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<title>Iterator Facade</title>
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<div class="document" id="iterator-facade">
<h1 class="title">Iterator Facade</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" 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;ive.uni-hannover.de">witt&#64;ive.uni-hannover.de</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>, 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>2004-01-11</td></tr>
<tr><th class="docinfo-name">Copyright:</th>
<td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All rights reserved</td></tr>
</tbody>
</table>
<table class="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"><tt class="literal"><span class="pre">iterator_facade</span></tt> is a base class template that implements the
interface of standard iterators in terms of a few core functions
and associated types, to be supplied by a derived iterator class.</td>
</tr>
</tbody>
</table>
<div class="contents topic" id="table-of-contents">
<p class="topic-title"><a name="table-of-contents">Table of Contents</a></p>
<ul class="simple">
<li><a class="reference" href="#overview" id="id20" name="id20">Overview</a><ul>
<li><a class="reference" href="#motivation" id="id21" name="id21">Motivation</a></li>
<li><a class="reference" href="#usage" id="id22" name="id22">Usage</a></li>
<li><a class="reference" href="#iterator-core-access" id="id23" name="id23">Iterator Core Access</a></li>
<li><a class="reference" href="#operator" id="id24" name="id24"><tt class="literal"><span class="pre">operator[]</span></tt></a></li>
<li><a class="reference" href="#id2" id="id25" name="id25"><tt class="literal"><span class="pre">operator-&gt;</span></tt></a></li>
</ul>
</li>
<li><a class="reference" href="#tutorial-example" id="id26" name="id26">Tutorial Example</a><ul>
<li><a class="reference" href="#introduction" id="id27" name="id27">Introduction</a></li>
<li><a class="reference" href="#the-problem" id="id28" name="id28">The Problem</a></li>
<li><a class="reference" href="#a-basic-iterator-using-iterator-facade" id="id29" name="id29">A Basic Iterator Using <tt class="literal"><span class="pre">iterator_facade</span></tt></a><ul>
<li><a class="reference" href="#template-parameters" id="id30" name="id30">Template Parameters</a><ul>
<li><a class="reference" href="#derived" id="id31" name="id31"><tt class="literal"><span class="pre">Derived</span></tt></a></li>
<li><a class="reference" href="#value" id="id32" name="id32"><tt class="literal"><span class="pre">Value</span></tt></a></li>
<li><a class="reference" href="#categoryortraversal" id="id33" name="id33"><tt class="literal"><span class="pre">CategoryOrTraversal</span></tt></a></li>
<li><a class="reference" href="#reference" id="id34" name="id34"><tt class="literal"><span class="pre">Reference</span></tt></a></li>
<li><a class="reference" href="#difference" id="id35" name="id35"><tt class="literal"><span class="pre">Difference</span></tt></a></li>
</ul>
</li>
<li><a class="reference" href="#constructors-and-data-members" id="id36" name="id36">Constructors and Data Members</a></li>
<li><a class="reference" href="#core-operations" id="id37" name="id37">Core Operations</a></li>
</ul>
</li>
<li><a class="reference" href="#a-constant-node-iterator" id="id38" name="id38">A constant <tt class="literal"><span class="pre">node_iterator</span></tt></a></li>
<li><a class="reference" href="#interoperability" id="id39" name="id39">Interoperability</a></li>
<li><a class="reference" href="#telling-the-truth" id="id40" name="id40">Telling the Truth</a></li>
<li><a class="reference" href="#wrap-up" id="id41" name="id41">Wrap Up</a></li>
</ul>
</li>
<li><a class="reference" href="#id14" id="id42" name="id42">Reference</a><ul>
<li><a class="reference" href="#ref" id="id43" name="id43">Ref</a><ul>
<li><a class="reference" href="#iterator-facade-usage" id="id44" name="id44"><tt class="literal"><span class="pre">iterator_facade</span></tt> usage</a><ul>
<li><a class="reference" href="#iterator-facade-core-operations" id="id45" name="id45"><tt class="literal"><span class="pre">iterator_facade</span></tt> Core Operations</a></li>
</ul>
</li>
<li><a class="reference" href="#iterator-facade-iterator-category" id="id46" name="id46"><tt class="literal"><span class="pre">iterator_facade</span></tt> iterator category</a></li>
<li><a class="reference" href="#iterator-facade-operations" id="id47" name="id47"><tt class="literal"><span class="pre">iterator_facade</span></tt> operations</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
<div class="section" id="overview">
<h1><a class="toc-backref" href="#id20" name="overview">Overview</a></h1>
<div class="section" id="motivation">
<h2><a class="toc-backref" href="#id21" name="motivation">Motivation</a></h2>
<!-- Version 1.1 of this ReStructuredText document corresponds to
n1530_, the paper accepted by the LWG for TR1. -->
<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All
rights reserved -->
<p>While the iterator interface is rich, there is a core subset of the
interface that is necessary for all the functionality. We have
identified the following core behaviors for iterators:</p>
<ul class="simple">
<li>dereferencing</li>
<li>incrementing</li>
<li>decrementing</li>
<li>equality comparison</li>
<li>random-access motion</li>
<li>distance measurement</li>
</ul>
<p>In addition to the behaviors listed above, the core interface elements
include the associated types exposed through iterator traits:
<tt class="literal"><span class="pre">value_type</span></tt>, <tt class="literal"><span class="pre">reference</span></tt>, <tt class="literal"><span class="pre">difference_type</span></tt>, and
<tt class="literal"><span class="pre">iterator_category</span></tt>.</p>
<p>Iterator facade uses the Curiously Recurring Template Pattern (CRTP)
<a class="citation-reference" href="#cop95" id="id1" name="id1">[Cop95]</a> so that the user can specify the behavior of
<tt class="literal"><span class="pre">iterator_facade</span></tt> in a derived class. Former designs used policy
objects to specify the behavior. <tt class="literal"><span class="pre">iterator_facade</span></tt> does not use policy
objects for several reasons:</p>
<blockquote>
<ol class="arabic simple">
<li>the creation and eventual copying of the policy object may create
overhead that can be avoided with the current approach.</li>
<li>The policy object approach does not allow for custom constructors
on the created iterator types, an essential feature if
<tt class="literal"><span class="pre">iterator_facade</span></tt> should be used in other library
implementations.</li>
<li>Without the use of CRTP, the standard requirement that an
iterator's <tt class="literal"><span class="pre">operator++</span></tt> returns the iterator type itself means
that all iterators generated by <tt class="literal"><span class="pre">iterator_facade</span></tt> would be
specializations of <tt class="literal"><span class="pre">iterator_facade</span></tt>. Cumbersome type generator
metafunctions would be needed to build new parameterized
iterators, and a separate <tt class="literal"><span class="pre">iterator_adaptor</span></tt> layer would be
impossible.</li>
</ol>
</blockquote>
</div>
<div class="section" id="usage">
<h2><a class="toc-backref" href="#id22" name="usage">Usage</a></h2>
<p>The user of <tt class="literal"><span class="pre">iterator_facade</span></tt> derives his iterator class from a
specialization of <tt class="literal"><span class="pre">iterator_facade</span></tt> and passes the derived
iterator class as <tt class="literal"><span class="pre">iterator_facade</span></tt>'s first template parameter.
The order of the other template parameters have been carefully
chosen to take advantage of useful defaults. For example, when
defining a constant lvalue iterator, the user can pass a
const-qualified version of the iterator's <tt class="literal"><span class="pre">value_type</span></tt> as
<tt class="literal"><span class="pre">iterator_facade</span></tt>'s <tt class="literal"><span class="pre">Value</span></tt> parameter and omit the
<tt class="literal"><span class="pre">Reference</span></tt> parameter which follows.</p>
<p>The derived iterator class must define member functions implementing
the iterator's core behaviors. The following table describes
expressions which are required to be valid depending on the category
of the derived iterator type. These member functions are described
briefly below and in more detail in the iterator facade
requirements.</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="44%" />
<col width="56%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Effects</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">i.dereference()</span></tt></td>
<td>Access the value referred to</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.equal(j)</span></tt></td>
<td>Compare for equality with <tt class="literal"><span class="pre">j</span></tt></td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.increment()</span></tt></td>
<td>Advance by one position</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.decrement()</span></tt></td>
<td>Retreat by one position</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.advance(n)</span></tt></td>
<td>Advance by <tt class="literal"><span class="pre">n</span></tt> positions</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.distance_to(j)</span></tt></td>
<td>Measure the distance to <tt class="literal"><span class="pre">j</span></tt></td>
</tr>
</tbody>
</table>
</blockquote>
<!-- Should we add a comment that a zero overhead implementation of iterator_facade
is possible with proper inlining? -->
<p>In addition to implementing the core interface functions, an iterator
derived from <tt class="literal"><span class="pre">iterator_facade</span></tt> typically defines several
constructors. To model any of the standard iterator concepts, the
iterator must at least have a copy constructor. Also, if the iterator
type <tt class="literal"><span class="pre">X</span></tt> is meant to be automatically interoperate with another
iterator type <tt class="literal"><span class="pre">Y</span></tt> (as with constant and mutable iterators) then
there must be an implicit conversion from <tt class="literal"><span class="pre">X</span></tt> to <tt class="literal"><span class="pre">Y</span></tt> or from <tt class="literal"><span class="pre">Y</span></tt>
to <tt class="literal"><span class="pre">X</span></tt> (but not both), typically implemented as a conversion
constructor. Finally, if the iterator is to model Forward Traversal
Iterator or a more-refined iterator concept, a default constructor is
required.</p>
</div>
<div class="section" id="iterator-core-access">
<h2><a class="toc-backref" href="#id23" name="iterator-core-access">Iterator Core Access</a></h2>
<p><tt class="literal"><span class="pre">iterator_facade</span></tt> and the operator implementations need to be able
to access the core member functions in the derived class. Making the
core member functions public would expose an implementation detail to
the user. The design used here ensures that implementation details do
not appear in the public interface of the derived iterator type.</p>
<p>Preventing direct access to the core member functions has two
advantages. First, there is no possibility for the user to accidently
use a member function of the iterator when a member of the value_type
was intended. This has been an issue with smart pointer
implementations in the past. The second and main advantage is that
library implementers can freely exchange a hand-rolled iterator
implementation for one based on <tt class="literal"><span class="pre">iterator_facade</span></tt> without fear of
breaking code that was accessing the public core member functions
directly.</p>
<p>In a naive implementation, keeping the derived class' core member
functions private would require it to grant friendship to
<tt class="literal"><span class="pre">iterator_facade</span></tt> and each of the seven operators. In order to
reduce the burden of limiting access, <tt class="literal"><span class="pre">iterator_core_access</span></tt> is
provided, a class that acts as a gateway to the core member functions
in the derived iterator class. The author of the derived class only
needs to grant friendship to <tt class="literal"><span class="pre">iterator_core_access</span></tt> to make his core
member functions available to the library.</p>
<!-- This is no long uptodate -thw -->
<!-- Yes it is; I made sure of it! -DWA -->
<p><tt class="literal"><span class="pre">iterator_core_access</span></tt> will be typically implemented as an empty
class containing only private static member functions which invoke the
iterator core member functions. There is, however, no need to
standardize the gateway protocol. Note that even if
<tt class="literal"><span class="pre">iterator_core_access</span></tt> used public member functions it would not
open a safety loophole, as every core member function preserves the
invariants of the iterator.</p>
</div>
<div class="section" id="operator">
<h2><a class="toc-backref" href="#id24" name="operator"><tt class="literal"><span class="pre">operator[]</span></tt></a></h2>
<p>The indexing operator for a generalized iterator presents special
challenges. A random access iterator's <tt class="literal"><span class="pre">operator[]</span></tt> is only
required to return something convertible to its <tt class="literal"><span class="pre">value_type</span></tt>.
Requiring that it return an lvalue would rule out currently-legal
random-access iterators which hold the referenced value in a data
member (e.g. <a class="reference" href="counting_iterator.html"><tt class="literal"><span class="pre">counting_iterator</span></tt></a>), because <tt class="literal"><span class="pre">*(p+n)</span></tt> is a reference
into the temporary iterator <tt class="literal"><span class="pre">p+n</span></tt>, which is destroyed when
<tt class="literal"><span class="pre">operator[]</span></tt> returns.</p>
<p>Writable iterators built with <tt class="literal"><span class="pre">iterator_facade</span></tt> implement the
semantics required by the preferred resolution to <a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#299">issue 299</a> and
adopted by proposal <a class="reference" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html">n1550</a>: the result of <tt class="literal"><span class="pre">p[n]</span></tt> is an object
convertible to the iterator's <tt class="literal"><span class="pre">value_type</span></tt>, and <tt class="literal"><span class="pre">p[n]</span> <span class="pre">=</span> <span class="pre">x</span></tt> is
equivalent to <tt class="literal"><span class="pre">*(p</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">x</span></tt> (Note: This result object may be
implemented as a proxy containing a copy of <tt class="literal"><span class="pre">p+n</span></tt>). This approach
will work properly for any random-access iterator regardless of the
other details of its implementation. A user who knows more about
the implementation of her iterator is free to implement an
<tt class="literal"><span class="pre">operator[]</span></tt> that returns an lvalue in the derived iterator
class; it will hide the one supplied by <tt class="literal"><span class="pre">iterator_facade</span></tt> from
clients of her iterator.</p>
<a class="target" id="operator-arrow" name="operator-arrow"></a></div>
<div class="section" id="id2">
<h2><a class="toc-backref" href="#id25" name="id2"><tt class="literal"><span class="pre">operator-&gt;</span></tt></a></h2>
<p>The <tt class="literal"><span class="pre">reference</span></tt> type of a readable iterator (and today's input
iterator) need not in fact be a reference, so long as it is
convertible to the iterator's <tt class="literal"><span class="pre">value_type</span></tt>. When the <tt class="literal"><span class="pre">value_type</span></tt>
is a class, however, it must still be possible to access members
through <tt class="literal"><span class="pre">operator-&gt;</span></tt>. Therefore, an iterator whose <tt class="literal"><span class="pre">reference</span></tt>
type is not in fact a reference must return a proxy containing a copy
of the referenced value from its <tt class="literal"><span class="pre">operator-&gt;</span></tt>.</p>
<p>The return type for <tt class="literal"><span class="pre">operator-&gt;</span></tt> and <tt class="literal"><span class="pre">operator[]</span></tt> is not
explicitly specified. Instead it requires each <tt class="literal"><span class="pre">iterator_facade</span></tt>
specialization to meet the requirements of its <tt class="literal"><span class="pre">iterator_category</span></tt>.</p>
<table class="citation" frame="void" id="cop95" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<col />
<tbody valign="top">
<tr><td class="label"><a name="cop95">[Cop95]</a></td><td><em>(<a class="fn-backref" href="#id1">1</a>, <a class="fn-backref" href="#id3">2</a>)</em> [Coplien, 1995] Coplien, J., Curiously Recurring Template
Patterns, C++ Report, February 1995, pp. 24-27.</td></tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="tutorial-example">
<h1><a class="toc-backref" href="#id26" name="tutorial-example">Tutorial Example</a></h1>
<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id27" name="introduction">Introduction</a></h2>
<p>In this section we'll walk through the implementation of a few
iterators using <tt class="literal"><span class="pre">iterator_facade</span></tt>, based around the simple
example of a linked list of polymorphic objects.</p>
</div>
<div class="section" id="the-problem">
<h2><a class="toc-backref" href="#id28" name="the-problem">The Problem</a></h2>
<p>Say we've written a polymorphic linked list node base class:</p>
<pre class="literal-block">
# include &lt;iostream&gt;
struct node_base
{
node_base() : m_next(0) {}
// Each node manages all of its tail nodes
virtual ~node_base() { delete m_next; }
// Access the rest of the list
node_base* next() const { return m_next; }
// print to the stream
virtual void print(std::ostream&amp; s) const = 0;
// double the value
virtual void double_me() = 0;
void append(node_base* p)
{
if (m_next)
m_next-&gt;append(p);
else
m_next = p;
}
private:
node_base* m_next;
};
</pre>
<p>Lists can hold objects of different types by linking together
specializations of the following template:</p>
<pre class="literal-block">
template &lt;class T&gt;
struct node : node_base
{
node(T x)
: m_value(x)
{}
void print(std::ostream&amp; s) const { s &lt;&lt; this-&gt;m_value; }
void double_me() { m_value += m_value; }
private:
T m_value;
};
</pre>
<p>And we can print any node using the following streaming operator:</p>
<pre class="literal-block">
inline std::ostream&amp; operator&lt;&lt;(std::ostream&amp; s, node_base const&amp; n)
{
n.print(s);
return s;
}
</pre>
<p>Our first challenge is to build an appropriate iterator over these
lists.</p>
</div>
<div class="section" id="a-basic-iterator-using-iterator-facade">
<h2><a class="toc-backref" href="#id29" name="a-basic-iterator-using-iterator-facade">A Basic Iterator Using <tt class="literal"><span class="pre">iterator_facade</span></tt></a></h2>
<div class="section" id="template-parameters">
<h3><a class="toc-backref" href="#id30" name="template-parameters">Template Parameters</a></h3>
<p>The first step in building a concrete iterator with iterator_facade
is to decide what its template parameters will be.</p>
<div class="section" id="derived">
<h4><a class="toc-backref" href="#id31" name="derived"><tt class="literal"><span class="pre">Derived</span></tt></a></h4>
<p>Because <tt class="literal"><span class="pre">iterator_facade</span></tt> is meant to be used with the CRTP
<a class="citation-reference" href="#cop95" id="id3" name="id3">[Cop95]</a> the first parameter is the iterator class name itself,
<tt class="literal"><span class="pre">node_iterator</span></tt>.</p>
</div>
<div class="section" id="value">
<h4><a class="toc-backref" href="#id32" name="value"><tt class="literal"><span class="pre">Value</span></tt></a></h4>
<p>The <tt class="literal"><span class="pre">Value</span></tt> parameter determines the <tt class="literal"><span class="pre">node_iterator</span></tt>'s
<tt class="literal"><span class="pre">value_type</span></tt>. In this case, we are iterating over <tt class="literal"><span class="pre">node_base</span></tt>
objects, so <tt class="literal"><span class="pre">Value</span></tt> will be <tt class="literal"><span class="pre">node_base</span></tt>.</p>
</div>
<div class="section" id="categoryortraversal">
<h4><a class="toc-backref" href="#id33" name="categoryortraversal"><tt class="literal"><span class="pre">CategoryOrTraversal</span></tt></a></h4>
<p>Now we have to determine which <a class="reference" href="new-iter-concepts.html#iterator-traversal-concepts-lib-iterator-traversal">iterator traversal concept</a> our
<tt class="literal"><span class="pre">node_iterator</span></tt> is going to model. Singly-linked lists only have
forward links, so our iterator can't can't be a <a class="reference" href="new-iter-concepts.html#bidirectional-traversal-iterators-lib-bidirectional-traversal-iterators">bidirectional
traversal iterator</a>. Our iterator should be able to make multiple
passes over the same linked list (unlike, say, an
<tt class="literal"><span class="pre">istream_iterator</span></tt> which consumes the stream it traverses), so it
must be a <a class="reference" href="new-iter-concepts.html#forward-traversal-iterators-lib-forward-traversal-iterators">forward traversal iterator</a>. Therefore, we'll pass
<tt class="literal"><span class="pre">boost::forward_traversal_tag</span></tt> in this position <a class="footnote-reference" href="#category" id="id4" name="id4"><sup>1</sup></a>.</p>
<table class="footnote" frame="void" id="category" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id4" name="category">[1]</a></td><td><tt class="literal"><span class="pre">iterator_facade</span></tt> also supports old-style category
tags, so we could've passed <tt class="literal"><span class="pre">std::forward_iterator_tag</span></tt> here;
either way, the resulting iterator's <tt class="literal"><span class="pre">iterator_category</span></tt> will
end up being <tt class="literal"><span class="pre">std::forward_iterator_tag</span></tt>.</td></tr>
</tbody>
</table>
</div>
<div class="section" id="reference">
<h4><a class="toc-backref" href="#id34" name="reference"><tt class="literal"><span class="pre">Reference</span></tt></a></h4>
<p>The <tt class="literal"><span class="pre">Reference</span></tt> argument becomes the type returned by
<tt class="literal"><span class="pre">node_iterator</span></tt>'s dereference operation, and will also be the
same as <tt class="literal"><span class="pre">std::iterator_traits&lt;node_iterator&gt;::reference</span></tt>. The
library's default for this parameter is <tt class="literal"><span class="pre">Value&amp;</span></tt>; since
<tt class="literal"><span class="pre">node_base&amp;</span></tt> is a good choice for the iterator's <tt class="literal"><span class="pre">reference</span></tt>
type, we can omit this argument, or pass <tt class="literal"><span class="pre">use_default</span></tt>.</p>
</div>
<div class="section" id="difference">
<h4><a class="toc-backref" href="#id35" name="difference"><tt class="literal"><span class="pre">Difference</span></tt></a></h4>
<p>The <tt class="literal"><span class="pre">Difference</span></tt> argument determines how the distance between
two <tt class="literal"><span class="pre">node_iterator</span></tt>s will be measured and will also be the
same as <tt class="literal"><span class="pre">std::iterator_traits&lt;node_iterator&gt;::difference_type</span></tt>.
The library's default for <tt class="literal"><span class="pre">Difference</span></tt> is <tt class="literal"><span class="pre">std::ptrdiff_t</span></tt>, an
appropriate type for measuring the distance between any two
addresses in memory, and one that works for almost any iterator,
so we can omit this argument, too.</p>
<p>The declaration of <tt class="literal"><span class="pre">node_iterator</span></tt> will therefore look something
like:</p>
<pre class="literal-block">
# include &quot;node.hpp&quot;
# include &lt;boost/iterator/iterator_facade.hpp&gt;
class node_iterator
: public boost::iterator_facade&lt;
node_iterator
, node_base
, boost::forward_traversal_tag
&gt;
{
...
};
</pre>
</div>
</div>
<div class="section" id="constructors-and-data-members">
<h3><a class="toc-backref" href="#id36" name="constructors-and-data-members">Constructors and Data Members</a></h3>
<p>Next we need to decide how to represent the iterator's position.
This representation will take the form of data members, so we'll
also need to write constructors to initialize them. The
<tt class="literal"><span class="pre">node_iterator</span></tt>'s position is quite naturally represented using
a pointer to a <tt class="literal"><span class="pre">node_base</span></tt>. We'll need a constructor to build an
iterator from a <tt class="literal"><span class="pre">node_base*</span></tt>, and a default constructor to
satisfy the <a class="reference" href="new-iter-concepts.html#forward-traversal-iterators-lib-forward-traversal-iterators">forward traversal iterator</a> requirements <a class="footnote-reference" href="#default" id="id5" name="id5"><sup>2</sup></a>.
Our <tt class="literal"><span class="pre">node_iterator</span></tt> then becomes:</p>
<pre class="literal-block">
# include &quot;node.hpp&quot;
# include &lt;boost/iterator/iterator_facade.hpp&gt;
class node_iterator
: public boost::iterator_facade&lt;
node_iterator
, node_base
, boost::forward_traversal_tag
&gt;
{
public:
node_iterator()
: m_node(0)
{}
explicit node_iterator(node_base* p)
: m_node(p)
{}
private:
...
node_base* m_node;
};
</pre>
<table class="footnote" frame="void" id="default" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id5" name="default">[2]</a></td><td>Technically, the C++ standard places almost no
requirements on a default-constructed iterator, so if we were
really concerned with efficiency, we could've written the
default constructor to leave <tt class="literal"><span class="pre">m_node</span></tt> uninitialized.</td></tr>
</tbody>
</table>
</div>
<div class="section" id="core-operations">
<h3><a class="toc-backref" href="#id37" name="core-operations">Core Operations</a></h3>
<p>The last step is to implement the <a class="reference" href="#id18">core operations</a> required by
the concepts we want our iterator to model. Referring to the
<a class="reference" href="#id18">table</a>, we can see that the first three rows are applicable
because <tt class="literal"><span class="pre">node_iterator</span></tt> needs to satisfy the requirements for
<a class="reference" href="new-iter-concepts.html#readable-iterators-lib-readable-iterators">readable iterator</a>, <a class="reference" href="new-iter-concepts.html#single-pass-iterators-lib-single-pass-iterators">single pass iterator</a>, and <a class="reference" href="new-iter-concepts.html#incrementable-iterators-lib-incrementable-iterators">incrementable
iterator</a>.</p>
<p>We therefore need to supply <tt class="literal"><span class="pre">dereference</span></tt>,
<tt class="literal"><span class="pre">equal</span></tt>, and <tt class="literal"><span class="pre">increment</span></tt> members. We don't want these members
to become part of <tt class="literal"><span class="pre">node_iterator</span></tt>'s public interface, so we can
make them private and grant friendship to
<tt class="literal"><span class="pre">boost::iterator_core_access</span></tt>, a &quot;back-door&quot; that
<tt class="literal"><span class="pre">iterator_facade</span></tt> uses to get access to the core operations:</p>
<pre class="literal-block">
# include &quot;node.hpp&quot;
# include &lt;boost/iterator/iterator_facade.hpp&gt;
class node_iterator
: public boost::iterator_facade&lt;
node_iterator
, node_base
, boost::forward_traversal_tag
&gt;
{
public:
node_iterator()
: m_node(0) {}
explicit node_iterator(node_base* p)
: m_node(p) {}
private:
friend class boost::iterator_core_access;
void increment() { m_node = m_node-&gt;next(); }
bool equal(node_iterator const&amp; other) const
{
return this-&gt;m_node == other.m_node;
}
node_base&amp; dereference() const { return *m_node; }
node_base* m_node;
};
</pre>
<p>Voilà; a complete and conforming readable, forward-traversal
iterator! For a working example of its use, see <a class="reference" href="../example/node_iterator1.cpp">this program</a>.</p>
</div>
</div>
<div class="section" id="a-constant-node-iterator">
<h2><a class="toc-backref" href="#id38" name="a-constant-node-iterator">A constant <tt class="literal"><span class="pre">node_iterator</span></tt></a></h2>
<div class="sidebar">
<p class="sidebar-title">Constant and Mutable iterators</p>
<p>The term <strong>mutable iterator</strong> means an iterator through which
the object it references (its &quot;referent&quot;) can be modified. A
<strong>constant iterator</strong> is one which doesn't allow modification of
its referent.</p>
<p>The words <em>constant</em> and <em>mutable</em> don't refer to the ability to
modify the iterator itself. For example, an <tt class="literal"><span class="pre">int</span> <span class="pre">const*</span></tt> is a
non-<tt class="literal"><span class="pre">const</span></tt> <em>constant iterator</em>, which can be incremented
but doesn't allow modification of its referent, and <tt class="literal"><span class="pre">int*</span>
<span class="pre">const</span></tt> is a <tt class="literal"><span class="pre">const</span></tt> <em>mutable iterator</em>, which cannot be
modified but which allows modification of its referent.</p>
<p>Confusing? We agree, but those are the standard terms. It
probably doesn't help much that a container's constant iterator
is called <tt class="literal"><span class="pre">const_iterator</span></tt>.</p>
</div>
<p>Now, our <tt class="literal"><span class="pre">node_iterator</span></tt> gives clients access to both <tt class="literal"><span class="pre">node</span></tt>'s <tt class="literal"><span class="pre">print(std::ostream&amp;)</span> <span class="pre">const</span></tt> member function, but also its
mutating <tt class="literal"><span class="pre">double_me()</span></tt> member. If we wanted to build a
<em>constant</em> <tt class="literal"><span class="pre">node_iterator</span></tt>, we'd only have to make three
changes:</p>
<pre class="literal-block">
class const_node_iterator
: public boost::iterator_facade&lt;
node_iterator
, node_base <strong>const</strong>
, boost::forward_traversal_tag
&gt;
{
public:
const_node_iterator()
: m_node(0) {}
explicit const_node_iterator(node_base* p)
: m_node(p) {}
private:
friend class boost::iterator_core_access;
void increment() { m_node = m_node-&gt;next(); }
bool equal(const_node_iterator const&amp; other) const
{
return this-&gt;m_node == other.m_node;
}
node_base <strong>const</strong>&amp; dereference() const { return *m_node; }
node_base <strong>const</strong>* m_node;
};
</pre>
<div class="sidebar">
<p class="sidebar-title"><tt class="literal"><span class="pre">const</span></tt> and an iterator's <tt class="literal"><span class="pre">value_type</span></tt></p>
<p>The C++ standard requires an iterator's <tt class="literal"><span class="pre">value_type</span></tt> <em>not</em> be
<tt class="literal"><span class="pre">const</span></tt>-qualified, so <tt class="literal"><span class="pre">iterator_facade</span></tt> strips the
<tt class="literal"><span class="pre">const</span></tt> from its <tt class="literal"><span class="pre">Value</span></tt> parameter in order to produce the
iterator's <tt class="literal"><span class="pre">value_type</span></tt>. Making the <tt class="literal"><span class="pre">Value</span></tt> argument
<tt class="literal"><span class="pre">const</span></tt> provides a useful hint to <tt class="literal"><span class="pre">iterator_facade</span></tt> that the
iterator is a <em>constant iterator</em>, and the default <tt class="literal"><span class="pre">Reference</span></tt>
argument will be correct for all lvalue iterators.</p>
</div>
<p>As a matter of fact, <tt class="literal"><span class="pre">node_iterator</span></tt> and <tt class="literal"><span class="pre">const_node_iterator</span></tt>
are so similar that it makes sense to factor the common code out
into a template as follows:</p>
<pre class="literal-block">
template &lt;class Value&gt;
class node_iter
: public boost::iterator_facade&lt;
node_iter&lt;Value&gt;
, Value
, boost::forward_traversal_tag
&gt;
{
public:
node_iter()
: m_node(0) {}
explicit node_iter(Value* p)
: m_node(p) {}
private:
friend class boost::iterator_core_access;
bool equal(node_iter&lt;Value&gt; const&amp; other) const
{
return this-&gt;m_node == other.m_node;
}
void increment()
{ m_node = m_node-&gt;next(); }
Value&amp; dereference() const
{ return *m_node; }
Value* m_node;
};
typedef node_iter&lt;node_base&gt; node_iterator;
typedef node_iter&lt;node_base const&gt; node_const_iterator;
</pre>
</div>
<div class="section" id="interoperability">
<h2><a class="toc-backref" href="#id39" name="interoperability">Interoperability</a></h2>
<p>Our <tt class="literal"><span class="pre">const_node_iterator</span></tt> works perfectly well on its own, but
taken together with <tt class="literal"><span class="pre">node_iterator</span></tt> it doesn't quite meet
expectations. For example, we'd like to be able to pass a
<tt class="literal"><span class="pre">node_iterator</span></tt> where a <tt class="literal"><span class="pre">node_const_iterator</span></tt> was expected,
just as you can with <tt class="literal"><span class="pre">std::list&lt;int&gt;</span></tt>'s <tt class="literal"><span class="pre">iterator</span></tt> and
<tt class="literal"><span class="pre">const_iterator</span></tt>. Furthermore, given a <tt class="literal"><span class="pre">node_iterator</span></tt> and a
<tt class="literal"><span class="pre">node_const_iterator</span></tt> into the same list, we should be able to
compare them for equality.</p>
<p>This expected ability to use two different iterator types together
is known as <strong>interoperability</strong>. Achieving interoperability in
our case is as simple as templatizing the <tt class="literal"><span class="pre">equal</span></tt> function and
adding a templatized converting constructor <a class="footnote-reference" href="#broken" id="id8" name="id8"><sup>3</sup></a> <a class="footnote-reference" href="#random" id="id9" name="id9"><sup>4</sup></a>:</p>
<pre class="literal-block">
template &lt;class Value&gt;
class node_iter
: public boost::iterator_facade&lt;
node_iter&lt;Value&gt;
, Value
, boost::forward_traversal_tag
&gt;
{
public:
node_iter()
: m_node(0) {}
explicit node_iter(Value* p)
: m_node(p) {}
template &lt;class OtherValue&gt;
node_iter(node_iter&lt;OtherValue&gt; const&amp; other)
: m_node(other.m_node) {}
private:
friend class boost::iterator_core_access;
template &lt;class&gt; friend class node_iter;
template &lt;class OtherValue&gt;
bool equal(node_iter&lt;OtherValue&gt; const&amp; other) const
{
return this-&gt;m_node == other.m_node;
}
void increment()
{ m_node = m_node-&gt;next(); }
Value&amp; dereference() const
{ return *m_node; }
Value* m_node;
};
typedef impl::node_iterator&lt;node_base&gt; node_iterator;
typedef impl::node_iterator&lt;node_base const&gt; node_const_iterator;
</pre>
<table class="footnote" frame="void" id="broken" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id8" name="broken">[3]</a></td><td>If you're using an older compiler and it can't handle
this example, see the <a class="reference" href="../example/node_iterator2.hpp">example code</a> for workarounds.</td></tr>
</tbody>
</table>
<table class="footnote" frame="void" id="random" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id9" name="random">[4]</a></td><td>If <tt class="literal"><span class="pre">node_iterator</span></tt> had been a <a class="reference" href="new-iter-concepts.html#random-access-traversal-iterators-lib-random-access-traversal-iterators">random access
traversal iterator</a>, we'd have had to templatize its
<tt class="literal"><span class="pre">distance_to</span></tt> function as well.</td></tr>
</tbody>
</table>
<p>You can see an example program which exercises our interoperable
iterators <a class="reference" href="../example/node_iterator2.cpp">here</a>.</p>
</div>
<div class="section" id="telling-the-truth">
<h2><a class="toc-backref" href="#id40" name="telling-the-truth">Telling the Truth</a></h2>
<p>Now <tt class="literal"><span class="pre">node_iterator</span></tt> and <tt class="literal"><span class="pre">node_const_iterator</span></tt> behave exactly as
you'd expect... almost. We can compare them and we can convert in
one direction: from <tt class="literal"><span class="pre">node_iterator</span></tt> to <tt class="literal"><span class="pre">node_const_iterator</span></tt>.
If we try to convert from <tt class="literal"><span class="pre">node_const_iterator</span></tt> to
<tt class="literal"><span class="pre">node_iterator</span></tt>, we'll get an error when the converting
constructor tries to initialize <tt class="literal"><span class="pre">node_iterator</span></tt>'s <tt class="literal"><span class="pre">m_node</span></tt>, a
<tt class="literal"><span class="pre">node*</span></tt> with a <tt class="literal"><span class="pre">node</span> <span class="pre">const*</span></tt>. So what's the problem?</p>
<p>The problem is that
<tt class="literal"><span class="pre">boost::</span></tt><a class="reference" href="../../type_traits/index.html#relationships"><tt class="literal"><span class="pre">is_convertible</span></tt></a><tt class="literal"><span class="pre">&lt;node_const_iterator,node_iterator&gt;::value</span></tt>
will be <tt class="literal"><span class="pre">true</span></tt>, but it should be <tt class="literal"><span class="pre">false</span></tt>. <a class="reference" href="../../type_traits/index.html#relationships"><tt class="literal"><span class="pre">is_convertible</span></tt></a>
lies because it can only see as far as the <em>declaration</em> of
<tt class="literal"><span class="pre">node_iter</span></tt>'s converting constructor, but can't look inside at
the <em>definition</em> to make sure it will compile. A perfect solution
would make <tt class="literal"><span class="pre">node_iter</span></tt>'s converting constructor disappear when
the <tt class="literal"><span class="pre">m_node</span></tt> conversion would fail.</p>
<p>In fact, that sort of magic is possible using
<a class="reference" href="../../utility/enable_if.html"><tt class="literal"><span class="pre">boost::enable_if</span></tt></a>. By rewriting the converting constructor as
follows, we can remove it from the overload set when it's not
appropriate:</p>
<pre class="literal-block">
template &lt;class OtherValue&gt;
node_iter(
node_iter&lt;OtherValue&gt; const&amp; other
, typename boost::enable_if&lt;
boost::is_convertible&lt;OtherValue*,Value*&gt;
, enabler
&gt;::type = enabler()
)
: m_node(other.m_node) {}
</pre>
</div>
<div class="section" id="wrap-up">
<h2><a class="toc-backref" href="#id41" name="wrap-up">Wrap Up</a></h2>
<p>This concludes our <tt class="literal"><span class="pre">iterator_facade</span></tt> tutorial, but before you
stop reading we urge you to take a look at <a class="reference" href="iterator_adaptor.html"><tt class="literal"><span class="pre">iterator_adaptor</span></tt></a>.
There's another way to approach writing these iterators which might
even be superior.</p>
</div>
</div>
<div class="section" id="id14">
<h1><a class="toc-backref" href="#id42" name="id14">Reference</a></h1>
<div class="section" id="ref">
<h2><a class="toc-backref" href="#id43" name="ref">Ref</a></h2>
<p>We need to resolve the title levels here.</p>
<!-- Version 1.3 of this ReStructuredText document corresponds to
n1530_, the paper accepted by the LWG for TR1. -->
<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All
rights reserved -->
<pre class="literal-block">
template &lt;
class Derived
, class Value
, class CategoryOrTraversal
, class Reference = Value&amp;
, class Difference = ptrdiff_t
&gt;
class iterator_facade {
public:
typedef remove_const&lt;Value&gt;::type value_type;
typedef Reference reference;
typedef Value* pointer;
typedef Difference difference_type;
typedef /* see <a class="reference" href="#facade-iterator-category">below</a> */ iterator_category;
reference operator*() const;
/* see <a class="reference" href="#operator-arrow">below</a> */ operator-&gt;() const;
/* see <a class="reference" href="#brackets">below</a> */ operator[](difference_type n) const;
Derived&amp; operator++();
Derived operator++(int);
Derived&amp; operator--();
Derived operator--(int);
Derived&amp; operator+=(difference_type n);
Derived&amp; operator-=(difference_type n);
Derived operator-(difference_type n) const;
};
// Comparison operators
template &lt;class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2&gt;
typename enable_if_interoperable&lt;Dr1, Dr2, bool&gt;::type // exposition
operator ==(iterator_facade&lt;Dr1, V1, TC1, R1, D1&gt; const&amp; lhs,
iterator_facade&lt;Dr2, V2, TC2, R2, D2&gt; const&amp; rhs);
template &lt;class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2&gt;
typename enable_if_interoperable&lt;Dr1, Dr2, bool&gt;::type
operator !=(iterator_facade&lt;Dr1, V1, TC1, R1, D1&gt; const&amp; lhs,
iterator_facade&lt;Dr2, V2, TC2, R2, D2&gt; const&amp; rhs);
template &lt;class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2&gt;
typename enable_if_interoperable&lt;Dr1, Dr2, bool&gt;::type
operator &lt;(iterator_facade&lt;Dr1, V1, TC1, R1, D1&gt; const&amp; lhs,
iterator_facade&lt;Dr2, V2, TC2, R2, D2&gt; const&amp; rhs);
template &lt;class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2&gt;
typename enable_if_interoperable&lt;Dr1, Dr2, bool&gt;::type
operator &lt;=(iterator_facade&lt;Dr1, V1, TC1, R1, D1&gt; const&amp; lhs,
iterator_facade&lt;Dr2, V2, TC2, R2, D2&gt; const&amp; rhs);
template &lt;class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2&gt;
typename enable_if_interoperable&lt;Dr1, Dr2, bool&gt;::type
operator &gt;(iterator_facade&lt;Dr1, V1, TC1, R1, D1&gt; const&amp; lhs,
iterator_facade&lt;Dr2, V2, TC2, R2, D2&gt; const&amp; rhs);
template &lt;class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2&gt;
typename enable_if_interoperable&lt;Dr1, Dr2, bool&gt;::type
operator &gt;=(iterator_facade&lt;Dr1, V1, TC1, R1, D1&gt; const&amp; lhs,
iterator_facade&lt;Dr2, V2, TC2, R2, D2&gt; const&amp; rhs);
template &lt;class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2&gt;
typename enable_if_interoperable&lt;Dr1, Dr2, bool&gt;::type
operator &gt;=(iterator_facade&lt;Dr1, V1, TC1, R1, D1&gt; const&amp; lhs,
iterator_facade&lt;Dr2, V2, TC2, R2, D2&gt; const&amp; rhs);
// Iterator difference
template &lt;class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2&gt;
typename enable_if_interoperable&lt;Dr1, Dr2, bool&gt;::type
operator -(iterator_facade&lt;Dr1, V1, TC1, R1, D1&gt; const&amp; lhs,
iterator_facade&lt;Dr2, V2, TC2, R2, D2&gt; const&amp; rhs);
// Iterator addition
template &lt;class Derived, class V, class TC, class R, class D&gt;
Derived operator+ (iterator_facade&lt;Derived, V, TC, R, D&gt; const&amp;,
typename Derived::difference_type n)
</pre>
<p>The <tt class="literal"><span class="pre">enable_if_interoperable</span></tt> template used above is for exposition
purposes. The member operators should be only be in an overload set
provided the derived types <tt class="literal"><span class="pre">Dr1</span></tt> and <tt class="literal"><span class="pre">Dr2</span></tt> are interoperable,
meaning that at least one of the types is convertible to the other. The
<tt class="literal"><span class="pre">enable_if_interoperable</span></tt> approach uses SFINAE to take the operators
out of the overload set when the types are not interoperable.
The operators should behave <em>as-if</em> <tt class="literal"><span class="pre">enable_if_interoperable</span></tt>
were defined to be:</p>
<pre class="literal-block">
template &lt;bool, typename&gt; enable_if_interoperable_impl
{};
template &lt;typename T&gt; enable_if_interoperable_impl&lt;true,T&gt;
{ typedef T type; };
template&lt;typename Dr1, typename Dr2, typename T&gt;
struct enable_if_interoperable
: enable_if_interoperable_impl&lt;
is_convertible&lt;Dr1,Dr2&gt;::value || is_convertible&lt;Dr2,Dr1&gt;::value
, T
&gt;
{};
</pre>
<div class="section" id="iterator-facade-usage">
<h3><a class="toc-backref" href="#id44" name="iterator-facade-usage"><tt class="literal"><span class="pre">iterator_facade</span></tt> usage</a></h3>
<p>The following table describes the typical valid expressions on
<tt class="literal"><span class="pre">iterator_facade</span></tt>'s <tt class="literal"><span class="pre">Derived</span></tt> parameter, depending on the
iterator concept(s) it will model. The operations in the first
column must be made accessible to member functions of class
<tt class="literal"><span class="pre">iterator_core_access</span></tt>.</p>
<p>In the table below, <tt class="literal"><span class="pre">F</span></tt> is <tt class="literal"><span class="pre">iterator_facade&lt;X,V,C,R,D&gt;</span></tt>, <tt class="literal"><span class="pre">a</span></tt> is an
object of type <tt class="literal"><span class="pre">X</span></tt>, <tt class="literal"><span class="pre">b</span></tt> and <tt class="literal"><span class="pre">c</span></tt> are objects of type <tt class="literal"><span class="pre">const</span> <span class="pre">X</span></tt>,
<tt class="literal"><span class="pre">n</span></tt> is an object of <tt class="literal"><span class="pre">F::difference_type</span></tt>, <tt class="literal"><span class="pre">y</span></tt> is a constant
object of a single pass iterator type interoperable with <tt class="literal"><span class="pre">X</span></tt>, and <tt class="literal"><span class="pre">z</span></tt>
is a constant object of a random access traversal iterator type
interoperable with <tt class="literal"><span class="pre">X</span></tt>.</p>
<a class="target" id="id18" name="id18"></a><div class="section" id="iterator-facade-core-operations">
<h4><a class="toc-backref" href="#id45" name="iterator-facade-core-operations"><tt class="literal"><span class="pre">iterator_facade</span></tt> Core Operations</a></h4>
<table border class="table">
<colgroup>
<col width="19%" />
<col width="21%" />
<col width="35%" />
<col width="25%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion/Note</th>
<th>Used to implement Iterator
Concept(s)</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">c.dereference()</span></tt></td>
<td><tt class="literal"><span class="pre">F::reference</span></tt></td>
<td>&nbsp;</td>
<td>Readable Iterator, Writable
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.equal(y)</span></tt></td>
<td>convertible to bool</td>
<td>true iff <tt class="literal"><span class="pre">c</span></tt> and <tt class="literal"><span class="pre">y</span></tt> refer to the
same position. Implements <tt class="literal"><span class="pre">c</span> <span class="pre">==</span> <span class="pre">y</span></tt>
and <tt class="literal"><span class="pre">c</span> <span class="pre">!=</span> <span class="pre">y</span></tt>.</td>
<td>Single Pass Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.increment()</span></tt></td>
<td>unused</td>
<td>&nbsp;</td>
<td>Incrementable Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.decrement()</span></tt></td>
<td>unused</td>
<td>&nbsp;</td>
<td>Bidirectional Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.advance(n)</span></tt></td>
<td>unused</td>
<td>&nbsp;</td>
<td>Random Access Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.distance_to(b)</span></tt></td>
<td>convertible to
<tt class="literal"><span class="pre">F::difference_type</span></tt></td>
<td>equivalent to <tt class="literal"><span class="pre">distance(c,</span> <span class="pre">b)</span></tt></td>
<td>Random Access Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.distance_to(z)</span></tt></td>
<td>convertible to
<tt class="literal"><span class="pre">F::difference_type</span></tt></td>
<td>equivalent to <tt class="literal"><span class="pre">distance(c,</span> <span class="pre">z)</span></tt>.
Implements <tt class="literal"><span class="pre">c</span> <span class="pre">-</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span> <span class="pre">&lt;</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span>
<span class="pre">&lt;=</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span> <span class="pre">&gt;</span> <span class="pre">z</span></tt>, and <tt class="literal"><span class="pre">c</span> <span class="pre">&gt;=</span> <span class="pre">c</span></tt>.</td>
<td>Random Access Traversal
Iterator</td>
</tr>
</tbody>
</table>
<a class="target" id="facade-iterator-category" name="facade-iterator-category"></a></div>
</div>
<div class="section" id="iterator-facade-iterator-category">
<h3><a class="toc-backref" href="#id46" name="iterator-facade-iterator-category"><tt class="literal"><span class="pre">iterator_facade</span></tt> iterator category</a></h3>
<p>The <tt class="literal"><span class="pre">iterator_category</span></tt> member of <tt class="literal"><span class="pre">iterator_facade&lt;X,V,R,C,D&gt;</span></tt>
is a type which satisfies the following conditions:</p>
<blockquote>
<ul>
<li><p class="first">if <tt class="literal"><span class="pre">C</span></tt> is convertible to <tt class="literal"><span class="pre">std::input_iterator_tag</span></tt> or
<tt class="literal"><span class="pre">C</span></tt> is convertible to <tt class="literal"><span class="pre">std::output_iterator_tag</span></tt>,
<tt class="literal"><span class="pre">iterator_category</span></tt> is the same as <tt class="literal"><span class="pre">C</span></tt>.</p>
</li>
<li><p class="first">Otherwise, if <tt class="literal"><span class="pre">C</span></tt> is not convertible to
<tt class="literal"><span class="pre">incrementable_traversal_tag</span></tt>, the program is ill-formed</p>
</li>
<li><p class="first">Otherwise:</p>
<ul>
<li><p class="first"><tt class="literal"><span class="pre">iterator_category</span></tt> is convertible to the iterator
category tag or tags given by the following algorithm, and
not to any more-derived iterator category tag or tags:</p>
<pre class="literal-block">
if (R is a reference type
&amp;&amp; C is convertible to forward_traversal_tag)
{
if (C is convertible to random_access_traversal_tag)
return random_access_iterator_tag
else if (C is convertible to bidirectional_traversal_tag)
return bidirectional_iterator_tag
else
return forward_traversal_tag
}
else
{
if (C is convertible to single_pass_traversal_tag
&amp;&amp; R is convertible to V)
{
if (V is const)
return input_iterator_tag
else
return input_iterator_tag and output_iterator_tag
}
else
return output_iterator_tag
}
</pre>
</li>
<li><p class="first"><tt class="literal"><span class="pre">iterator_traversal&lt;X&gt;::type</span></tt> is convertible to the most
derived traversal tag type to which <tt class="literal"><span class="pre">C</span></tt> is also
convertible, and not to any more-derived traversal tag type.</p>
</li>
</ul>
</li>
</ul>
</blockquote>
</div>
<div class="section" id="iterator-facade-operations">
<h3><a class="toc-backref" href="#id47" name="iterator-facade-operations"><tt class="literal"><span class="pre">iterator_facade</span></tt> operations</a></h3>
<p>The operations in this section are described in terms of operations on
the core interface of <tt class="literal"><span class="pre">Derived</span></tt> which may be inaccessible
(i.e. private). The implementation should access these operations
through member functions of class <tt class="literal"><span class="pre">iterator_core_access</span></tt>.</p>
<p><tt class="literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">static_cast&lt;Derived</span> <span class="pre">const*&gt;(this)-&gt;dereference()</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">operator-&gt;()</span> <span class="pre">const;</span></tt> (see <a class="reference" href="#operator-arrow">below</a>)</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">If <tt class="literal"><span class="pre">reference</span></tt> is a reference type, an object
of type <tt class="literal"><span class="pre">pointer</span></tt> equal to:</p>
<pre class="literal-block">
&amp;static_cast&lt;Derived const*&gt;(this)-&gt;dereference()
</pre>
<p class="last">Otherwise returns an object of unspecified type such that,
<tt class="literal"><span class="pre">(*static_cast&lt;Derived</span> <span class="pre">const*&gt;(this))-&gt;m</span></tt> is equivalent to <tt class="literal"><span class="pre">(w</span> <span class="pre">=</span> <span class="pre">**static_cast&lt;Derived</span> <span class="pre">const*&gt;(this),</span>
<span class="pre">w.m)</span></tt> for some temporary object <tt class="literal"><span class="pre">w</span></tt> of type <tt class="literal"><span class="pre">value_type</span></tt>.</p>
</td>
</tr>
</tbody>
</table>
<a class="target" id="brackets" name="brackets"></a><p><em>unspecified</em> <tt class="literal"><span class="pre">operator[](difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">an object convertible to <tt class="literal"><span class="pre">value_type</span></tt>. For constant
objects <tt class="literal"><span class="pre">v</span></tt> of type <tt class="literal"><span class="pre">value_type</span></tt>, and <tt class="literal"><span class="pre">n</span></tt> of type
<tt class="literal"><span class="pre">difference_type</span></tt>, and reference <tt class="literal"><span class="pre">p</span></tt> equal to
<tt class="literal"><span class="pre">*static_cast&lt;Derived</span> <span class="pre">const*&gt;(this)</span></tt>, <tt class="literal"><span class="pre">(*this)[n]</span> <span class="pre">=</span> <span class="pre">v</span></tt> is
equivalent to <tt class="literal"><span class="pre">*(p+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">v</span></tt>, and <tt class="literal"><span class="pre">static_cast&lt;value_type</span>
<span class="pre">const&amp;&gt;((*this)[n])</span></tt> is equivalent to
<tt class="literal"><span class="pre">static_cast&lt;value_type</span> <span class="pre">const&amp;&gt;(*(p+n))</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&amp;</span> <span class="pre">operator++();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast&lt;Derived*&gt;(this)-&gt;increment();
return *static_cast&lt;Derived*&gt;(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived</span> <span class="pre">operator++(int);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast&lt;Derived const*&gt;(this));
++*this;
return tmp;
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&amp;</span> <span class="pre">operator--();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast&lt;Derived*&gt;(this)-&gt;decrement();
return static_cast&lt;Derived*&gt;(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived</span> <span class="pre">operator--(int);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast&lt;Derived const*&gt;(this));
--*this;
return tmp;
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&amp;</span> <span class="pre">operator+=(difference_type</span> <span class="pre">n);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast&lt;Derived*&gt;(this)-&gt;advance(n);
return static_cast&lt;Derived*&gt;(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&amp;</span> <span class="pre">operator-=(difference_type</span> <span class="pre">n);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast&lt;Derived*&gt;(this)-&gt;advance(-n);
return static_cast&lt;Derived*&gt;(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived</span> <span class="pre">operator-(difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast&lt;Derived const*&gt;(this));
return tmp -= n;
</pre>
</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
</div>
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