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<div class="document" id="new-iterator-concepts">
@ -303,16 +21,16 @@ ul.auto-toc {
<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" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="reference" href="mailto:jsiek&#64;osl.iu.edu">jsiek&#64;osl.iu.edu</a>, <a class="last reference" href="mailto:witt&#64;styleadvisor.com">witt&#64;styleadvisor.com</a></td></tr>
<td><a class="first reference external" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek&#64;osl.iu.edu">jsiek&#64;osl.iu.edu</a>, <a class="last reference external" href="mailto:witt&#64;styleadvisor.com">witt&#64;styleadvisor.com</a></td></tr>
<tr><th class="docinfo-name">Organization:</th>
<td><a class="first reference" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference" href="http://www.osl.iu.edu">Open Systems
Lab</a>, <a class="last reference" href="http://www.styleadvisor.com">Zephyr Associates, Inc.</a></td></tr>
<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>2006-04-30</td></tr>
<tr class="field"><th class="docinfo-name">Number:</th><td class="field-body">This is a revised version of <a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1550.html">n1550</a>=03-0133, which was
<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" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2001/n1297.html">n1297</a>, <a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1477.html">n1477</a>, and <a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1531.html">n1531</a>.</td>
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
@ -337,48 +55,48 @@ of iterators that are used in practice.</td>
</tbody>
</table>
<div class="contents topic" id="table-of-contents">
<p class="topic-title first"><a name="table-of-contents">Table of Contents</a></p>
<p class="topic-title first">Table of Contents</p>
<ul class="simple">
<li><a class="reference" href="#motivation" id="id1" name="id1">Motivation</a></li>
<li><a class="reference" href="#impact-on-the-standard" id="id2" name="id2">Impact on the Standard</a><ul>
<li><a class="reference" href="#possible-but-not-proposed-changes-to-the-working-paper" id="id3" name="id3">Possible (but not proposed) Changes to the Working Paper</a><ul>
<li><a class="reference" href="#changes-to-algorithm-requirements" id="id4" name="id4">Changes to Algorithm Requirements</a></li>
<li><a class="reference" href="#deprecations" id="id5" name="id5">Deprecations</a></li>
<li><a class="reference" href="#vector-bool" id="id6" name="id6"><tt class="docutils literal"><span class="pre">vector&lt;bool&gt;</span></tt></a></li>
<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&lt;bool&gt;</span></tt></a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference" href="#design" id="id7" name="id7">Design</a></li>
<li><a class="reference" href="#proposed-text" id="id8" name="id8">Proposed Text</a><ul>
<li><a class="reference" href="#addition-to-lib-iterator-requirements" id="id9" name="id9">Addition to [lib.iterator.requirements]</a><ul>
<li><a class="reference" href="#iterator-value-access-concepts-lib-iterator-value-access" id="id10" name="id10">Iterator Value Access Concepts [lib.iterator.value.access]</a><ul>
<li><a class="reference" href="#readable-iterators-lib-readable-iterators" id="id11" name="id11">Readable Iterators [lib.readable.iterators]</a></li>
<li><a class="reference" href="#writable-iterators-lib-writable-iterators" id="id12" name="id12">Writable Iterators [lib.writable.iterators]</a></li>
<li><a class="reference" href="#swappable-iterators-lib-swappable-iterators" id="id13" name="id13">Swappable Iterators [lib.swappable.iterators]</a></li>
<li><a class="reference" href="#lvalue-iterators-lib-lvalue-iterators" id="id14" name="id14">Lvalue Iterators [lib.lvalue.iterators]</a></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" href="#iterator-traversal-concepts-lib-iterator-traversal" id="id15" name="id15">Iterator Traversal Concepts [lib.iterator.traversal]</a><ul>
<li><a class="reference" href="#incrementable-iterators-lib-incrementable-iterators" id="id16" name="id16">Incrementable Iterators [lib.incrementable.iterators]</a></li>
<li><a class="reference" href="#single-pass-iterators-lib-single-pass-iterators" id="id17" name="id17">Single Pass Iterators [lib.single.pass.iterators]</a></li>
<li><a class="reference" href="#forward-traversal-iterators-lib-forward-traversal-iterators" id="id18" name="id18">Forward Traversal Iterators [lib.forward.traversal.iterators]</a></li>
<li><a class="reference" href="#bidirectional-traversal-iterators-lib-bidirectional-traversal-iterators" id="id19" name="id19">Bidirectional Traversal Iterators [lib.bidirectional.traversal.iterators]</a></li>
<li><a class="reference" href="#random-access-traversal-iterators-lib-random-access-traversal-iterators" id="id20" name="id20">Random Access Traversal Iterators [lib.random.access.traversal.iterators]</a></li>
<li><a class="reference" href="#interoperable-iterators-lib-interoperable-iterators" id="id21" name="id21">Interoperable Iterators [lib.interoperable.iterators]</a></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" href="#addition-to-lib-iterator-synopsis" id="id22" name="id22">Addition to [lib.iterator.synopsis]</a></li>
<li><a class="reference" href="#addition-to-lib-iterator-traits" id="id23" name="id23">Addition to [lib.iterator.traits]</a></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" href="#footnotes" id="id24" name="id24">Footnotes</a></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" name="motivation">Motivation</a></h1>
<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
@ -398,7 +116,7 @@ requirements in the iterator categories.</p>
<col width="69%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="2">Value Access Requirements in Existing Iterator Categories</th>
<tr><th class="head" colspan="2">Value Access Requirements in Existing Iterator Categories</th>
</tr>
</thead>
<tbody valign="top">
@ -409,12 +127,12 @@ requirements in the iterator categories.</p>
<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&amp;</span></tt> (or <tt class="docutils literal"><span class="pre">const</span> <span class="pre">T&amp;</span></tt> once <a class="reference" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/lwg-active.html#200">issue 200</a>
<td><tt class="docutils literal"><span class="pre">*i</span></tt> is <tt class="docutils literal"><span class="pre">T&amp;</span></tt> (or <tt class="docutils literal"><span class="pre">const</span> <span class="pre">T&amp;</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" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/lwg-active.html#299">issue 299</a>
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>
@ -423,7 +141,7 @@ is resolved)</td>
single hierarchy, many useful iterators can not be appropriately
categorized. For example, <tt class="docutils literal"><span class="pre">vector&lt;bool&gt;::iterator</span></tt> is almost a
random access iterator, but the return type is not <tt class="docutils literal"><span class="pre">bool&amp;</span></tt> (see
<a class="reference" 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
<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&lt;bool&gt;</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.
@ -431,7 +149,7 @@ In paragraph 23.2.4/1 it says that a <tt class="docutils literal"><span class="p
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" href="http://www.boost.org/libs/iterator/doc/transform_iterator.html">transform_iterator</a>).
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
@ -439,13 +157,13 @@ iterators are required to return lvalues from <tt class="docutils literal"><span
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" href="http://www.boost.org/libs/graph/doc/table_of_contents.html">Boost Graph Library</a>. These iterators return vertex and edge
<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" href="http://www.boost.org/libs/utility/MultiPassInputIterator.html">Multi-Pass Input Iterator</a> was introduced to describe the vertex and
<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
@ -459,7 +177,7 @@ 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" name="impact-on-the-standard">Impact on the Standard</a></h1>
<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
@ -475,12 +193,12 @@ 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" name="possible-but-not-proposed-changes-to-the-working-paper">Possible (but not proposed) Changes to the Working Paper</a></h2>
<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" name="changes-to-algorithm-requirements">Changes to Algorithm Requirements</a></h3>
<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
@ -524,7 +242,7 @@ Forward Iterator (2) -&gt; Swappable Iterator and Incrementable Iterator</p>
<dt>Bidirectional Iterator -&gt; 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) -&gt; Bidirectional Traversal Iterator and Readable Iterator,
<p>Bidirectional Iterator (1) -&gt; Bidirectional Traversal Iterator and Readable Iterator,
Bidirectional Iterator (2) -&gt; Bidirectional Traversal Iterator and Writable Iterator</p>
<blockquote>
<tt class="docutils literal"><span class="pre">copy_backwards</span></tt></blockquote>
@ -545,14 +263,14 @@ Bidirectional Iterator (2) -&gt; Bidirectional Traversal Iterator and Writable I
</dl>
</div>
<div class="section" id="deprecations">
<h3><a class="toc-backref" href="#id5" name="deprecations">Deprecations</a></h3>
<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" name="vector-bool"><tt class="docutils literal"><span class="pre">vector&lt;bool&gt;</span></tt></a></h3>
<h3><a class="toc-backref" href="#id6"><tt class="docutils literal"><span class="pre">vector&lt;bool&gt;</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&lt;bool&gt;::iterator</span></tt> as a Random
Access Traversal Iterator and Readable Iterator and Writable
@ -561,7 +279,7 @@ Iterator.</p>
</div>
</div>
<div class="section" id="design">
<h1><a class="toc-backref" href="#id7" name="design">Design</a></h1>
<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">
@ -583,7 +301,7 @@ of concepts handles the syntax and semantics of value access:</p>
</ul>
<p>The refinement relationships for the traversal concepts are in the
following diagram.</p>
<div class="image"><img alt="traversal.png" src="traversal.png" /></div>
<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&lt;</span></tt>. The
@ -598,7 +316,7 @@ interoperable with <tt class="docutils literal"><span class="pre">int</span> <sp
</ul>
<p>The relationship between the new iterator concepts and the old are
given in the following diagram.</p>
<div class="image"><img alt="oldeqnew.png" src="oldeqnew.png" /></div>
<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
@ -626,24 +344,24 @@ approach for specifying <tt class="docutils literal"><span class="pre">operator[
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" 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
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" name="proposed-text">Proposed Text</a></h1>
<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" name="addition-to-lib-iterator-requirements">Addition to [lib.iterator.requirements]</a></h2>
<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" name="iterator-value-access-concepts-lib-iterator-value-access">Iterator Value Access Concepts [lib.iterator.value.access]</a></h3>
<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&lt;X&gt;::reference</span></tt>, <tt class="docutils literal"><span class="pre">T</span></tt> is
<tt class="docutils literal"><span class="pre">std::iterator_traits&lt;X&gt;::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>
<a class="target" id="readable-iterator" name="readable-iterator"></a><div class="section" id="readable-iterators-lib-readable-iterators">
<h4><a class="toc-backref" href="#id11" name="readable-iterators-lib-readable-iterators">Readable Iterators [lib.readable.iterators]</a></h4>
<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
@ -656,11 +374,11 @@ type <tt class="docutils literal"><span class="pre">T</span></tt>.</p>
<col width="52%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="3">Readable Iterator Requirements (in addition to Assignable and Copy Constructible)</th>
<tr><th class="head" colspan="3">Readable Iterator Requirements (in addition to Assignable and Copy Constructible)</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Note/Precondition</th>
<tr><th class="head">Expression</th>
<th class="head">Return Type</th>
<th class="head">Note/Precondition</th>
</tr>
</thead>
<tbody valign="top">
@ -684,9 +402,9 @@ non-cv-qualified type</td>
</tbody>
</table>
<!-- We won't say anything about iterator_traits<X>::reference until the DR is resolved. -JGS -->
<a class="target" id="writable-iterator" name="writable-iterator"></a></div>
</div>
<div class="section" id="writable-iterators-lib-writable-iterators">
<h4><a class="toc-backref" href="#id12" name="writable-iterators-lib-writable-iterators">Writable Iterators [lib.writable.iterators]</a></h4>
<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
@ -698,11 +416,11 @@ Iterators have an associated <em>set of value types</em>.</p>
<col width="42%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="3">Writable Iterator Requirements (in addition to Copy Constructible)</th>
<tr><th class="head" colspan="3">Writable Iterator Requirements (in addition to Copy Constructible)</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Precondition</th>
<tr><th class="head">Expression</th>
<th class="head">Return Type</th>
<th class="head">Precondition</th>
</tr>
</thead>
<tbody valign="top">
@ -716,7 +434,7 @@ value types of <tt class="docutils literal"><span class="pre">X</span></tt></td>
</table>
</div>
<div class="section" id="swappable-iterators-lib-swappable-iterators">
<h4><a class="toc-backref" href="#id13" name="swappable-iterators-lib-swappable-iterators">Swappable Iterators [lib.swappable.iterators]</a></h4>
<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>
@ -727,11 +445,11 @@ expressions are valid and respect the stated semantics.</p>
<col width="43%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="3">Swappable Iterator Requirements (in addition to Copy Constructible)</th>
<tr><th class="head" colspan="3">Swappable Iterator Requirements (in addition to Copy Constructible)</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Postcondition</th>
<tr><th class="head">Expression</th>
<th class="head">Return Type</th>
<th class="head">Postcondition</th>
</tr>
</thead>
<tbody valign="top">
@ -742,12 +460,12 @@ exchanged</td>
</tr>
</tbody>
</table>
<p>[<em>Note:</em> An iterator that is a model of the <a class="reference" href="#readable-iterator">Readable Iterator</a> and
<a class="reference" href="#writable-iterator">Writable Iterator</a> concepts is also a model of <em>Swappable
<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" name="lvalue-iterators-lib-lvalue-iterators">Lvalue Iterators [lib.lvalue.iterators]</a></h4>
<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>
@ -758,11 +476,11 @@ iterator.</p>
<col width="59%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="3">Lvalue Iterator Requirements</th>
<tr><th class="head" colspan="3">Lvalue Iterator Requirements</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Note/Assertion</th>
<tr><th class="head">Expression</th>
<th class="head">Return Type</th>
<th class="head">Note/Assertion</th>
</tr>
</thead>
<tbody valign="top">
@ -776,20 +494,20 @@ dereferenceable.</td>
</tr>
</tbody>
</table>
<p>If <tt class="docutils literal"><span class="pre">X</span></tt> is a <a class="reference" 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" href="#readable-iterator">Readable
<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" name="iterator-traversal-concepts-lib-iterator-traversal">Iterator Traversal Concepts [lib.iterator.traversal]</a></h3>
<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&lt;X&gt;::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" name="incrementable-iterators-lib-incrementable-iterators">Incrementable Iterators [lib.incrementable.iterators]</a></h4>
<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
@ -801,11 +519,11 @@ stated semantics.</p>
<col width="23%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="3">Incrementable Iterator Requirements (in addition to Assignable, Copy Constructible)</th>
<tr><th class="head" colspan="3">Incrementable Iterator Requirements (in addition to Assignable, Copy Constructible)</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion</th>
<tr><th class="head">Expression</th>
<th class="head">Return Type</th>
<th class="head">Assertion</th>
</tr>
</thead>
<tbody valign="top">
@ -828,16 +546,16 @@ stated semantics.</p>
</tr>
</tbody>
</table>
<p>If <tt class="docutils literal"><span class="pre">X</span></tt> is a <a class="reference" 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
<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" 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
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" name="single-pass-iterators-lib-single-pass-iterators">Single Pass Iterators [lib.single.pass.iterators]</a></h4>
<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>
@ -849,14 +567,14 @@ semantics.</p>
<col width="27%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="4">Single Pass Iterator Requirements (in addition to Incrementable Iterator and Equality
<tr><th class="head" colspan="4">Single Pass Iterator Requirements (in addition to Incrementable Iterator and Equality
Comparable)</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Operational
<tr><th class="head">Expression</th>
<th class="head">Return Type</th>
<th class="head">Operational
Semantics</th>
<th>Assertion/
<th class="head">Assertion/
Pre-/Post-condition</th>
</tr>
</thead>
@ -892,7 +610,7 @@ relation over its domain</td>
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" name="forward-traversal-iterators-lib-forward-traversal-iterators">Forward Traversal Iterators [lib.forward.traversal.iterators]</a></h4>
<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
@ -904,11 +622,11 @@ valid and respect the stated semantics.</p>
<col width="27%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="3">Forward Traversal Iterator Requirements (in addition to Default Constructible and Single Pass Iterator)</th>
<tr><th class="head" colspan="3">Forward Traversal Iterator Requirements (in addition to Default Constructible and Single Pass Iterator)</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion/Note</th>
<tr><th class="head">Expression</th>
<th class="head">Return Type</th>
<th class="head">Assertion/Note</th>
</tr>
</thead>
<tbody valign="top">
@ -939,7 +657,7 @@ the distance between iterators</td>
forward_traversal_tag for consistency -->
</div>
<div class="section" id="bidirectional-traversal-iterators-lib-bidirectional-traversal-iterators">
<h4><a class="toc-backref" href="#id19" name="bidirectional-traversal-iterators-lib-bidirectional-traversal-iterators">Bidirectional Traversal Iterators [lib.bidirectional.traversal.iterators]</a></h4>
<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
@ -952,14 +670,14 @@ respect the stated semantics.</p>
<col width="21%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="4">Bidirectional Traversal Iterator Requirements (in addition to Forward Traversal
<tr><th class="head" colspan="4">Bidirectional Traversal Iterator Requirements (in addition to Forward Traversal
Iterator)</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Operational
<tr><th class="head">Expression</th>
<th class="head">Return Type</th>
<th class="head">Operational
Semantics</th>
<th>Assertion/
<th class="head">Assertion/
Pre-/Post-condition</th>
</tr>
</thead>
@ -1002,7 +720,7 @@ implies <tt class="docutils literal"><span class="pre">r</span> <span class="pre
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" name="random-access-traversal-iterators-lib-random-access-traversal-iterators">Random Access Traversal Iterators [lib.random.access.traversal.iterators]</a></h4>
<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
@ -1016,12 +734,12 @@ constant object of type <tt class="docutils literal"><span class="pre">Distance<
<col width="20%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="4">Random Access Traversal Iterator Requirements (in addition to Bidirectional Traversal Iterator)</th>
<tr><th class="head" colspan="4">Random Access Traversal Iterator Requirements (in addition to Bidirectional Traversal Iterator)</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Operational Semantics</th>
<th>Assertion/
<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>
@ -1073,13 +791,13 @@ value <tt class="docutils literal"><span class="pre">n</span></tt> of
<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" href="#readable-iterator">Readable
<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" href="#writable-iterator">Writable
<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">&lt;</span> <span class="pre">b</span></tt></td>
@ -1116,7 +834,7 @@ ordering relation</td>
random_access_traversal_tag for consistency -->
</div>
<div class="section" id="interoperable-iterators-lib-interoperable-iterators">
<h4><a class="toc-backref" href="#id21" name="interoperable-iterators-lib-interoperable-iterators">Interoperable Iterators [lib.interoperable.iterators]</a></h4>
<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
@ -1131,9 +849,9 @@ constant object of type <tt class="docutils literal"><span class="pre">Distance<
<col width="60%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion/Precondition/Postcondition</th>
<tr><th class="head">Expression</th>
<th class="head">Return Type</th>
<th class="head">Assertion/Precondition/Postcondition</th>
</tr>
</thead>
<tbody valign="top">
@ -1173,10 +891,10 @@ the following additional requirements must be met.</p>
<col width="41%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Return Type</th>
<th>Operational Semantics</th>
<th>Assertion/ Precondition</th>
<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">
@ -1240,7 +958,7 @@ the following additional requirements must be met.</p>
</div>
</div>
<div class="section" id="addition-to-lib-iterator-synopsis">
<h2><a class="toc-backref" href="#id22" name="addition-to-lib-iterator-synopsis">Addition to [lib.iterator.synopsis]</a></h2>
<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 &lt;class Iterator&gt; struct is_readable_iterator;
@ -1254,19 +972,19 @@ 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" name="addition-to-lib-iterator-traits">Addition to [lib.iterator.traits]</a></h2>
<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" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1519.htm">UnaryTypeTrait</a> requirements.</p>
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&lt;X&gt;::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&lt;X&gt;::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&lt;X&gt;::type</span></tt> is</p>
<pre class="literal-block">
<em>category-to-traversal</em>(iterator_traits&lt;X&gt;::iterator_category)
<em>category-to-traversal</em>(iterator_traits&lt;X&gt;::iterator_category)
</pre>
<p>where <em>category-to-traversal</em> is defined as follows</p>
<a class="target" id="category-to-traversal" name="category-to-traversal"></a><pre class="literal-block">
<pre class="literal-block" id="category-to-traversal">
<em>category-to-traversal</em>(C) =
if (C is convertible to incrementable_traversal_tag)
return C;
@ -1286,8 +1004,8 @@ otherwise.</p>
</div>
</div>
<div class="section" id="footnotes">
<h1><a class="toc-backref" href="#id24" name="footnotes">Footnotes</a></h1>
<p>The UnaryTypeTrait concept is defined in <a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1519.htm">n1519</a>; the LWG is
<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
@ -1298,6 +1016,12 @@ LocalWords: RandomAccessTraversalIterator dereferenceable Incrementable tmp
LocalWords: incrementable xxx min prev inplace png oldeqnew AccessTag struct
LocalWords: TraversalTag typename lvalues DWA Hmm JGS mis enum -->
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