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 12<h1 class="title">The Boost.Iterator Library <a class="reference external" href="../../../index.htm"><img alt="Boost" src="../../../boost.png" /></a></h1>
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 18<table class="docutils field-list" frame="void" rules="none">
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 21<tbody valign="top">
 22<tr class="field"><th class="field-name">Authors:</th><td class="field-body">David Abrahams, Jeremy Siek, Thomas Witt</td>
 24<tr class="field"><th class="field-name">Contact:</th><td class="field-body"><a class="reference external" href="mailto:dave&#64;">dave&#64;</a>, <a class="reference external" href="mailto:jsiek&#64;">jsiek&#64;</a>, <a class="reference external" href="mailto:witt&#64;">witt&#64;</a></td>
 26<tr class="field"><th class="field-name">organizations:</th><td class="field-body"><a class="reference external" href="">Boost Consulting</a>, Indiana University <a class="reference external" href="">Open Systems
 27Lab</a>, <a class="reference external" href="">Zephyr Associates, Inc.</a></td>
 29<tr class="field"><th class="field-name">date:</th><td class="field-body">$Date: 2011-03-30 08:17:11 +1100 (Wed, 30 Mar 2011) $</td>
 31<tr class="field"><th class="field-name">copyright:</th><td class="field-body">Copyright David Abrahams, Jeremy Siek, Thomas Witt 2003.</td>
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 39<tr class="field"><th class="field-name">Abstract:</th><td class="field-body">The Boost Iterator Library contains two parts. The first
 40is a system of <a class="reference external" href="">concepts</a> which extend the C++ standard
 41iterator requirements. The second is a framework of
 42components for building iterators based on these
 43extended concepts and includes several useful iterator
 44adaptors. The extended iterator concepts have been
 45carefully designed so that old-style iterators
 46can fit in the new concepts and so that new-style
 47iterators will be compatible with old-style algorithms,
 48though algorithms may need to be updated if they want to
 49take full advantage of the new-style iterator
 50capabilities.  Several components of this library have
 51been accepted into the C++ standard technical report.
 52The components of the Boost Iterator Library replace the
 53older Boost Iterator Adaptor Library.</td>
 57<div class="contents topic" id="table-of-contents">
 58<p class="topic-title first"><strong>Table of Contents</strong></p>
 59<ul class="simple">
 60<li><a class="reference internal" href="#new-style-iterators" id="id23">New-Style Iterators</a></li>
 61<li><a class="reference internal" href="#iterator-facade-and-adaptor" id="id24">Iterator Facade and Adaptor</a></li>
 62<li><a class="reference internal" href="#specialized-adaptors" id="id25">Specialized Adaptors</a></li>
 63<li><a class="reference internal" href="#iterator-utilities" id="id26">Iterator Utilities</a><ul>
 64<li><a class="reference internal" href="#traits" id="id27">Traits</a></li>
 65<li><a class="reference internal" href="#testing-and-concept-checking" id="id28">Testing and Concept Checking</a></li>
 68<li><a class="reference internal" href="#upgrading-from-the-old-boost-iterator-adaptor-library" id="id29">Upgrading from the old Boost Iterator Adaptor Library</a></li>
 69<li><a class="reference internal" href="#history" id="id30">History</a></li>
 72<hr class="docutils" />
 73<div class="section" id="new-style-iterators">
 74<h1><a class="toc-backref" href="#id23">New-Style Iterators</a></h1>
 75<p>The iterator categories defined in C++98 are extremely limiting
 76because they bind together two orthogonal concepts: traversal and
 77element access.  For example, because a random access iterator is
 78required to return a reference (and not a proxy) when dereferenced,
 79it is impossible to capture the capabilities of
 80<tt class="docutils literal"><span class="pre">vector&lt;bool&gt;::iterator</span></tt> using the C++98 categories.  This is the
 81infamous &quot;<tt class="docutils literal"><span class="pre">vector&lt;bool&gt;</span></tt> is not a container, and its iterators
 82aren't random access iterators&quot;, debacle about which Herb Sutter
 83wrote two papers for the standards comittee (<a class="reference external" href="">n1185</a> and <a class="reference external" href="">n1211</a>),
 84and a <a class="reference external" href="">Guru of the Week</a>.  New-style iterators go well beyond
 85patching up <tt class="docutils literal"><span class="pre">vector&lt;bool&gt;</span></tt>, though: there are lots of other
 86iterators already in use which can't be adequately represented by
 87the existing concepts.  For details about the new iterator
 88concepts, see our</p>
 90<a class="reference external" href="new-iter-concepts.html">Standard Proposal For New-Style Iterators</a> (<a class="reference external" href="new-iter-concepts.pdf">PDF</a>)</blockquote>
 92<div class="section" id="iterator-facade-and-adaptor">
 93<h1><a class="toc-backref" href="#id24">Iterator Facade and Adaptor</a></h1>
 94<p>Writing standard-conforming iterators is tricky, but the need comes
 95up often.  In order to ease the implementation of new iterators,
 96the Boost.Iterator library provides the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> class template,
 97which implements many useful defaults and compile-time checks
 98designed to help the iterator author ensure that his iterator is
100<p>It is also common to define a new iterator that is similar to some
101underlying iterator or iterator-like type, but that modifies some
102aspect of the underlying type's behavior.  For that purpose, the
103library supplies the <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template, which is specially
104designed to take advantage of as much of the underlying type's
105behavior as possible.</p>
106<p>The documentation for these two classes can be found at the following
107web pages:</p>
108<ul class="simple">
109<li><a class="reference external" href="iterator_facade.html"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a> (<a class="reference external" href="iterator_facade.pdf">PDF</a>)</li>
110<li><a class="reference external" href="iterator_adaptor.html"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt></a> (<a class="reference external" href="iterator_adaptor.pdf">PDF</a>)</li>
112<p>Both <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> as well as many of the <a class="reference internal" href="#specialized-adaptors">specialized
113adaptors</a> mentioned below have been proposed for standardization,
114and accepted into the first C++ technical report; see our</p>
116<a class="reference external" href="facade-and-adaptor.html">Standard Proposal For Iterator Facade and Adaptor</a> (<a class="reference external" href="facade-and-adaptor.pdf">PDF</a>)</blockquote>
117<p>for more details.</p>
119<div class="section" id="specialized-adaptors">
120<h1><a class="toc-backref" href="#id25">Specialized Adaptors</a></h1>
121<p>The iterator library supplies a useful suite of standard-conforming
122iterator templates based on the Boost <a class="reference internal" href="#iterator-facade-and-adaptor">iterator facade and adaptor</a>.</p>
123<ul class="simple">
124<li><a class="reference external" href="counting_iterator.html"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a> (<a class="reference external" href="counting_iterator.pdf">PDF</a>): an iterator over a sequence of consecutive values.
125Implements a &quot;lazy sequence&quot;</li>
126<li><a class="reference external" href="filter_iterator.html"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt></a> (<a class="reference external" href="filter_iterator.pdf">PDF</a>): an iterator over the subset of elements of some
127sequence which satisfy a given predicate</li>
128<li><a class="reference external" href="function_input_iterator.html"><tt class="docutils literal"><span class="pre">function_input_iterator</span></tt></a> (<a class="reference external" href="function_input_iterator.pdf">PDF</a>): an input iterator wrapping a generator (nullary
129function object); each time the iterator is dereferenced, the function object
130is called to get the value to return.</li>
131<li><a class="reference external" href="function_output_iterator.html"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt></a> (<a class="reference external" href="function_output_iterator.pdf">PDF</a>): an output iterator wrapping a unary function
132object; each time an element is written into the dereferenced
133iterator, it is passed as a parameter to the function object.</li>
134<li><a class="reference external" href="indirect_iterator.html"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt></a> (<a class="reference external" href="indirect_iterator.pdf">PDF</a>): an iterator over the objects <em>pointed-to</em> by the
135elements of some sequence.</li>
136<li><a class="reference external" href="permutation_iterator.html"><tt class="docutils literal"><span class="pre">permutation_iterator</span></tt></a> (<a class="reference external" href="permutation_iterator.pdf">PDF</a>): an iterator over the elements of some random-access
137sequence, rearranged according to some sequence of integer indices.</li>
138<li><a class="reference external" href="reverse_iterator.html"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt></a> (<a class="reference external" href="reverse_iterator.pdf">PDF</a>): an iterator which traverses the elements of some
139bidirectional sequence in reverse.  Corrects many of the
140shortcomings of C++98's <tt class="docutils literal"><span class="pre">std::reverse_iterator</span></tt>.</li>
141<li><a class="reference external" href="../../utility/shared_container_iterator.html"><tt class="docutils literal"><span class="pre">shared_container_iterator</span></tt></a>: an iterator over elements of a container whose
142lifetime is maintained by a <a class="reference external" href="../../smart_ptr/shared_ptr.htm"><tt class="docutils literal"><span class="pre">shared_ptr</span></tt></a> stored in the iterator.</li>
143<li><a class="reference external" href="transform_iterator.html"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt></a> (<a class="reference external" href="transform_iterator.pdf">PDF</a>): an iterator over elements which are the result of
144applying some functional transformation to the elements of an
145underlying sequence.  This component also replaces the old
146<tt class="docutils literal"><span class="pre">projection_iterator_adaptor</span></tt>.</li>
147<li><a class="reference external" href="zip_iterator.html"><tt class="docutils literal"><span class="pre">zip_iterator</span></tt></a> (<a class="reference external" href="zip_iterator.pdf">PDF</a>): an iterator over tuples of the elements at corresponding
148positions of heterogeneous underlying iterators.</li>
151<div class="section" id="iterator-utilities">
152<h1><a class="toc-backref" href="#id26">Iterator Utilities</a></h1>
153<div class="section" id="traits">
154<h2><a class="toc-backref" href="#id27">Traits</a></h2>
155<ul class="simple">
156<li><a class="reference external" href="pointee.html"><tt class="docutils literal"><span class="pre">pointee.hpp</span></tt></a> (<a class="reference external" href="pointee.pdf">PDF</a>): Provides the capability to deduce the referent types
157of pointers, smart pointers and iterators in generic code.  Used
158in <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt>.</li>
159<li><a class="reference external" href="iterator_traits.html"><tt class="docutils literal"><span class="pre">iterator_traits.hpp</span></tt></a> (<a class="reference external" href="iterator_traits.pdf">PDF</a>): Provides <a class="reference external" href="../../mpl/doc/index.html">MPL</a>-compatible metafunctions which
160retrieve an iterator's traits.  Also corrects for the deficiencies
161of broken implementations of <tt class="docutils literal"><span class="pre">std::iterator_traits</span></tt>.</li>
163<!-- * |interoperable|_ (PDF__): Provides an MPL_\ -compatible metafunction for
164testing iterator interoperability -->
165<!-- comment! __ interoperable.pdf -->
167<div class="section" id="testing-and-concept-checking">
168<h2><a class="toc-backref" href="#id28">Testing and Concept Checking</a></h2>
169<ul class="simple">
170<li><a class="reference external" href="iterator_concepts.html"><tt class="docutils literal"><span class="pre">iterator_concepts.hpp</span></tt></a> (<a class="reference external" href="iterator_concepts.pdf">PDF</a>): Concept checking classes for the new iterator concepts.</li>
171<li><a class="reference external" href="iterator_archetypes.html"><tt class="docutils literal"><span class="pre">iterator_archetypes.hpp</span></tt></a> (<a class="reference external" href="iterator_archetypes.pdf">PDF</a>): Concept archetype classes for the new iterators concepts.</li>
175<div class="section" id="upgrading-from-the-old-boost-iterator-adaptor-library">
176<h1><a class="toc-backref" href="#id29">Upgrading from the old Boost Iterator Adaptor Library</a></h1>
177<p id="upgrading">If you have been using the old Boost Iterator Adaptor library to
178implement iterators, you probably wrote a <tt class="docutils literal"><span class="pre">Policies</span></tt> class which
179captures the core operations of your iterator.  In the new library
180design, you'll move those same core operations into the body of the
181iterator class itself.  If you were writing a family of iterators,
182you probably wrote a <a class="reference external" href="">type generator</a> to build the
183<tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> specialization you needed; in the new library
184design you don't need a type generator (though may want to keep it
185around as a compatibility aid for older code) because, due to the
186use of the Curiously Recurring Template Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id22">[Cop95]</a>,
187you can now define the iterator class yourself and acquire
188functionality through inheritance from <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> or
189<tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>.  As a result, you also get much finer control
190over how your iterator works: you can add additional constructors,
191or even override the iterator functionality provided by the
193<p>If you're looking for the old <tt class="docutils literal"><span class="pre">projection_iterator</span></tt> component,
194its functionality has been merged into <tt class="docutils literal"><span class="pre">transform_iterator</span></tt>: as
195long as the function object's <tt class="docutils literal"><span class="pre">result_type</span></tt> (or the <tt class="docutils literal"><span class="pre">Reference</span></tt>
196template argument, if explicitly specified) is a true reference
197type, <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> will behave like
198<tt class="docutils literal"><span class="pre">projection_iterator</span></tt> used to.</p>
200<div class="section" id="history">
201<h1><a class="toc-backref" href="#id30">History</a></h1>
202<p>In 2000 Dave Abrahams was writing an iterator for a container of
203pointers, which would access the pointed-to elements when
204dereferenced.  Naturally, being a library writer, he decided to
205generalize the idea and the Boost Iterator Adaptor library was born.
206Dave was inspired by some writings of Andrei Alexandrescu and chose a
207policy based design (though he probably didn't capture Andrei's idea
208very well - there was only one policy class for all the iterator's
209orthogonal properties).  Soon Jeremy Siek realized he would need the
210library and they worked together to produce a &quot;Boostified&quot; version,
211which was reviewed and accepted into the library.  They wrote a paper
212and made several important revisions of the code.</p>
213<p>Eventually, several shortcomings of the older library began to make
214the need for a rewrite apparent.  Dave and Jeremy started working
215at the Santa Cruz C++ committee meeting in 2002, and had quickly
216generated a working prototype.  At the urging of Mat Marcus, they
217decided to use the GenVoca/CRTP pattern approach, and moved the
218policies into the iterator class itself.  Thomas Witt expressed
219interest and became the voice of strict compile-time checking for
220the project, adding uses of the SFINAE technique to eliminate false
221converting constructors and operators from the overload set.  He
222also recognized the need for a separate <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, and
223factored it out of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>.  Finally, after a
224near-complete rewrite of the prototype, they came up with the
225library you see today.</p>
226<table class="docutils citation" frame="void" id="cop95" rules="none">
227<colgroup><col class="label" /><col /></colgroup>
228<tbody valign="top">
229<tr><td class="label"><a class="fn-backref" href="#id22">[Cop95]</a></td><td>[Coplien, 1995] Coplien, J., Curiously Recurring Template
230Patterns, C++ Report, February 1995, pp. 24-27.</td></tr>
233<!-- LocalWords:  Abrahams Siek Witt const bool Sutter's WG int UL LI href Lvalue
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236LocalWords:  ForwardTraversalIterator BidirectionalTraversalIterator lvalue
237LocalWords:  RandomAccessTraversalIterator dereferenceable Incrementable tmp
238LocalWords:  incrementable xxx min prev inplace png oldeqnew AccessTag struct
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