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