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<title>The Boost.Iterator Library Boost</title>
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<link rel="stylesheet" href="../../../rst.css" type="text/css" />
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<div class="document" id="the-boost-iterator-library-logo">
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<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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<!-- Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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<hr class="docutils" />
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">Authors:</th><td class="field-body">David Abrahams, Jeremy Siek, Thomas Witt</td>
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</tr>
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<tr class="field"><th class="field-name">Contact:</th><td class="field-body"><a class="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="reference external" href="mailto:witt@styleadvisor.com">witt@styleadvisor.com</a></td>
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</tr>
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<tr class="field"><th class="field-name">organizations:</th><td class="field-body"><a class="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
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Lab</a>, <a class="reference external" href="http://www.styleadvisor.com">Zephyr Associates, Inc.</a></td>
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</tr>
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<tr class="field"><th class="field-name">date:</th><td class="field-body">$Date$</td>
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<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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</tr>
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</tbody>
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</table>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<tr class="field"><th class="field-name">Abstract:</th><td class="field-body">The Boost Iterator Library contains two parts. The first
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is a system of <a class="reference external" href="http://www.boost.org/more/generic_programming.html#concept">concepts</a> which extend the C++ standard
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iterator requirements. The second is a framework of
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components for building iterators based on these
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extended concepts and includes several useful iterator
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adaptors. The extended iterator concepts have been
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carefully designed so that old-style iterators
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can fit in the new concepts and so that new-style
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iterators will be compatible with old-style algorithms,
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though algorithms may need to be updated if they want to
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take full advantage of the new-style iterator
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capabilities. Several components of this library have
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been accepted into the C++ standard technical report.
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The components of the Boost Iterator Library replace the
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older Boost Iterator Adaptor Library.</td>
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</tr>
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</tbody>
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</table>
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<div class="contents topic" id="table-of-contents">
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<p class="topic-title first"><strong>Table of Contents</strong></p>
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<ul class="simple">
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<li><a class="reference internal" href="#new-style-iterators" id="id23">New-Style Iterators</a></li>
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<li><a class="reference internal" href="#iterator-facade-and-adaptor" id="id24">Iterator Facade and Adaptor</a></li>
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<li><a class="reference internal" href="#specialized-adaptors" id="id25">Specialized Adaptors</a></li>
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<li><a class="reference internal" href="#iterator-utilities" id="id26">Iterator Utilities</a><ul>
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<li><a class="reference internal" href="#traits" id="id27">Traits</a></li>
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<li><a class="reference internal" href="#testing-and-concept-checking" id="id28">Testing and Concept Checking</a></li>
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</ul>
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</li>
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<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>
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<li><a class="reference internal" href="#history" id="id30">History</a></li>
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</ul>
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</div>
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<hr class="docutils" />
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<div class="section" id="new-style-iterators">
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<h1><a class="toc-backref" href="#id23">New-Style Iterators</a></h1>
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<p>The iterator categories defined in C++98 are extremely limiting
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because they bind together two orthogonal concepts: traversal and
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element access. For example, because a random access iterator is
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required to return a reference (and not a proxy) when dereferenced,
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it is impossible to capture the capabilities of
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<tt class="docutils literal"><span class="pre">vector<bool>::iterator</span></tt> using the C++98 categories. This is the
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infamous "<tt class="docutils literal"><span class="pre">vector<bool></span></tt> is not a container, and its iterators
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aren't random access iterators", debacle about which Herb Sutter
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wrote two papers for the standards comittee (<a class="reference external" href="http://www.gotw.ca/publications/N1185.pdf">n1185</a> and <a class="reference external" href="http://www.gotw.ca/publications/N1211.pdf">n1211</a>),
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and a <a class="reference external" href="http://www.gotw.ca/gotw/050.htm">Guru of the Week</a>. New-style iterators go well beyond
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patching up <tt class="docutils literal"><span class="pre">vector<bool></span></tt>, though: there are lots of other
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iterators already in use which can't be adequately represented by
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the existing concepts. For details about the new iterator
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concepts, see our</p>
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<blockquote>
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<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>
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</div>
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<div class="section" id="iterator-facade-and-adaptor">
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<h1><a class="toc-backref" href="#id24">Iterator Facade and Adaptor</a></h1>
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<p>Writing standard-conforming iterators is tricky, but the need comes
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up often. In order to ease the implementation of new iterators,
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the Boost.Iterator library provides the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> class template,
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which implements many useful defaults and compile-time checks
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designed to help the iterator author ensure that his iterator is
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correct.</p>
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<p>It is also common to define a new iterator that is similar to some
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underlying iterator or iterator-like type, but that modifies some
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aspect of the underlying type's behavior. For that purpose, the
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library supplies the <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template, which is specially
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designed to take advantage of as much of the underlying type's
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behavior as possible.</p>
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<p>The documentation for these two classes can be found at the following
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web pages:</p>
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<ul class="simple">
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<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>
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<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>
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</ul>
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<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
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adaptors</a> mentioned below have been proposed for standardization,
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and accepted into the first C++ technical report; see our</p>
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<blockquote>
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<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>
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<p>for more details.</p>
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</div>
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<div class="section" id="specialized-adaptors">
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<h1><a class="toc-backref" href="#id25">Specialized Adaptors</a></h1>
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<p>The iterator library supplies a useful suite of standard-conforming
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iterator templates based on the Boost <a class="reference internal" href="#iterator-facade-and-adaptor">iterator facade and adaptor</a>.</p>
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<ul class="simple">
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<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.
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Implements a "lazy sequence"</li>
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<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
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sequence which satisfy a given predicate</li>
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<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
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function object); each time the iterator is dereferenced, the function object
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is called to get the value to return.</li>
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<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
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object; each time an element is written into the dereferenced
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iterator, it is passed as a parameter to the function object.</li>
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<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
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elements of some sequence.</li>
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<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
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sequence, rearranged according to some sequence of integer indices.</li>
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<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
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bidirectional sequence in reverse. Corrects many of the
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shortcomings of C++98's <tt class="docutils literal"><span class="pre">std::reverse_iterator</span></tt>.</li>
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<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
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lifetime 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>
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<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
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applying some functional transformation to the elements of an
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underlying sequence. This component also replaces the old
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<tt class="docutils literal"><span class="pre">projection_iterator_adaptor</span></tt>.</li>
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<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
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positions of heterogeneous underlying iterators.</li>
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</ul>
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</div>
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<div class="section" id="iterator-utilities">
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<h1><a class="toc-backref" href="#id26">Iterator Utilities</a></h1>
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<div class="section" id="traits">
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<h2><a class="toc-backref" href="#id27">Traits</a></h2>
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<ul class="simple">
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<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
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of pointers, smart pointers and iterators in generic code. Used
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in <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt>.</li>
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<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
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retrieve an iterator's traits. Also corrects for the deficiencies
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of broken implementations of <tt class="docutils literal"><span class="pre">std::iterator_traits</span></tt>.</li>
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</ul>
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<!-- * |interoperable|_ (PDF__): Provides an MPL_\ -compatible metafunction for
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testing iterator interoperability -->
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<!-- comment! __ interoperable.pdf -->
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</div>
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<div class="section" id="testing-and-concept-checking">
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<h2><a class="toc-backref" href="#id28">Testing and Concept Checking</a></h2>
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<ul class="simple">
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<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>
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<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>
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</ul>
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</div>
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</div>
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<div class="section" id="upgrading-from-the-old-boost-iterator-adaptor-library">
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<h1><a class="toc-backref" href="#id29">Upgrading from the old Boost Iterator Adaptor Library</a></h1>
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<p id="upgrading">If you have been using the old Boost Iterator Adaptor library to
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implement iterators, you probably wrote a <tt class="docutils literal"><span class="pre">Policies</span></tt> class which
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captures the core operations of your iterator. In the new library
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design, you'll move those same core operations into the body of the
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iterator class itself. If you were writing a family of iterators,
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you probably wrote a <a class="reference external" href="http://www.boost.org/more/generic_programming.html#type_generator">type generator</a> to build the
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<tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> specialization you needed; in the new library
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design you don't need a type generator (though may want to keep it
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around as a compatibility aid for older code) because, due to the
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use of the Curiously Recurring Template Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id22">[Cop95]</a>,
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you can now define the iterator class yourself and acquire
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functionality through inheritance from <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> or
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<tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>. As a result, you also get much finer control
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over how your iterator works: you can add additional constructors,
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or even override the iterator functionality provided by the
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library.</p>
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<p>If you're looking for the old <tt class="docutils literal"><span class="pre">projection_iterator</span></tt> component,
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its functionality has been merged into <tt class="docutils literal"><span class="pre">transform_iterator</span></tt>: as
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long 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>
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template argument, if explicitly specified) is a true reference
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type, <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> will behave like
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<tt class="docutils literal"><span class="pre">projection_iterator</span></tt> used to.</p>
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</div>
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<div class="section" id="history">
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<h1><a class="toc-backref" href="#id30">History</a></h1>
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<p>In 2000 Dave Abrahams was writing an iterator for a container of
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pointers, which would access the pointed-to elements when
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dereferenced. Naturally, being a library writer, he decided to
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generalize the idea and the Boost Iterator Adaptor library was born.
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Dave was inspired by some writings of Andrei Alexandrescu and chose a
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policy based design (though he probably didn't capture Andrei's idea
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very well - there was only one policy class for all the iterator's
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orthogonal properties). Soon Jeremy Siek realized he would need the
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library and they worked together to produce a "Boostified" version,
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which was reviewed and accepted into the library. They wrote a paper
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and made several important revisions of the code.</p>
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<p>Eventually, several shortcomings of the older library began to make
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the need for a rewrite apparent. Dave and Jeremy started working
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at the Santa Cruz C++ committee meeting in 2002, and had quickly
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generated a working prototype. At the urging of Mat Marcus, they
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decided to use the GenVoca/CRTP pattern approach, and moved the
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policies into the iterator class itself. Thomas Witt expressed
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interest and became the voice of strict compile-time checking for
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the project, adding uses of the SFINAE technique to eliminate false
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converting constructors and operators from the overload set. He
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also recognized the need for a separate <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, and
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factored it out of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>. Finally, after a
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near-complete rewrite of the prototype, they came up with the
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library you see today.</p>
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<table class="docutils citation" frame="void" id="cop95" rules="none">
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<colgroup><col class="label" /><col /></colgroup>
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<tbody valign="top">
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<tr><td class="label"><a class="fn-backref" href="#id22">[Cop95]</a></td><td>[Coplien, 1995] Coplien, J., Curiously Recurring Template
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Patterns, C++ Report, February 1995, pp. 24-27.</td></tr>
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</tbody>
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</table>
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<!-- LocalWords: Abrahams Siek Witt const bool Sutter's WG int UL LI href Lvalue
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LocalWords: ReadableIterator WritableIterator SwappableIterator cv pre iter
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LocalWords: ConstantLvalueIterator MutableLvalueIterator CopyConstructible TR
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LocalWords: ForwardTraversalIterator BidirectionalTraversalIterator lvalue
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LocalWords: RandomAccessTraversalIterator dereferenceable Incrementable tmp
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LocalWords: incrementable xxx min prev inplace png oldeqnew AccessTag struct
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LocalWords: TraversalTag typename lvalues DWA Hmm JGS -->
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