<?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>The Boost.Iterator Library Boost</title> <link rel="stylesheet" href="../../../rst.css" type="text/css" /> </head> <body> <div class="document" id="the-boost-iterator-library-logo"> <h1 class="title">The Boost.Iterator Library <a class="reference external" href="../../../index.htm"><img alt="Boost" src="../../../boost.png" /></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) --> <!-- 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) --> <hr class="docutils" /> <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">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: 2008-03-22 17:45:55 -0400 (Sat, 22 Mar 2008) $</td> </tr> <tr class="field"><th class="field-name">copyright:</th><td class="field-body">Copyright David Abrahams, Jeremy Siek, Thomas Witt 2003.</td> </tr> </tbody> </table> <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">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="id22">New-Style Iterators</a></li> <li><a class="reference internal" href="#iterator-facade-and-adaptor" id="id23">Iterator Facade and Adaptor</a></li> <li><a class="reference internal" href="#specialized-adaptors" id="id24">Specialized Adaptors</a></li> <li><a class="reference internal" href="#iterator-utilities" id="id25">Iterator Utilities</a><ul> <li><a class="reference internal" href="#traits" id="id26">Traits</a></li> <li><a class="reference internal" href="#testing-and-concept-checking" id="id27">Testing and Concept Checking</a></li> </ul> </li> <li><a class="reference internal" href="#upgrading-from-the-old-boost-iterator-adaptor-library" id="id28">Upgrading from the old Boost Iterator Adaptor Library</a></li> <li><a class="reference internal" href="#history" id="id29">History</a></li> </ul> </div> <hr class="docutils" /> <div class="section" id="new-style-iterators"> <h1><a class="toc-backref" href="#id22">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="#id23">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="#id24">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_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="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="#id25">Iterator Utilities</a></h1> <div class="section" id="traits"> <h2><a class="toc-backref" href="#id26">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="#id27">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="#id28">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="id21">[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="#id29">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="#id21">[Cop95]</a></td><td>[Coplien, 1995] Coplien, J., Curiously Recurring Template Patterns, C++ Report, February 1995, pp. 24-27.</td></tr> </tbody> </table> <!-- LocalWords: Abrahams Siek Witt const bool Sutter's WG int UL LI href Lvalue LocalWords: ReadableIterator WritableIterator SwappableIterator cv pre iter LocalWords: ConstantLvalueIterator MutableLvalueIterator CopyConstructible TR LocalWords: ForwardTraversalIterator BidirectionalTraversalIterator lvalue LocalWords: RandomAccessTraversalIterator dereferenceable Incrementable tmp LocalWords: incrementable xxx min prev inplace png oldeqnew AccessTag struct LocalWords: TraversalTag typename lvalues DWA Hmm JGS --> </div> </div> <div class="footer"> <hr class="footer" /> <a class="reference external" href="index.rst">View document source</a>. 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