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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" />
<meta name="date" content="2003-09-14" />
<meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All rights reserved" />
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<div class="document" id="iterator-adaptor">
<h1 class="title">Iterator Adaptor</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>2003-09-14</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"></td>
</tr>
</tbody>
</table>
<p>The <tt class="literal"><span class="pre">iterator_adaptor</span></tt> is a base class template derived from an
instantiation of <tt class="literal"><span class="pre">iterator_facade</span></tt>. The core interface functions
expected by <tt class="literal"><span class="pre">iterator_facade</span></tt> are implemented in terms of the
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="literal"><span class="pre">Base</span></tt> template parameter. A class derived
from <tt class="literal"><span class="pre">iterator_adaptor</span></tt> typically redefines some of the core
interface functions to adapt the behavior of the <tt class="literal"><span class="pre">Base</span></tt> type.
Whether the derived class models any of the standard iterator concepts
depends on the operations supported by the <tt class="literal"><span class="pre">Base</span></tt> type and which
core interface functions of <tt class="literal"><span class="pre">iterator_facade</span></tt> are redefined in the
<tt class="literal"><span class="pre">Derived</span></tt> class.</p>
<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-public-operations" id="id6" name="id6"><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="id7" name="id7"><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="id8" name="id8"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="introduction">
<h1><a class="toc-backref" href="#id3" name="introduction">Introduction</a></h1>
<p>The <tt class="literal"><span class="pre">iterator_adaptor</span></tt> class template adapts some <tt class="literal"><span class="pre">Base</span></tt> <a class="footnote-reference" href="#base" id="id1" name="id1"><sup>1</sup></a>
type to create a new iterator. Instantiations of <tt class="literal"><span class="pre">iterator_adaptor</span></tt>
are derived from a corresponding instantiation of <tt class="literal"><span class="pre">iterator_facade</span></tt>
and implement the core behaviors in terms of the <tt class="literal"><span class="pre">Base</span></tt> type. In
essence, <tt class="literal"><span class="pre">iterator_adaptor</span></tt> merely forwards all operations to an
instance of the <tt class="literal"><span class="pre">Base</span></tt> type, which it stores as a member.</p>
<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
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>
</tbody>
</table>
<p>The user of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> creates a class derived from an
instantiation of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> and then selectively
redefines some of the core member functions described in the table
above. The <tt class="literal"><span class="pre">Base</span></tt> type need not meet the full requirements for an
iterator. It need only support the operations used by the core
interface functions of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> that have not been
redefined in the user's derived class.</p>
<p>Several of the template parameters of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> default to
<tt class="literal"><span class="pre">use_default</span></tt>. This allows the user to make use of a default
parameter even when the user wants to specify a parameter later in the
parameter list. Also, the defaults for the corresponding associated
types are fairly complicated, so metaprogramming is required to
compute them, and <tt class="literal"><span class="pre">use_default</span></tt> can help to simplify the
implementation. Finally, <tt class="literal"><span class="pre">use_default</span></tt> is not left unspecified
because specification helps to highlight that the <tt class="literal"><span class="pre">Reference</span></tt>
template parameter may not always be identical to the iterator's
<tt class="literal"><span class="pre">reference</span></tt> type, and will keep users making mistakes based on that
assumption.</p>
</div>
<div class="section" id="reference">
<h1><a class="toc-backref" href="#id4" name="reference">Reference</a></h1>
<pre class="literal-block">
template &lt;
class Derived
, class Base
, class Value = use_default
, class Category = use_default
, class Reference = use_default
, class Difference = use_default
&gt;
class iterator_adaptor
: public iterator_facade&lt;Derived, /* see <a class="reference" href=":">details</a> ...*/&gt;
{
friend class iterator_core_access;
public:
iterator_adaptor();
explicit iterator_adaptor(Base iter);
Base base() const;
protected:
Base const&amp; base_reference() const;
Base&amp; base_reference();
private: // Core iterator interface for iterator_facade.
typename iterator_adaptor::reference dereference() const;
template &lt;
class OtherDerived, class OtherIterator, class V, class C, class R, class D
&gt;
bool equal(iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; x) const;
void advance(typename iterator_adaptor::difference_type n);
void increment();
void decrement();
template &lt;
class OtherDerived, class OtherIterator, class V, class C, class R, class D
&gt;
typename iterator_adaptor::difference_type distance_to(
iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; y) const;
private:
Base m_iterator;
};
</pre>
<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>
<p>The <tt class="literal"><span class="pre">Derived</span></tt> template parameter must be a derived class of
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>. The <tt class="literal"><span class="pre">Base</span></tt> type must implement the
expressions involving <tt class="literal"><span class="pre">m_iterator</span></tt> in the specifications of those
private member functions of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> that are not
redefined by the <tt class="literal"><span class="pre">Derived</span></tt> class and that are needed to model the
concept corresponding to the <tt class="literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="literal"><span class="pre">category</span></tt>
typedef according to the requirements of <tt class="literal"><span class="pre">iterator_facade</span></tt>. The
rest of the template parameters specify the types for the member
typedefs in <tt class="literal"><span class="pre">iterator_facade</span></tt>. The following pseudo-code
specifies the traits types for <tt class="literal"><span class="pre">iterator_adaptor</span></tt>.</p>
<pre class="literal-block">
if (Value == use_default)
value_type = iterator_traits&lt;Base&gt;::value_type;
else
value_type = remove_cv&lt;Value&gt;::type;
if (Reference == use_default) {
if (Value == use_default)
reference = iterator_traits&lt;Base&gt;::reference;
else
reference = Value&amp;;
} else
reference = Reference;
if (Distance == use_default)
difference_type = iterator_traits&lt;Base&gt;::difference_type;
else
difference_type = Distance;
if (Category == use_default)
iterator_category = iterator_tag&lt;
access_category&lt; Base &gt;,
traversal_category&lt; Base &gt;
&gt;
else if (Category is an access tag)
iterator_category = iterator_tag&lt;
Category
...
else if (Category is a traversal tag)
...
else
iterator_category = Category;
// Actually the above is wrong. See the use of
// access_category_tag and
// new_category_to_access/iter_category_to_access.
</pre>
<!-- Replaced with new semantics - -thw
if (Category == use_default)
iterator_category = iterator_traits<Base>::iterator_category;
else
iterator_category = Category;
Fix this up!! -->
</div>
<div class="section" id="iterator-adaptor-public-operations">
<h2><a class="toc-backref" href="#id6" 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" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body">The <tt class="literal"><span class="pre">Base</span></tt> type must be Default Constructible.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> with
<tt class="literal"><span class="pre">m_iterator</span></tt> default constructed.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">explicit</span> <span class="pre">iterator_adaptor(Base</span> <span class="pre">iter);</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 instance of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> with
<tt class="literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="literal"><span class="pre">iter</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Base</span> <span class="pre">base()</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">m_iterator</span></tt></td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-adaptor-protected-member-functions">
<h2><a class="toc-backref" href="#id7" 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" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A const reference to <tt class="literal"><span class="pre">m_iterator</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Base&amp;</span> <span class="pre">base_reference();</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">A non-const reference to <tt class="literal"><span class="pre">m_iterator</span></tt>.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-adaptor-private-member-functions">
<h2><a class="toc-backref" href="#id8" 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" />
<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">*m_iterator</span></tt></td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template &lt;
class OtherDerived, class OtherIterator, class V, class C, class R, class D
&gt;
bool equal(iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; x) const;
</pre>
<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">m_iterator</span> <span class="pre">==</span> <span class="pre">x.base()</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">void</span> <span class="pre">advance(typename</span> <span class="pre">iterator_adaptor::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"><tt class="literal"><span class="pre">m_iterator</span> <span class="pre">+=</span> <span class="pre">n;</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">void</span> <span class="pre">increment();</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"><tt class="literal"><span class="pre">++m_iterator;</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">void</span> <span class="pre">decrement();</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"><tt class="literal"><span class="pre">--m_iterator;</span></tt></td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template &lt;
class OtherDerived, class OtherIterator, class V, class C, class R, class D
&gt;
typename iterator_adaptor::difference_type distance_to(
iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; y) const;
</pre>
<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">y.base()</span> <span class="pre">-</span> <span class="pre">m_iterator</span></tt></td>
</tr>
</tbody>
</table>
</div>
</div>
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