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Finished iterator_adaptor tutorial

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Added example test code
Rolled forward old counting_iterator abstract for boost docs.


[SVN r21649]
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Dave Abrahams
2004-01-12 20:58:22 +00:00
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@ -3,7 +3,7 @@
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<title>Iterator Adaptor</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" />
@ -57,20 +57,21 @@ core interface functions of <tt class="literal"><span class="pre">iterator_facad
<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="#introduction" id="id3" name="id3">Introduction</a></li>
<li><a class="reference" href="#reference" id="id4" name="id4">Reference</a><ul>
<li><a class="reference" href="#iterator-adaptor-requirements" id="id5" name="id5"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></li>
<li><a class="reference" href="#iterator-adaptor-base-class-parameters" id="id6" name="id6"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></li>
<li><a class="reference" href="#iterator-adaptor-models" id="id7" name="id7"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> models</a></li>
<li><a class="reference" href="#iterator-adaptor-public-operations" id="id8" name="id8"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></li>
<li><a class="reference" href="#iterator-adaptor-protected-member-functions" id="id9" name="id9"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></li>
<li><a class="reference" href="#iterator-adaptor-private-member-functions" id="id10" name="id10"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></li>
<li><a class="reference" href="#overview" id="id6" name="id6">Overview</a></li>
<li><a class="reference" href="#reference" id="id7" name="id7">Reference</a><ul>
<li><a class="reference" href="#iterator-adaptor-requirements" id="id8" name="id8"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></li>
<li><a class="reference" href="#iterator-adaptor-base-class-parameters" id="id9" name="id9"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></li>
<li><a class="reference" href="#iterator-adaptor-models" id="id10" name="id10"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> models</a></li>
<li><a class="reference" href="#iterator-adaptor-public-operations" id="id11" name="id11"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></li>
<li><a class="reference" href="#iterator-adaptor-protected-member-functions" id="id12" name="id12"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></li>
<li><a class="reference" href="#iterator-adaptor-private-member-functions" id="id13" name="id13"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></li>
</ul>
</li>
<li><a class="reference" href="#tutorial-example" id="id14" name="id14">Tutorial Example</a></li>
</ul>
</div>
<div class="section" id="introduction">
<h1><a class="toc-backref" href="#id3" name="introduction">Introduction</a></h1>
<div class="section" id="overview">
<h1><a class="toc-backref" href="#id6" name="overview">Overview</a></h1>
<!-- Version 1.2 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
@ -84,7 +85,7 @@ instance of the <tt class="literal"><span class="pre">Base</span></tt> type, whi
<table class="footnote" frame="void" id="base" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1" name="base">[1]</a></td><td>The term &quot;Base&quot; here does not refer to a base class and is
<tr><td class="label"><a name="base">[1]</a></td><td><em>(<a class="fn-backref" href="#id1">1</a>, <a class="fn-backref" href="#id3">2</a>)</em> The term &quot;Base&quot; here does not refer to a base class and is
not meant to imply the use of derivation. We have followed the lead
of the standard library, which provides a base() function to access
the underlying iterator object of a <tt class="literal"><span class="pre">reverse_iterator</span></tt> adaptor.</td></tr>
@ -111,7 +112,7 @@ template parameter may not always be identical to the iterator's
that assumption.</p>
</div>
<div class="section" id="reference">
<h1><a class="toc-backref" href="#id4" name="reference">Reference</a></h1>
<h1><a class="toc-backref" href="#id7" name="reference">Reference</a></h1>
<!-- Version 1.4 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
@ -159,13 +160,13 @@ class iterator_adaptor
};
</pre>
<a class="target" id="requirements" name="requirements"></a><div class="section" id="iterator-adaptor-requirements">
<h2><a class="toc-backref" href="#id5" name="iterator-adaptor-requirements"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></h2>
<h2><a class="toc-backref" href="#id8" name="iterator-adaptor-requirements"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></h2>
<p>The <tt class="literal"><span class="pre">Derived</span></tt> template argument must be a publicly derived from
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>.</p>
<p>The <tt class="literal"><span class="pre">Base</span></tt> argument shall be Assignable and Copy Constructible.</p>
<a class="target" id="base-parameters" name="base-parameters"></a></div>
<div class="section" id="iterator-adaptor-base-class-parameters">
<h2><a class="toc-backref" href="#id6" name="iterator-adaptor-base-class-parameters"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></h2>
<h2><a class="toc-backref" href="#id9" name="iterator-adaptor-base-class-parameters"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></h2>
<p>The <em>V</em>, <em>C</em>, <em>R</em>, and <em>D</em> parameters of the <tt class="literal"><span class="pre">iterator_facade</span></tt>
used as a base class in the summary of <tt class="literal"><span class="pre">iterator_adaptor</span></tt>
above are defined as follows:</p>
@ -195,7 +196,7 @@ above are defined as follows:</p>
</pre>
</div>
<div class="section" id="iterator-adaptor-models">
<h2><a class="toc-backref" href="#id7" name="iterator-adaptor-models"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> models</a></h2>
<h2><a class="toc-backref" href="#id10" name="iterator-adaptor-models"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> models</a></h2>
<p>In order for <tt class="literal"><span class="pre">Derived</span></tt> to model the iterator concepts corresponding
to <tt class="literal"><span class="pre">iterator_traits&lt;Derived&gt;::iterator_category</span></tt>, the expressions
involving <tt class="literal"><span class="pre">m_iterator</span></tt> in the specifications of those private member
@ -205,7 +206,7 @@ expression involving <tt class="literal"><span class="pre">Derived</span></tt> i
<!-- The above is confusing and needs a rewrite. -JGS -->
</div>
<div class="section" id="iterator-adaptor-public-operations">
<h2><a class="toc-backref" href="#id8" name="iterator-adaptor-public-operations"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></h2>
<h2><a class="toc-backref" href="#id11" name="iterator-adaptor-public-operations"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></h2>
<p><tt class="literal"><span class="pre">iterator_adaptor();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
@ -239,7 +240,7 @@ expression involving <tt class="literal"><span class="pre">Derived</span></tt> i
</table>
</div>
<div class="section" id="iterator-adaptor-protected-member-functions">
<h2><a class="toc-backref" href="#id9" name="iterator-adaptor-protected-member-functions"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></h2>
<h2><a class="toc-backref" href="#id12" name="iterator-adaptor-protected-member-functions"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></h2>
<p><tt class="literal"><span class="pre">Base</span> <span class="pre">const&amp;</span> <span class="pre">base_reference()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
@ -260,7 +261,7 @@ expression involving <tt class="literal"><span class="pre">Derived</span></tt> i
</table>
</div>
<div class="section" id="iterator-adaptor-private-member-functions">
<h2><a class="toc-backref" href="#id10" name="iterator-adaptor-private-member-functions"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></h2>
<h2><a class="toc-backref" href="#id13" name="iterator-adaptor-private-member-functions"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></h2>
<p><tt class="literal"><span class="pre">typename</span> <span class="pre">iterator_adaptor::reference</span> <span class="pre">dereference()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
@ -328,11 +329,114 @@ typename iterator_adaptor::difference_type distance_to(
</table>
</div>
</div>
<div class="section" id="tutorial-example">
<h1><a class="toc-backref" href="#id14" 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) -->
<p>In this section we'll further refine the <tt class="literal"><span class="pre">node_iter</span></tt> class
template we developed in the <a class="reference" href="iterator_facade.html#tutorial-example"><tt class="literal"><span class="pre">iterator_facade</span></tt> tutorial</a>. If you haven't already
read that material, you should go back now and check it out because
we're going to pick up right where it left off.</p>
<div class="sidebar">
<p class="sidebar-title"><tt class="literal"><span class="pre">node_base*</span></tt> really <em>is</em> an iterator</p>
<p>It's not really a very interesting iterator, since <tt class="literal"><span class="pre">node_base</span></tt>
is an abstract class: a pointer to a <tt class="literal"><span class="pre">node_base</span></tt> just points
at some base subobject of an instance of some other class, and
incrementing a <tt class="literal"><span class="pre">node_base*</span></tt> moves it past this base subobject
to who-knows-where? The most we can do with that incremented
position is to compare another <tt class="literal"><span class="pre">node_base*</span></tt> to it. In other
words, the original iterator traverses a one-element array.</p>
</div>
<hr class="footer"/>
<p>You probably didn't think of it that way, but the <tt class="literal"><span class="pre">node_base*</span></tt>
object which underlies <tt class="literal"><span class="pre">node_iterator</span></tt> is itself an iterator,
just like all other pointers. If we examine that pointer closely
from an iterator perspective, we can see that it has much in common
with the <tt class="literal"><span class="pre">node_iterator</span></tt> we're building. First, they share most
of the same associated types (<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">pointer</span></tt>, and <tt class="literal"><span class="pre">difference_type</span></tt>). Second, even much of the
core functionality is the same: <tt class="literal"><span class="pre">operator*</span></tt> and <tt class="literal"><span class="pre">operator==</span></tt> on
the <tt class="literal"><span class="pre">node_iterator</span></tt> just return the result of invoking the same
operations on the underlying pointer, via the <tt class="literal"><span class="pre">node_iterator</span></tt>'s
<a class="reference" href="iterator_facade.html#implementing-the-core-operations"><tt class="literal"><span class="pre">dereference</span></tt> and <tt class="literal"><span class="pre">equal</span></tt> member functions</a>)</p>
<p>It turns out that the pattern of building an iterator on another
iterator-like type (the <tt class="literal"><span class="pre">Base</span></tt> <a class="footnote-reference" href="#base" id="id3" name="id3"><sup>1</sup></a> type) while modifying
just a few aspects of the underlying type's behavior is an
extremely common one, and it's the pattern addressed by
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>. Using <tt class="literal"><span class="pre">iterator_adaptor</span></tt> is very much like
using <tt class="literal"><span class="pre">iterator_facade</span></tt>, but because iterator_adaptor tries to
mimic as much of the <tt class="literal"><span class="pre">Base</span></tt> type's behavior as possible, we
neither have to supply a <tt class="literal"><span class="pre">Value</span></tt> argument, nor implement any core
behaviors other than <tt class="literal"><span class="pre">increment</span></tt>. The implementation of
<tt class="literal"><span class="pre">node_iter</span></tt> is thus reduced to:</p>
<pre class="literal-block">
template &lt;class Value&gt;
class node_iter
: public boost::iterator_adaptor&lt;
node_iter&lt;Value&gt; // Derived
, Value* // Base
, boost::use_default // Value
, boost::forward_traversal_tag // CategoryOrTraversal
&gt;
{
private:
struct enabler {}; // a private type avoids misuse
typedef boost::iterator_adaptor&lt;
node_iter&lt;Value&gt;, Value*, boost::use_default, boost::forward_traversal_tag
&gt; super_t;
public:
node_iter()
: super_t(0) {}
explicit node_iter(Value* p)
: super_t(p) {}
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()
)
: super_t(other.base()) {}
private:
friend class boost::iterator_core_access;
void increment() { this-&gt;base_reference() = this-&gt;base()-&gt;next(); }
};
</pre>
<p>You can see an example program which exercises this version of the
node iterators <a class="reference" href="../example/node_iterator3.cpp">here</a>.</p>
<p>In the case of <tt class="literal"><span class="pre">node_iter</span></tt>, it's not very compelling to pass
<tt class="literal"><span class="pre">boost::use_default</span></tt> as <tt class="literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="literal"><span class="pre">Value</span></tt>
argument; we could have just passed <tt class="literal"><span class="pre">node_iter</span></tt>'s <tt class="literal"><span class="pre">Value</span></tt>
along to <tt class="literal"><span class="pre">iterator_adaptor</span></tt>, and that'd even be shorter! Most
iterator class templates built with <tt class="literal"><span class="pre">iterator_adaptor</span></tt> are
parameterized on another iterator type, rather than on its
<tt class="literal"><span class="pre">value_type</span></tt>. For example, <tt class="literal"><span class="pre">boost::reverse_iterator</span></tt> takes an
iterator type argument and reverses its direction of traversal,
since the original iterator and the reversed one have all the same
associated types, <tt class="literal"><span class="pre">iterator_adaptor</span></tt>'s delegation of default
types to its <tt class="literal"><span class="pre">Base</span></tt> saves the implementor of
<tt class="literal"><span class="pre">boost::reverse_iterator</span></tt> from writing</p>
<pre class="literal-block">
std::iterator_traits&lt;Iterator&gt;::<em>some-associated-type</em>
</pre>
<p>at least four times.</p>
<p>We urge you to review the documentation and implementations of
<a class="reference" href="reverse_iterator.html"><tt class="literal"><span class="pre">reverse_iterator</span></tt></a> and the other Boost <a class="reference" href="index.html#specialized-adaptors">specialized iterator
adaptors</a> to get an idea of the sorts of things you can do with
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>. In particular, have a look at
<a class="reference" href="counting_iterator.html"><tt class="literal"><span class="pre">counting_iterator</span></tt></a>, which demonstrates that <tt class="literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="literal"><span class="pre">Base</span></tt> type needn't be an iterator.</p>
</div>
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