<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="generator" content="Docutils 0.5: http://docutils.sourceforge.net/" /> <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="2006-09-11" /> <meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." /> <link rel="stylesheet" href="../../../rst.css" type="text/css" /> </head> <body> <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 external" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>, <a class="last reference external" href="mailto:witt@ive.uni-hannover.de">witt@ive.uni-hannover.de</a></td></tr> <tr><th class="docinfo-name">Organization:</th> <td><a class="first reference external" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference external" href="http://www.osl.iu.edu">Open Systems Lab</a>, University of Hanover <a class="last reference external" 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>2006-09-11</td></tr> <tr><th class="docinfo-name">Copyright:</th> <td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr> </tbody> </table> <!-- Distributed under 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) --> <table class="docutils 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> <!-- Distributed under 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) --> <!-- Version 1.1 of this ReStructuredText document corresponds to n1530_, the paper accepted by the LWG. --> <!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. --> <p>Each specialization of the <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template is derived from a specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>. The core interface functions expected by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> are implemented in terms of the <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="docutils literal"><span class="pre">Base</span></tt> template parameter. A class derived from <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> typically redefines some of the core interface functions to adapt the behavior of the <tt class="docutils 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="docutils literal"><span class="pre">Base</span></tt> type and which core interface functions of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> are redefined in the <tt class="docutils literal"><span class="pre">Derived</span></tt> class.</p> <div class="contents topic" id="table-of-contents"> <p class="topic-title first">Table of Contents</p> <ul class="simple"> <li><a class="reference internal" href="#overview" id="id6">Overview</a></li> <li><a class="reference internal" href="#reference" id="id7">Reference</a><ul> <li><a class="reference internal" href="#iterator-adaptor-requirements" id="id8"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></li> <li><a class="reference internal" href="#iterator-adaptor-base-class-parameters" id="id9"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></li> <li><a class="reference internal" href="#iterator-adaptor-public-operations" id="id10"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></li> <li><a class="reference internal" href="#iterator-adaptor-protected-member-functions" id="id11"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></li> <li><a class="reference internal" href="#iterator-adaptor-private-member-functions" id="id12"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></li> </ul> </li> <li><a class="reference internal" href="#tutorial-example" id="id13">Tutorial Example</a></li> </ul> </div> <div class="section" id="overview"> <h1><a class="toc-backref" href="#id6">Overview</a></h1> <!-- Distributed under 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) --> <!-- 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. --> <p>The <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template adapts some <tt class="docutils literal"><span class="pre">Base</span></tt><a class="footnote-reference" href="#base" id="id1"><sup>1</sup></a> type to create a new iterator. Instantiations of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> are derived from a corresponding instantiation of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and implement the core behaviors in terms of the <tt class="docutils literal"><span class="pre">Base</span></tt> type. In essence, <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> merely forwards all operations to an instance of the <tt class="docutils literal"><span class="pre">Base</span></tt> type, which it stores as a member.</p> <table class="docutils footnote" frame="void" id="base" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label">[1]</td><td><em>(<a class="fn-backref" href="#id1">1</a>, <a class="fn-backref" href="#id3">2</a>)</em> The term "Base" 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="docutils literal"><span class="pre">reverse_iterator</span></tt> adaptor.</td></tr> </tbody> </table> <p>The user of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> creates a class derived from an instantiation of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> and then selectively redefines some of the core member functions described in the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> core requirements table. The <tt class="docutils 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="docutils 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="docutils literal"><span class="pre">iterator_adaptor</span></tt> default to <tt class="docutils literal"><span class="pre">use_default</span></tt>. This allows the user to make use of a default parameter even when she wants to specify a parameter later in the parameter list. Also, the defaults for the corresponding associated types are somewhat complicated, so metaprogramming is required to compute them, and <tt class="docutils literal"><span class="pre">use_default</span></tt> can help to simplify the implementation. Finally, the identity of the <tt class="docutils literal"><span class="pre">use_default</span></tt> type is not left unspecified because specification helps to highlight that the <tt class="docutils literal"><span class="pre">Reference</span></tt> template parameter may not always be identical to the iterator's <tt class="docutils literal"><span class="pre">reference</span></tt> type, and will keep users from making mistakes based on that assumption.</p> </div> <div class="section" id="reference"> <h1><a class="toc-backref" href="#id7">Reference</a></h1> <!-- Distributed under 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) --> <!-- 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. --> <pre class="literal-block"> template < class Derived , class Base , class Value = use_default , class CategoryOrTraversal = use_default , class Reference = use_default , class Difference = use_default > class iterator_adaptor : public iterator_facade<Derived, <em>V'</em>, <em>C'</em>, <em>R'</em>, <em>D'</em>> // see <a class="reference internal" href="#base-parameters">details</a> { friend class iterator_core_access; public: iterator_adaptor(); explicit iterator_adaptor(Base const& iter); typedef Base base_type; Base const& base() const; protected: typedef iterator_adaptor iterator_adaptor_; Base const& base_reference() const; Base& base_reference(); private: // Core iterator interface for iterator_facade. typename iterator_adaptor::reference dereference() const; template < class OtherDerived, class OtherIterator, class V, class C, class R, class D > bool equal(iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& x) const; void advance(typename iterator_adaptor::difference_type n); void increment(); void decrement(); template < class OtherDerived, class OtherIterator, class V, class C, class R, class D > typename iterator_adaptor::difference_type distance_to( iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& y) const; private: Base m_iterator; // exposition only }; </pre> <div class="section" id="iterator-adaptor-requirements"> <span id="requirements"></span><h2><a class="toc-backref" href="#id8"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> requirements</a></h2> <p><tt class="docutils literal"><span class="pre">static_cast<Derived*>(iterator_adaptor*)</span></tt> shall be well-formed. The <tt class="docutils literal"><span class="pre">Base</span></tt> argument shall be Assignable and Copy Constructible.</p> </div> <div class="section" id="iterator-adaptor-base-class-parameters"> <span id="base-parameters"></span><h2><a class="toc-backref" href="#id9"><tt class="docutils 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="docutils literal"><span class="pre">iterator_facade</span></tt> used as a base class in the summary of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> above are defined as follows:</p> <pre class="literal-block"> <em>V'</em> = if (Value is use_default) return iterator_traits<Base>::value_type else return Value <em>C'</em> = if (CategoryOrTraversal is use_default) return iterator_traversal<Base>::type else return CategoryOrTraversal <em>R'</em> = if (Reference is use_default) if (Value is use_default) return iterator_traits<Base>::reference else return Value& else return Reference <em>D'</em> = if (Difference is use_default) return iterator_traits<Base>::difference_type else return Difference </pre> <!-- ``iterator_adaptor`` models - - - - - - - - - - - - - - - - - - - - - - - - - - - In order for ``Derived`` to model the iterator concepts corresponding to ``iterator_traits<Derived>::iterator_category``, the expressions involving ``m_iterator`` in the specifications of those private member functions of ``iterator_adaptor`` that may be called by ``iterator_facade<Derived, V, C, R, D>`` in evaluating any valid expression involving ``Derived`` in those concepts' requirements. --> <!-- The above is confusing and needs a rewrite. -JGS --> <!-- That's why it's removed. We're embracing inheritance, remember? --> </div> <div class="section" id="iterator-adaptor-public-operations"> <h2><a class="toc-backref" href="#id10"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></h2> <p><tt class="docutils literal"><span class="pre">iterator_adaptor();</span></tt></p> <table class="docutils 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="docutils 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="docutils literal"><span class="pre">iterator_adaptor</span></tt> with <tt class="docutils literal"><span class="pre">m_iterator</span></tt> default constructed.</td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">iterator_adaptor(Base</span> <span class="pre">const&</span> <span class="pre">iter);</span></tt></p> <table class="docutils 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="docutils literal"><span class="pre">iterator_adaptor</span></tt> with <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">iter</span></tt>.</td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">Base</span> <span class="pre">const&</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p> <table class="docutils 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="docutils 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="#id11"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></h2> <p><tt class="docutils literal"><span class="pre">Base</span> <span class="pre">const&</span> <span class="pre">base_reference()</span> <span class="pre">const;</span></tt></p> <table class="docutils 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="docutils literal"><span class="pre">m_iterator</span></tt>.</td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">Base&</span> <span class="pre">base_reference();</span></tt></p> <table class="docutils 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="docutils 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="#id12"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></h2> <p><tt class="docutils 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="docutils 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="docutils literal"><span class="pre">*m_iterator</span></tt></td> </tr> </tbody> </table> <pre class="literal-block"> template < class OtherDerived, class OtherIterator, class V, class C, class R, class D > bool equal(iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& x) const; </pre> <table class="docutils 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="docutils 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="docutils 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="docutils 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="docutils literal"><span class="pre">m_iterator</span> <span class="pre">+=</span> <span class="pre">n;</span></tt></td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">increment();</span></tt></p> <table class="docutils 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="docutils literal"><span class="pre">++m_iterator;</span></tt></td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">decrement();</span></tt></p> <table class="docutils 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="docutils literal"><span class="pre">--m_iterator;</span></tt></td> </tr> </tbody> </table> <pre class="literal-block"> template < class OtherDerived, class OtherIterator, class V, class C, class R, class D > typename iterator_adaptor::difference_type distance_to( iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& y) const; </pre> <table class="docutils 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="docutils literal"><span class="pre">y.base()</span> <span class="pre">-</span> <span class="pre">m_iterator</span></tt></td> </tr> </tbody> </table> </div> </div> <div class="section" id="tutorial-example"> <h1><a class="toc-backref" href="#id13">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="docutils literal"><span class="pre">node_iter</span></tt> class template we developed in the <a class="reference external" href="iterator_facade.html#tutorial-example"><tt class="docutils 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="first sidebar-title"><tt class="docutils literal"><span class="pre">node_base*</span></tt> really <em>is</em> an iterator</p> <p class="last">It's not really a very interesting iterator, since <tt class="docutils literal"><span class="pre">node_base</span></tt> is an abstract class: a pointer to a <tt class="docutils 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="docutils 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="docutils literal"><span class="pre">node_base*</span></tt> to it. In other words, the original iterator traverses a one-element array.</p> </div> <p>You probably didn't think of it this way, but the <tt class="docutils literal"><span class="pre">node_base*</span></tt> object that underlies <tt class="docutils 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="docutils literal"><span class="pre">node_iterator</span></tt> we're building. First, they share most of the same associated types (<tt class="docutils literal"><span class="pre">value_type</span></tt>, <tt class="docutils literal"><span class="pre">reference</span></tt>, <tt class="docutils literal"><span class="pre">pointer</span></tt>, and <tt class="docutils literal"><span class="pre">difference_type</span></tt>). Second, even some of the core functionality is the same: <tt class="docutils literal"><span class="pre">operator*</span></tt> and <tt class="docutils literal"><span class="pre">operator==</span></tt> on the <tt class="docutils literal"><span class="pre">node_iterator</span></tt> return the result of invoking the same operations on the underlying pointer, via the <tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s <a class="reference external" href="iterator_facade.html#implementing-the-core-operations"><tt class="docutils literal"><span class="pre">dereference</span></tt> and <tt class="docutils literal"><span class="pre">equal</span></tt> member functions</a>). The only real behavioral difference between <tt class="docutils literal"><span class="pre">node_base*</span></tt> and <tt class="docutils literal"><span class="pre">node_iterator</span></tt> can be observed when they are incremented: <tt class="docutils literal"><span class="pre">node_iterator</span></tt> follows the <tt class="docutils literal"><span class="pre">m_next</span></tt> pointer, while <tt class="docutils literal"><span class="pre">node_base*</span></tt> just applies an address offset.</p> <p>It turns out that the pattern of building an iterator on another iterator-like type (the <tt class="docutils literal"><span class="pre">Base</span></tt><a class="footnote-reference" href="#base" id="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="docutils literal"><span class="pre">iterator_adaptor</span></tt>. Using <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> is very much like using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, but because iterator_adaptor tries to mimic as much of the <tt class="docutils literal"><span class="pre">Base</span></tt> type's behavior as possible, we neither have to supply a <tt class="docutils literal"><span class="pre">Value</span></tt> argument, nor implement any core behaviors other than <tt class="docutils literal"><span class="pre">increment</span></tt>. The implementation of <tt class="docutils literal"><span class="pre">node_iter</span></tt> is thus reduced to:</p> <pre class="literal-block"> template <class Value> class node_iter : public boost::iterator_adaptor< node_iter<Value> // Derived , Value* // Base , boost::use_default // Value , boost::forward_traversal_tag // CategoryOrTraversal > { private: struct enabler {}; // a private type avoids misuse public: node_iter() : node_iter::iterator_adaptor_(0) {} explicit node_iter(Value* p) : node_iter::iterator_adaptor_(p) {} template <class OtherValue> node_iter( node_iter<OtherValue> const& other , typename boost::enable_if< boost::is_convertible<OtherValue*,Value*> , enabler >::type = enabler() ) : node_iter::iterator_adaptor_(other.base()) {} private: friend class boost::iterator_core_access; void increment() { this->base_reference() = this->base()->next(); } }; </pre> <p>Note the use of <tt class="docutils literal"><span class="pre">node_iter::iterator_adaptor_</span></tt> here: because <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> defines a nested <tt class="docutils literal"><span class="pre">iterator_adaptor_</span></tt> type that refers to itself, that gives us a convenient way to refer to the complicated base class type of <tt class="docutils literal"><span class="pre">node_iter<Value></span></tt>. [Note: this technique is known not to work with Borland C++ 5.6.4 and Metrowerks CodeWarrior versions prior to 9.0]</p> <p>You can see an example program that exercises this version of the node iterators <a class="reference external" href="../example/node_iterator3.cpp">here</a>.</p> <p>In the case of <tt class="docutils literal"><span class="pre">node_iter</span></tt>, it's not very compelling to pass <tt class="docutils literal"><span class="pre">boost::use_default</span></tt> as <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="docutils literal"><span class="pre">Value</span></tt> argument; we could have just passed <tt class="docutils literal"><span class="pre">node_iter</span></tt>'s <tt class="docutils literal"><span class="pre">Value</span></tt> along to <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>, and that'd even be shorter! Most iterator class templates built with <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> are parameterized on another iterator type, rather than on its <tt class="docutils literal"><span class="pre">value_type</span></tt>. For example, <tt class="docutils 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="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s delegation of default types to its <tt class="docutils literal"><span class="pre">Base</span></tt> saves the implementor of <tt class="docutils literal"><span class="pre">boost::reverse_iterator</span></tt> from writing:</p> <pre class="literal-block"> std::iterator_traits<Iterator>::<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 external" href="reverse_iterator.html"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt></a> and the other Boost <a class="reference external" href="index.html#specialized-adaptors">specialized iterator adaptors</a> to get an idea of the sorts of things you can do with <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>. In particular, have a look at <a class="reference external" href="transform_iterator.html"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt></a>, which is perhaps the most straightforward adaptor, and also <a class="reference external" href="counting_iterator.html"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a>, which demonstrates that <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="docutils literal"><span class="pre">Base</span></tt> type needn't be an iterator.</p> </div> </div> <div class="footer"> <hr class="footer" /> <a class="reference external" href="iterator_adaptor.rst">View document source</a>. Generated by <a class="reference external" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference external" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source. </div> </body> </html>