement relationships for the traversal concepts are in the following diagram.</p> <img alt="traversal.png" src="traversal.png" /> <p>In addition to the iterator movement operators, such as <tt class="docutils literal"><span class="pre">operator++</span></tt>, the traversal concepts also include requirements on position comparison such as <tt class="docutils literal"><span class="pre">operator==</span></tt> and <tt class="docutils literal"><span class="pre">operator<</span></tt>. The reason for the fine grain slicing of the concepts into the Incrementable and Single Pass is to provide concepts that are exact matches with the original input and output iterator requirements.</p> <p>This proposal also includes a concept for specifying when an iterator is interoperable with another iterator, in the sense that <tt class="docutils literal"><span class="pre">int*</span></tt> is interoperable with <tt class="docutils literal"><span class="pre">int</span> <span class="pre">const*</span></tt>.</p> <ul class="simple"> <li>Interoperable Iterators</li> </ul> <p>The relationship between the new iterator concepts and the old are given in the following diagram.</p> <img alt="oldeqnew.png" src="oldeqnew.png" /> <p>Like the old iterator requirements, we provide tags for purposes of dispatching based on the traversal concepts. The tags are related via inheritance so that a tag is convertible to another tag if the concept associated with the first tag is a refinement of the second tag.</p> <p>Our design reuses <tt class="docutils literal"><span class="pre">iterator_traits<Iter>::iterator_category</span></tt> to indicate an iterator's traversal capability. To specify capabilities not captured by any old-style iterator category, an iterator designer can use an <tt class="docutils literal"><span class="pre">iterator_category</span></tt> type that is convertible to both the the most-derived old iterator category tag which fits, and the appropriate new iterator traversal tag.</p> <!-- dwa2003/1/2: Note that we are not *requiring* convertibility to a new-style traversal tag in order to meet new concepts. Old-style iterators still fit, after all. --> <p>We do not provide tags for the purposes of dispatching based on the access concepts, in part because we could not find a way to automatically infer the right access tags for old-style iterators. An iterator's writability may be dependent on the assignability of its <tt class="docutils literal"><span class="pre">value_type</span></tt> and there's no known way to detect whether an arbitrary type is assignable. Fortunately, the need for dispatching based on access capability is not as great as the need for dispatching based on traversal capability.</p> <p>A difficult design decision concerned the <tt class="docutils literal"><span class="pre">operator[]</span></tt>. The direct approach for specifying <tt class="docutils literal"><span class="pre">operator[]</span></tt> would have a return type of <tt class="docutils literal"><span class="pre">reference</span></tt>; the same as <tt class="docutils literal"><span class="pre">operator*</span></tt>. However, going in this direction would mean that an iterator satisfying the old Random Access Iterator requirements would not necessarily be a model of Readable or Writable Lvalue Iterator. Instead we have chosen a design that matches the preferred resolution of <a class="reference external" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/lwg-active.html#299">issue 299</a>: <tt class="docutils literal"><span class="pre">operator[]</span></tt> is only required to return something convertible to the <tt class="docutils literal"><span class="pre">value_type</span></tt> (for a Readable Iterator), and is required to support assignment <tt class="docutils literal"><span class="pre">i[n]</span> <span class="pre">=</span> <span class="pre">t</span></tt> (for a Writable Iterator).</p> </div> <div class="section" id="proposed-text"> <h1><a class="toc-backref" href="#id8">Proposed Text</a></h1> <div class="section" id="addition-to-lib-iterator-requirements"> <h2><a class="toc-backref" href="#id9">Addition to [lib.iterator.requirements]</a></h2> <div class="section" id="iterator-value-access-concepts-lib-iterator-value-access"> <h3><a class="toc-backref" href="#id10">Iterator Value Access Concepts [lib.iterator.value.access]</a></h3> <p>In the tables below, <tt class="docutils literal"><span class="pre">X</span></tt> is an iterator type, <tt class="docutils literal"><span class="pre">a</span></tt> is a constant object of type <tt class="docutils literal"><span class="pre">X</span></tt>, <tt class="docutils literal"><span class="pre">R</span></tt> is <tt class="docutils literal"><span class="pre">std::iterator_traits<X>::reference</span></tt>, <tt class="docutils literal"><span class="pre">T</span></tt> is <tt class="docutils literal"><span class="pre">std::iterator_traits<X>::value_type</span></tt>, and <tt class="docutils literal"><span class="pre">v</span></tt> is a constant object of type <tt class="docutils literal"><span class="pre">T</span></tt>.</p> <div class="section" id="readable-iterators-lib-readable-iterators"> <span id="readable-iterator"></span><h4><a class="toc-backref" href="#id11">Readable Iterators [lib.readable.iterators]</a></h4> <p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> models the <em>Readable Iterator</em> concept for value type <tt class="docutils literal"><span class="pre">T</span></tt> if, in addition to <tt class="docutils literal"><span class="pre">X</span></tt> being Assignable and Copy Constructible, the following expressions are valid and respect the stated semantics. <tt class="docutils literal"><span class="pre">U</span></tt> is the type of any specified member of type <tt class="docutils literal"><span class="pre">T</span></tt>.</p> <table border="1" class="docutils"> <colgroup> <col width="28%" /> <col width="20%" /> <col width="52%" /> </colgroup> <thead valign="bottom"> <tr><th class="head" colspan="3">Readable Iterator Requirements (in addition to Assignable and Copy Constructible)</th> </tr> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Note/Precondition</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">iterator_traits<X>::value_type</span></tt></td> <td><tt class="docutils literal"><span class="pre">T</span></tt></td> <td>Any non-reference, non-cv-qualified type</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">*a</span></tt></td> <td>Convertible to <tt class="docutils literal"><span class="pre">T</span></tt></td> <td><dl class="first last docutils"> <dt>pre: <tt class="docutils literal"><span class="pre">a</span></tt> is dereferenceable. If <tt class="docutils literal"><span class="pre">a</span> <span class="pre">==</span> <span class="pre">b</span></tt> then <tt class="docutils literal"><span class="pre">*a</span></tt></dt> <dd>is equivalent to <tt class="docutils literal"><span class="pre">*b</span></tt>.</dd> </dl> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a->m</span></tt></td> <td><tt class="docutils literal"><span class="pre">U&</span></tt></td> <td>pre: <tt class="docutils literal"><span class="pre">pre:</span> <span class="pre">(*a).m</span></tt> is well-defined. Equivalent to <tt class="docutils literal"><span class="pre">(*a).m</span></tt>.</td> </tr> </tbody> </table> <!-- We won't say anything about iterator_traits<X>::reference until the DR is resolved. -JGS --> </div> <div class="section" id="writable-iterators-lib-writable-iterators"> <span id="writable-iterator"></span><h4><a class="toc-backref" href="#id12">Writable Iterators [lib.writable.iterators]</a></h4> <p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> models the <em>Writable Iterator</em> concept if, in addition to <tt class="docutils literal"><span class="pre">X</span></tt> being Copy Constructible, the following expressions are valid and respect the stated semantics. Writable Iterators have an associated <em>set of value types</em>.</p> <table border="1" class="docutils"> <colgroup> <col width="37%" /> <col width="21%" /> <col width="42%" /> </colgroup> <thead valign="bottom"> <tr><th class="head" colspan="3">Writable Iterator Requirements (in addition to Copy Constructible)</th> </tr> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Precondition</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">*a</span> <span class="pre">=</span> <span class="pre">o</span></tt></td> <td> </td> <td>pre: The type of <tt class="docutils literal"><span class="pre">o</span></tt> is in the set of value types of <tt class="docutils literal"><span class="pre">X</span></tt></td> </tr> </tbody> </table> </div> <div class="section" id="swappable-iterators-lib-swappable-iterators"> <h4><a class="toc-backref" href="#id13">Swappable Iterators [lib.swappable.iterators]</a></h4> <p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> models the <em>Swappable Iterator</em> concept if, in addition to <tt class="docutils literal"><span class="pre">X</span></tt> being Copy Constructible, the following expressions are valid and respect the stated semantics.</p> <table border="1" class="docutils"> <colgroup> <col width="37%" /> <col width="19%" /> <col width="43%" /> </colgroup> <thead valign="bottom"> <tr><th class="head" colspan="3">Swappable Iterator Requirements (in addition to Copy Constructible)</th> </tr> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Postcondition</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">iter_swap(a,</span> <span class="pre">b)</span></tt></td> <td><tt class="docutils literal"><span class="pre">void</span></tt></td> <td>the pointed to values are exchanged</td> </tr> </tbody> </table> <p>[<em>Note:</em> An iterator that is a model of the <a class="reference internal" href="#readable-iterator">Readable Iterator</a> and <a class="reference internal" href="#writable-iterator">Writable Iterator</a> concepts is also a model of <em>Swappable Iterator</em>. <em>--end note</em>]</p> </div> <div class="section" id="lvalue-iterators-lib-lvalue-iterators"> <h4><a class="toc-backref" href="#id14">Lvalue Iterators [lib.lvalue.iterators]</a></h4> <p>The <em>Lvalue Iterator</em> concept adds the requirement that the return type of <tt class="docutils literal"><span class="pre">operator*</span></tt> type be a reference to the value type of the iterator.</p> <table border="1" class="docutils"> <colgroup> <col width="22%" /> <col width="19%" /> <col width="59%" /> </colgroup> <thead valign="bottom"> <tr><th class="head" colspan="3">Lvalue Iterator Requirements</th> </tr> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Note/Assertion</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">*a</span></tt></td> <td><tt class="docutils literal"><span class="pre">T&</span></tt></td> <td><tt class="docutils literal"><span class="pre">T</span></tt> is <em>cv</em> <tt class="docutils literal"><span class="pre">iterator_traits<X>::value_type</span></tt> where <em>cv</em> is an optional cv-qualification. pre: <tt class="docutils literal"><span class="pre">a</span></tt> is dereferenceable.</td> </tr> </tbody> </table> <p>If <tt class="docutils literal"><span class="pre">X</span></tt> is a <a class="reference internal" href="#writable-iterator">Writable Iterator</a> then <tt class="docutils literal"><span class="pre">a</span> <span class="pre">==</span> <span class="pre">b</span></tt> if and only if <tt class="docutils literal"><span class="pre">*a</span></tt> is the same object as <tt class="docutils literal"><span class="pre">*b</span></tt>. If <tt class="docutils literal"><span class="pre">X</span></tt> is a <a class="reference internal" href="#readable-iterator">Readable Iterator</a> then <tt class="docutils literal"><span class="pre">a</span> <span class="pre">==</span> <span class="pre">b</span></tt> implies <tt class="docutils literal"><span class="pre">*a</span></tt> is the same object as <tt class="docutils literal"><span class="pre">*b</span></tt>.</p> </div> </div> <div class="section" id="iterator-traversal-concepts-lib-iterator-traversal"> <h3><a class="toc-backref" href="#id15">Iterator Traversal Concepts [lib.iterator.traversal]</a></h3> <p>In the tables below, <tt class="docutils literal"><span class="pre">X</span></tt> is an iterator type, <tt class="docutils literal"><span class="pre">a</span></tt> and <tt class="docutils literal"><span class="pre">b</span></tt> are constant objects of type <tt class="docutils literal"><span class="pre">X</span></tt>, <tt class="docutils literal"><span class="pre">r</span></tt> and <tt class="docutils literal"><span class="pre">s</span></tt> are mutable objects of type <tt class="docutils literal"><span class="pre">X</span></tt>, <tt class="docutils literal"><span class="pre">T</span></tt> is <tt class="docutils literal"><span class="pre">std::iterator_traits<X>::value_type</span></tt>, and <tt class="docutils literal"><span class="pre">v</span></tt> is a constant object of type <tt class="docutils literal"><span class="pre">T</span></tt>.</p> <div class="section" id="incrementable-iterators-lib-incrementable-iterators"> <h4><a class="toc-backref" href="#id16">Incrementable Iterators [lib.incrementable.iterators]</a></h4> <p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> models the <em>Incrementable Iterator</em> concept if, in addition to <tt class="docutils literal"><span class="pre">X</span></tt> being Assignable and Copy Constructible, the following expressions are valid and respect the stated semantics.</p> <table border="1" class="docutils"> <colgroup> <col width="39%" /> <col width="38%" /> <col width="23%" /> </colgroup> <thead valign="bottom"> <tr><th class="head" colspan="3">Incrementable Iterator Requirements (in addition to Assignable, Copy Constructible)</th> </tr> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Assertion</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">++r</span></tt></td> <td><tt class="docutils literal"><span class="pre">X&</span></tt></td> <td><tt class="docutils literal"><span class="pre">&r</span> <span class="pre">==</span> <span class="pre">&++r</span></tt></td> </tr> <tr><td><tt class="docutils literal"><span class="pre">r++</span></tt></td> <td> </td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">*r++</span></tt></td> <td> </td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">iterator_traversal<X>::type</span></tt></td> <td>Convertible to <tt class="docutils literal"><span class="pre">incrementable_traversal_tag</span></tt></td> <td> </td> </tr> </tbody> </table> <p>If <tt class="docutils literal"><span class="pre">X</span></tt> is a <a class="reference internal" href="#writable-iterator">Writable Iterator</a> then <tt class="docutils literal"><span class="pre">X</span> <span class="pre">a(r++);</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">X</span> <span class="pre">a(r);</span> <span class="pre">++r;</span></tt> and <tt class="docutils literal"><span class="pre">*r++</span> <span class="pre">=</span> <span class="pre">o</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">*r</span> <span class="pre">=</span> <span class="pre">o;</span> <span class="pre">++r</span></tt>. If <tt class="docutils literal"><span class="pre">X</span></tt> is a <a class="reference internal" href="#readable-iterator">Readable Iterator</a> then <tt class="docutils literal"><span class="pre">T</span> <span class="pre">z(*r++);</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">T</span> <span class="pre">z(*r);</span> <span class="pre">++r;</span></tt>.</p> <!-- TR1: incrementable_iterator_tag changed to incrementable_traversal_tag for consistency. --> </div> <div class="section" id="single-pass-iterators-lib-single-pass-iterators"> <h4><a class="toc-backref" href="#id17">Single Pass Iterators [lib.single.pass.iterators]</a></h4> <p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> models the <em>Single Pass Iterator</em> concept if the following expressions are valid and respect the stated semantics.</p> <table border="1" class="docutils"> <colgroup> <col width="37%" /> <col width="27%" /> <col width="12%" /> <col width="25%" /> </colgroup> <thead valign="bottom"> <tr><th class="head" colspan="4">Single Pass Iterator Requirements (in addition to Incrementable Iterator and Equality Comparable)</th> </tr> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Operational Semantics</th> <th class="head">Assertion/ Pre-/Post-condition</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">++r</span></tt></td> <td><tt class="docutils literal"><span class="pre">X&</span></tt></td> <td> </td> <td>pre: <tt class="docutils literal"><span class="pre">r</span></tt> is dereferenceable; post: <tt class="docutils literal"><span class="pre">r</span></tt> is dereferenceable or <tt class="docutils literal"><span class="pre">r</span></tt> is past-the-end</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre">==</span> <span class="pre">b</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td> </td> <td><tt class="docutils literal"><span class="pre">==</span></tt> is an equivalence relation over its domain</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre">!=</span> <span class="pre">b</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">!(a</span> <span class="pre">==</span> <span class="pre">b)</span></tt></td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">iterator_traits<X>::difference_type</span></tt></td> <td>A signed integral type representing the distance between iterators</td> <td> </td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">iterator_traversal<X>::type</span></tt></td> <td>Convertible to <tt class="docutils literal"><span class="pre">single_pass_traversal_tag</span></tt></td> <td> </td> <td> </td> </tr> </tbody> </table> <!-- TR1: single_pass_iterator_tag changed to single_pass_traversal_tag for consistency --> </div> <div class="section" id="forward-traversal-iterators-lib-forward-traversal-iterators"> <h4><a class="toc-backref" href="#id18">Forward Traversal Iterators [lib.forward.traversal.iterators]</a></h4> <p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> models the <em>Forward Traversal Iterator</em> concept if, in addition to <tt class="docutils literal"><span class="pre">X</span></tt> meeting the requirements of Default Constructible and Single Pass Iterator, the following expressions are valid and respect the stated semantics.</p> <table border="1" class="docutils"> <colgroup> <col width="38%" /> <col width="34%" /> <col width="27%" /> </colgroup> <thead valign="bottom"> <tr><th class="head" colspan="3">Forward Traversal Iterator Requirements (in addition to Default Constructible and Single Pass Iterator)</th> </tr> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Assertion/Note</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">X</span> <span class="pre">u;</span></tt></td> <td><tt class="docutils literal"><span class="pre">X&</span></tt></td> <td>note: <tt class="docutils literal"><span class="pre">u</span></tt> may have a singular value.</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">++r</span></tt></td> <td><tt class="docutils literal"><span class="pre">X&</span></tt></td> <td><tt class="docutils literal"><span class="pre">r</span> <span class="pre">==</span> <span class="pre">s</span></tt> and <tt class="docutils literal"><span class="pre">r</span></tt> is dereferenceable implies <tt class="docutils literal"><span class="pre">++r</span> <span class="pre">==</span> <span class="pre">++s.</span></tt></td> </tr> <tr><td><tt class="docutils literal"><span class="pre">iterator_traversal<X>::type</span></tt></td> <td>Convertible to <tt class="docutils literal"><span class="pre">forward_traversal_tag</span></tt></td> <td> </td> </tr> </tbody> </table> <!-- TR1: forward_traversal_iterator_tag changed to forward_traversal_tag for consistency --> </div> <div class="section" id="bidirectional-traversal-iterators-lib-bidirectional-traversal-iterators"> <h4><a class="toc-backref" href="#id19">Bidirectional Traversal Iterators [lib.bidirectional.traversal.iterators]</a></h4> <p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> models the <em>Bidirectional Traversal Iterator</em> concept if, in addition to <tt class="docutils literal"><span class="pre">X</span></tt> meeting the requirements of Forward Traversal Iterator, the following expressions are valid and respect the stated semantics.</p> <table border="1" class="docutils"> <colgroup> <col width="33%" /> <col width="32%" /> <col width="14%" /> <col width="21%" /> </colgroup> <thead valign="bottom"> <tr><th class="head" colspan="4">Bidirectional Traversal Iterator Requirements (in addition to Forward Traversal Iterator)</th> </tr> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Operational Semantics</th> <th class="head">Assertion/ Pre-/Post-condition</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">--r</span></tt></td> <td><tt class="docutils literal"><span class="pre">X&</span></tt></td> <td> </td> <td><p class="first">pre: there exists <tt class="docutils literal"><span class="pre">s</span></tt> such that <tt class="docutils literal"><span class="pre">r</span> <span class="pre">==</span> <span class="pre">++s</span></tt>. post: <tt class="docutils literal"><span class="pre">s</span></tt> is dereferenceable.</p> <p class="last"><tt class="docutils literal"><span class="pre">++(--r)</span> <span class="pre">==</span> <span class="pre">r</span></tt>. <tt class="docutils literal"><span class="pre">--r</span> <span class="pre">==</span> <span class="pre">--s</span></tt> implies <tt class="docutils literal"><span class="pre">r</span> <span class="pre">==</span> <span class="pre">s</span></tt>. <tt class="docutils literal"><span class="pre">&r</span> <span class="pre">==</span> <span class="pre">&--r</span></tt>.</p> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">r--</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">const</span> <span class="pre">X&</span></tt></td> <td><pre class="first last literal-block"> { X tmp = r; --r; return tmp; } </pre> </td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">iterator_traversal<X>::type</span></tt></td> <td>Convertible to <tt class="docutils literal"><span class="pre">bidirectional_traversal_tag</span></tt></td> <td> </td> <td> </td> </tr> </tbody> </table> <!-- TR1: bidirectional_traversal_iterator_tag changed to bidirectional_traversal_tag for consistency --> </div> <div class="section" id="random-access-traversal-iterators-lib-random-access-traversal-iterators"> <h4><a class="toc-backref" href="#id20">Random Access Traversal Iterators [lib.random.access.traversal.iterators]</a></h4> <p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> models the <em>Random Access Traversal Iterator</em> concept if the following expressions are valid and respect the stated semantics. In the table below, <tt class="docutils literal"><span class="pre">Distance</span></tt> is <tt class="docutils literal"><span class="pre">iterator_traits<X>::difference_type</span></tt> and <tt class="docutils literal"><span class="pre">n</span></tt> represents a constant object of type <tt class="docutils literal"><span class="pre">Distance</span></tt>.</p> <table border="1" class="docutils"> <colgroup> <col width="28%" /> <col width="30%" /> <col width="23%" /> <col width="20%" /> </colgroup> <thead valign="bottom"> <tr><th class="head" colspan="4">Random Access Traversal Iterator Requirements (in addition to Bidirectional Traversal Iterator)</th> </tr> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Operational Semantics</th> <th class="head">Assertion/ Precondition</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">r</span> <span class="pre">+=</span> <span class="pre">n</span></tt></td> <td><tt class="docutils literal"><span class="pre">X&</span></tt></td> <td><pre class="first last literal-block"> { Distance m = n; if (m >= 0) while (m--) ++r; else while (m++) --r; return r; } </pre> </td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre">+</span> <span class="pre">n</span></tt>, <tt class="docutils literal"><span class="pre">n</span> <span class="pre">+</span> <span class="pre">a</span></tt></td> <td><tt class="docutils literal"><span class="pre">X</span></tt></td> <td><tt class="docutils literal"><span class="pre">{</span> <span class="pre">X</span> <span class="pre">tmp</span> <span class="pre">=</span> <span class="pre">a;</span> <span class="pre">return</span> <span class="pre">tmp</span> <span class="pre">+=</span> <span class="pre">n;</span> <span class="pre">}</span></tt></td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">r</span> <span class="pre">-=</span> <span class="pre">n</span></tt></td> <td><tt class="docutils literal"><span class="pre">X&</span></tt></td> <td><tt class="docutils literal"><span class="pre">return</span> <span class="pre">r</span> <span class="pre">+=</span> <span class="pre">-n</span></tt></td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre">-</span> <span class="pre">n</span></tt></td> <td><tt class="docutils literal"><span class="pre">X</span></tt></td> <td><tt class="docutils literal"><span class="pre">{</span> <span class="pre">X</span> <span class="pre">tmp</span> <span class="pre">=</span> <span class="pre">a;</span> <span class="pre">return</span> <span class="pre">tmp</span> <span class="pre">-=</span> <span class="pre">n;</span> <span class="pre">}</span></tt></td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">b</span> <span class="pre">-</span> <span class="pre">a</span></tt></td> <td><tt class="docutils literal"><span class="pre">Distance</span></tt></td> <td><tt class="docutils literal"><span class="pre">a</span> <span class="pre"><</span> <span class="pre">b</span> <span class="pre">?</span> <span class="pre">distance(a,b)</span> <span class="pre">:</span> <span class="pre">-distance(b,a)</span></tt></td> <td>pre: there exists a value <tt class="docutils literal"><span class="pre">n</span></tt> of <tt class="docutils literal"><span class="pre">Distance</span></tt> such that <tt class="docutils literal"><span class="pre">a</span> <span class="pre">+</span> <span class="pre">n</span> <span class="pre">==</span> <span class="pre">b</span></tt>. <tt class="docutils literal"><span class="pre">b</span> <span class="pre">==</span> <span class="pre">a</span> <span class="pre">+</span> <span class="pre">(b</span> <span class="pre">-</span> <span class="pre">a)</span></tt>.</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a[n]</span></tt></td> <td>convertible to T</td> <td><tt class="docutils literal"><span class="pre">*(a</span> <span class="pre">+</span> <span class="pre">n)</span></tt></td> <td>pre: a is a <a class="reference internal" href="#readable-iterator">Readable Iterator</a></td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a[n]</span> <span class="pre">=</span> <span class="pre">v</span></tt></td> <td>convertible to T</td> <td><tt class="docutils literal"><span class="pre">*(a</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">v</span></tt></td> <td>pre: a is a <a class="reference internal" href="#writable-iterator">Writable Iterator</a></td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre"><</span> <span class="pre">b</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">b</span> <span class="pre">-</span> <span class="pre">a</span> <span class="pre">></span> <span class="pre">0</span></tt></td> <td><tt class="docutils literal"><span class="pre"><</span></tt> is a total ordering relation</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre">></span> <span class="pre">b</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">b</span> <span class="pre"><</span> <span class="pre">a</span></tt></td> <td><tt class="docutils literal"><span class="pre">></span></tt> is a total ordering relation</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre">>=</span> <span class="pre">b</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">!(a</span> <span class="pre"><</span> <span class="pre">b)</span></tt></td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">a</span> <span class="pre"><=</span> <span class="pre">b</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">!(a</span> <span class="pre">></span> <span class="pre">b)</span></tt></td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">iterator_traversal<X>::type</span></tt></td> <td>Convertible to <tt class="docutils literal"><span class="pre">random_access_traversal_tag</span></tt></td> <td> </td> <td> </td> </tr> </tbody> </table> <!-- TR1: random_access_traversal_iterator_tag changed to random_access_traversal_tag for consistency --> </div> <div class="section" id="interoperable-iterators-lib-interoperable-iterators"> <h4><a class="toc-backref" href="#id21">Interoperable Iterators [lib.interoperable.iterators]</a></h4> <p>A class or built-in type <tt class="docutils literal"><span class="pre">X</span></tt> that models Single Pass Iterator is <em>interoperable with</em> a class or built-in type <tt class="docutils literal"><span class="pre">Y</span></tt> that also models Single Pass Iterator if the following expressions are valid and respect the stated semantics. In the tables below, <tt class="docutils literal"><span class="pre">x</span></tt> is an object of type <tt class="docutils literal"><span class="pre">X</span></tt>, <tt class="docutils literal"><span class="pre">y</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Y</span></tt>, <tt class="docutils literal"><span class="pre">Distance</span></tt> is <tt class="docutils literal"><span class="pre">iterator_traits<Y>::difference_type</span></tt>, and <tt class="docutils literal"><span class="pre">n</span></tt> represents a constant object of type <tt class="docutils literal"><span class="pre">Distance</span></tt>.</p> <table border="1" class="docutils"> <colgroup> <col width="13%" /> <col width="27%" /> <col width="60%" /> </colgroup> <thead valign="bottom"> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Assertion/Precondition/Postcondition</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">y</span> <span class="pre">=</span> <span class="pre">x</span></tt></td> <td><tt class="docutils literal"><span class="pre">Y</span></tt></td> <td>post: <tt class="docutils literal"><span class="pre">y</span> <span class="pre">==</span> <span class="pre">x</span></tt></td> </tr> <tr><td><tt class="docutils literal"><span class="pre">Y(x)</span></tt></td> <td><tt class="docutils literal"><span class="pre">Y</span></tt></td> <td>post: <tt class="docutils literal"><span class="pre">Y(x)</span> <span class="pre">==</span> <span class="pre">x</span></tt></td> </tr> <tr><td><tt class="docutils literal"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">==</span></tt> is an equivalence relation over its domain.</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">y</span> <span class="pre">==</span> <span class="pre">x</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">==</span></tt> is an equivalence relation over its domain.</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">x</span> <span class="pre">!=</span> <span class="pre">y</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">bool(a==b)</span> <span class="pre">!=</span> <span class="pre">bool(a!=b)</span></tt> over its domain.</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">y</span> <span class="pre">!=</span> <span class="pre">x</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">bool(a==b)</span> <span class="pre">!=</span> <span class="pre">bool(a!=b)</span></tt> over its domain.</td> </tr> </tbody> </table> <p>If <tt class="docutils literal"><span class="pre">X</span></tt> and <tt class="docutils literal"><span class="pre">Y</span></tt> both model Random Access Traversal Iterator then the following additional requirements must be met.</p> <table border="1" class="docutils"> <colgroup> <col width="12%" /> <col width="25%" /> <col width="23%" /> <col width="41%" /> </colgroup> <thead valign="bottom"> <tr><th class="head">Expression</th> <th class="head">Return Type</th> <th class="head">Operational Semantics</th> <th class="head">Assertion/ Precondition</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="docutils literal"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">y</span> <span class="pre">-</span> <span class="pre">x</span> <span class="pre">></span> <span class="pre">0</span></tt></td> <td><tt class="docutils literal"><span class="pre"><</span></tt> is a total ordering relation</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">y</span> <span class="pre"><</span> <span class="pre">x</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">x</span> <span class="pre">-</span> <span class="pre">y</span> <span class="pre">></span> <span class="pre">0</span></tt></td> <td><tt class="docutils literal"><span class="pre"><</span></tt> is a total ordering relation</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">x</span> <span class="pre">></span> <span class="pre">y</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">y</span> <span class="pre"><</span> <span class="pre">x</span></tt></td> <td><tt class="docutils literal"><span class="pre">></span></tt> is a total ordering relation</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">y</span> <span class="pre">></span> <span class="pre">x</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span></tt></td> <td><tt class="docutils literal"><span class="pre">></span></tt> is a total ordering relation</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">x</span> <span class="pre">>=</span> <span class="pre">y</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">!(x</span> <span class="pre"><</span> <span class="pre">y)</span></tt></td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">y</span> <span class="pre">>=</span> <span class="pre">x</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">!(y</span> <span class="pre"><</span> <span class="pre">x)</span></tt></td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">x</span> <span class="pre"><=</span> <span class="pre">y</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">!(x</span> <span class="pre">></span> <span class="pre">y)</span></tt></td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">y</span> <span class="pre"><=</span> <span class="pre">x</span></tt></td> <td>convertible to <tt class="docutils literal"><span class="pre">bool</span></tt></td> <td><tt class="docutils literal"><span class="pre">!(y</span> <span class="pre">></span> <span class="pre">x)</span></tt></td> <td> </td> </tr> <tr><td><tt class="docutils literal"><span class="pre">y</span> <span class="pre">-</span> <span class="pre">x</span></tt></td> <td><tt class="docutils literal"><span class="pre">Distance</span></tt></td> <td><tt class="docutils literal"><span class="pre">distance(Y(x),y)</span></tt></td> <td>pre: there exists a value <tt class="docutils literal"><span class="pre">n</span></tt> of <tt class="docutils literal"><span class="pre">Distance</span></tt> such that <tt class="docutils literal"><span class="pre">x</span> <span class="pre">+</span> <span class="pre">n</span> <span class="pre">==</span> <span class="pre">y</span></tt>. <tt class="docutils literal"><span class="pre">y</span> <span class="pre">==</span> <span class="pre">x</span> <span class="pre">+</span> <span class="pre">(y</span> <span class="pre">-</span> <span class="pre">x)</span></tt>.</td> </tr> <tr><td><tt class="docutils literal"><span class="pre">x</span> <span class="pre">-</span> <span class="pre">y</span></tt></td> <td><tt class="docutils literal"><span class="pre">Distance</span></tt></td> <td><tt class="docutils literal"><span class="pre">distance(y,Y(x))</span></tt></td> <td>pre: there exists a value <tt class="docutils literal"><span class="pre">n</span></tt> of <tt class="docutils literal"><span class="pre">Distance</span></tt> such that <tt class="docutils literal"><span class="pre">y</span> <span class="pre">+</span> <span class="pre">n</span> <span class="pre">==</span> <span class="pre">x</span></tt>. <tt class="docutils literal"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span> <span class="pre">+</span> <span class="pre">(x</span> <span class="pre">-</span> <span class="pre">y)</span></tt>.</td> </tr> </tbody> </table> </div> </div> </div> <div class="section" id="addition-to-lib-iterator-synopsis"> <h2><a class="toc-backref" href="#id22">Addition to [lib.iterator.synopsis]</a></h2> <pre class="literal-block"> // lib.iterator.traits, traits and tags template <class Iterator> struct is_readable_iterator; template <class Iterator> struct iterator_traversal; struct incrementable_traversal_tag { }; struct single_pass_traversal_tag : incrementable_traversal_tag { }; struct forward_traversal_tag : single_pass_traversal_tag { }; struct bidirectional_traversal_tag : forward_traversal_tag { }; struct random_access_traversal_tag : bidirectional_traversal_tag { }; </pre> </div> <div class="section" id="addition-to-lib-iterator-traits"> <h2><a class="toc-backref" href="#id23">Addition to [lib.iterator.traits]</a></h2> <p>The <tt class="docutils literal"><span class="pre">is_readable_iterator</span></tt> class template satisfies the <a class="reference external" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1519.htm">UnaryTypeTrait</a> requirements.</p> <p>Given an iterator type <tt class="docutils literal"><span class="pre">X</span></tt>, <tt class="docutils literal"><span class="pre">is_readable_iterator<X>::value</span></tt> yields <tt class="docutils literal"><span class="pre">true</span></tt> if, for an object <tt class="docutils literal"><span class="pre">a</span></tt> of type <tt class="docutils literal"><span class="pre">X</span></tt>, <tt class="docutils literal"><span class="pre">*a</span></tt> is convertible to <tt class="docutils literal"><span class="pre">iterator_traits<X>::value_type</span></tt>, and <tt class="docutils literal"><span class="pre">false</span></tt> otherwise.</p> <p><tt class="docutils literal"><span class="pre">iterator_traversal<X>::type</span></tt> is</p> <pre class="literal-block"> <em>category-to-traversal</em>(iterator_traits<X>::iterator_category) </pre> <p>where <em>category-to-traversal</em> is defined as follows</p> <pre class="literal-block" id="category-to-traversal"> <em>category-to-traversal</em>(C) = if (C is convertible to incrementable_traversal_tag) return C; else if (C is convertible to random_access_iterator_tag) return random_access_traversal_tag; else if (C is convertible to bidirectional_iterator_tag) return bidirectional_traversal_tag; else if (C is convertible to forward_iterator_tag) return forward_traversal_tag; else if (C is convertible to input_iterator_tag) return single_pass_traversal_tag; else if (C is convertible to output_iterator_tag) return incrementable_traversal_tag; else <em>the program is ill-formed</em> </pre> </div> </div> <div class="section" id="footnotes"> <h1><a class="toc-backref" href="#id24">Footnotes</a></h1> <p>The UnaryTypeTrait concept is defined in <a class="reference external" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1519.htm">n1519</a>; the LWG is considering adding the requirement that specializations are derived from their nested <tt class="docutils literal"><span class="pre">::type</span></tt>.</p> <!-- LocalWords: Abrahams Siek Witt const bool Sutter's WG int UL LI href Lvalue LocalWords: ReadableIterator WritableIterator SwappableIterator cv pre iter LocalWords: ConstantLvalueIterator MutableLvalueIterator CopyConstructible TR LocalWords: ForwardTraversalIterator BidirectionalTraversalIterator lvalue LocalWords: RandomAccessTraversalIterator dereferenceable Incrementable tmp LocalWords: incrementable xxx min prev inplace png oldeqnew AccessTag struct LocalWords: TraversalTag typename lvalues DWA Hmm JGS mis enum --> </div> </div> <div class="footer"> <hr class="footer" /> <a class="reference external" href="new-iter-concepts.rst">View document source</a>. Generated by <a class="reference external" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference external" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source. </div> </body> </html>