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some edits based on comments from Howard Hinnant

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[SVN r20125]
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Jeremy Siek
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@ -3,11 +3,11 @@
<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 0.2.8: http://docutils.sourceforge.net/" />
<meta name="generator" content="Docutils 0.3.0: 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, University of Hanover Institute for Transport Railway Operation and Construction" />
<meta name="date" content="2003-09-14" />
<meta name="date" content="2003-09-20" />
<meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All rights reserved" />
<link rel="stylesheet" href="default.css" type="text/css" />
</head>
@ -25,7 +25,7 @@
<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>, University of Hanover <a class="last reference" href="http://www.ive.uni-hannover.de">Institute for Transport Railway Operation and Construction</a></td></tr>
<tr><th class="docinfo-name">Date:</th>
<td>2003-09-14</td></tr>
<td>2003-09-20</td></tr>
<tr class="field"><th class="docinfo-name">Number:</th><td class="field-body"><strong>This document is a revised version of the official</strong> N1477=03-0060</td>
</tr>
<tr><th class="docinfo-name">Copyright:</th>
@ -92,7 +92,7 @@ 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 class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="31%" />
<col width="69%" />
@ -121,9 +121,10 @@ categorized. For example, <tt class="literal"><span class="pre">vector&lt;bool&g
random access iterator, but the return type is not <tt class="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
N1185). Therefore, the iterators of <tt class="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 at least one implementation erroneously assigns
<tt class="literal"><span class="pre">random_access_iterator_tag</span></tt> as its <tt class="literal"><span class="pre">iterator_category</span></tt>.</p>
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="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" href="http://www.boost.org/libs/utility/transform_iterator.htm">transform_iterator</a>).
@ -218,11 +219,11 @@ Bidirectional Iterator (2) -&gt; Bidirectional Traversal Iterator and Writable I
<dd><tt class="literal"><span class="pre">stable_partition,</span> <span class="pre">inplace_merge</span></tt></dd>
<dt>Bidirectional Iterator -&gt; Bidirectional Traversal Iterator and Readable Iterator</dt>
<dd><tt class="literal"><span class="pre">reverse_copy</span></tt></dd>
<dt>Random Access Iterator -&gt; Random Access Traversal Iterator and Readable and Swappable Iterator</dt>
<dt>Random Access Iterator -&gt; Random Access Traversal Iterator and Readable and Writable Iterator</dt>
<dd><tt class="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) -&gt; Incrementable Iterator and Readable Iterator</dt>
<dd><tt class="literal"><span class="pre">equal</span></tt></dd>
<dd><tt class="literal"><span class="pre">equal,</span> <span class="pre">mismatch</span></tt></dd>
<dt>Input Iterator (2) -&gt; Incrementable Iterator and Readable Iterator</dt>
<dd><tt class="literal"><span class="pre">transform</span></tt></dd>
</dl>
@ -266,11 +267,17 @@ given in the following diagram.</p>
<p><img alt="oldeqnew.png" src="oldeqnew.png" /></p>
<p>Like the old iterator requirements, we provide tags for purposes of
dispatching. There are two hierarchies of tags, one for the access
concepts and one for the traversal concepts. We provide an access
mechanism for mapping iterator types to these new tags. Our design
reuses <tt class="literal"><span class="pre">iterator_traits&lt;Iter&gt;::iterator_category</span></tt> as the access
mechanism. To enable this, a pair of access and traversal tags are
combined into a single type using the following <cite>iterator_tag</cite> class.</p>
concepts and one for 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. We
use virtual inheritance of the diamonds in the current hierarchy, and
because of possible diamonds that could be created when programmers
define new iterator concepts and the corresponding tags.</p>
<p>We provide an access mechanism for mapping iterator types to the new
tags. Our design reuses <tt class="literal"><span class="pre">iterator_traits&lt;Iter&gt;::iterator_category</span></tt>
as the access mechanism. To enable this, a pair of access and
traversal tags are combined into a single type using the following
<cite>iterator_tag</cite> class.</p>
<pre class="literal-block">
template &lt;class AccessTag, class TraversalTag&gt;
struct iterator_tag : /* appropriate old category or categories */
@ -326,7 +333,7 @@ for the value type <tt class="literal"><span class="pre">T</span></tt> if the fo
respect the stated semantics. <tt class="literal"><span class="pre">U</span></tt> is the type of any specified
member of type <tt class="literal"><span class="pre">T</span></tt>.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="34%" />
<col width="30%" />
@ -377,7 +384,7 @@ semantics. In addition, a model of <em>Writable Iterator</em> must include
in its documentation the <em>set of value types</em> that it allows for
output.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="42%" />
<col width="27%" />
@ -413,7 +420,7 @@ value types of <tt class="literal"><span class="pre">X</span></tt></td>
if the following expressions are valid and respect the stated
semantics.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="38%" />
<col width="14%" />
@ -445,7 +452,7 @@ semantics.</p>
<p>The <em>Readable Lvalue Iterator</em> concept adds the requirement that the
<tt class="literal"><span class="pre">reference</span></tt> type be a reference to the value type of the iterator.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="35%" />
<col width="32%" />
@ -482,7 +489,7 @@ cv-qualification</td>
<tt class="literal"><span class="pre">reference</span></tt> type be a non-const reference to the value type of the
iterator.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="45%" />
<col width="55%" />
@ -518,7 +525,7 @@ type <tt class="literal"><span class="pre">X</span></tt>, <tt class="literal"><s
concept if the following expressions are valid and respect the stated
semantics.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="27%" />
<col width="38%" />
@ -555,7 +562,7 @@ semantics.</p>
concept if the following expressions are valid and respect the stated
semantics.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="33%" />
<col width="27%" />
@ -600,7 +607,7 @@ its domain</td>
concept if the following expressions are valid and respect the stated
semantics.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="39%" />
<col width="37%" />
@ -646,7 +653,7 @@ the distance between iterators</td>
Iterator</em> concept if the following expressions are valid and respect
the stated semantics.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="28%" />
<col width="36%" />
@ -690,7 +697,7 @@ the stated semantics. In the table below, <tt class="literal"><span class="pre"
<tt class="literal"><span class="pre">iterator_traits&lt;X&gt;::difference_type</span></tt> and <tt class="literal"><span class="pre">n</span></tt> represents a
constant object of type <tt class="literal"><span class="pre">Distance</span></tt>.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<table border class="table">
<colgroup>
<col width="26%" />
<col width="33%" />
@ -874,8 +881,7 @@ inherit-category(access-tag, traversal-tag) =
convertible to one or more of: <tt class="literal"><span class="pre">readable_iterator_tag</span></tt>,
<tt class="literal"><span class="pre">writable_iterator_tag</span></tt>, <tt class="literal"><span class="pre">swappable_iterator_tag</span></tt>, or if the
argument for <tt class="literal"><span class="pre">TraversalTag</span></tt> is not convertible to
<tt class="literal"><span class="pre">incrementable_iterator_tag</span></tt> then the behavior of <tt class="literal"><span class="pre">iterator_tag</span></tt>
is not defined.</p>
<tt class="literal"><span class="pre">incrementable_iterator_tag</span></tt> then the programm is ill-formed.</p>
<p>The <tt class="literal"><span class="pre">access_category</span></tt> and <tt class="literal"><span class="pre">traversal_category</span></tt> class templates are
traits classes. For iterators whose
<tt class="literal"><span class="pre">iterator_traits&lt;Iter&gt;::iterator_category</span></tt> type is <tt class="literal"><span class="pre">iterator_tag</span></tt>,