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522 lines
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<body>
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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" 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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<col class="field-body" />
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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" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a>, <a class="reference" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>, <a class="reference" 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" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference" href="http://www.osl.iu.edu">Open Systems
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Lab</a>, <a class="reference" 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>
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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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<col class="field-body" />
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<tbody valign="top">
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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" href="../../../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">
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<p class="topic-title first"><a id="table-of-contents" name="table-of-contents"><strong>Table of Contents</strong></a></p>
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<ul class="simple">
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<li><a class="reference" href="#new-style-iterators" id="id22" name="id22">New-Style Iterators</a></li>
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<li><a class="reference" href="#iterator-facade-and-adaptor" id="id23" name="id23">Iterator Facade and Adaptor</a></li>
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<li><a class="reference" href="#specialized-adaptors" id="id24" name="id24">Specialized Adaptors</a></li>
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<li><a class="reference" href="#iterator-utilities" id="id25" name="id25">Iterator Utilities</a><ul>
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<li><a class="reference" href="#traits" id="id26" name="id26">Traits</a></li>
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<li><a class="reference" href="#testing-and-concept-checking" id="id27" name="id27">Testing and Concept Checking</a></li>
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</ul>
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</li>
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<li><a class="reference" href="#upgrading-from-the-old-boost-iterator-adaptor-library" id="id28" name="id28">Upgrading from the old Boost Iterator Adaptor Library</a></li>
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<li><a class="reference" href="#history" id="id29" name="id29">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">
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<h1><a class="toc-backref" href="#id22" id="new-style-iterators" name="new-style-iterators">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" href="http://www.gotw.ca/publications/N1185.pdf">n1185</a> and <a class="reference" href="http://www.gotw.ca/publications/N1211.pdf">n1211</a>),
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and a <a class="reference" 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" href="new-iter-concepts.html">Standard Proposal For New-Style Iterators</a> (<a class="reference" href="new-iter-concepts.pdf">PDF</a>)</blockquote>
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</div>
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<div class="section">
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<h1><a class="toc-backref" href="#id23" id="iterator-facade-and-adaptor" name="iterator-facade-and-adaptor">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" href="iterator_facade.html"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a> (<a class="reference" href="iterator_facade.pdf">PDF</a>)</li>
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<li><a class="reference" href="iterator_adaptor.html"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt></a> (<a class="reference" 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" 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" href="facade-and-adaptor.html">Standard Proposal For Iterator Facade and Adaptor</a> (<a class="reference" 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">
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<h1><a class="toc-backref" href="#id24" id="specialized-adaptors" name="specialized-adaptors">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" 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" href="counting_iterator.html"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a> (<a class="reference" 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" href="filter_iterator.html"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt></a> (<a class="reference" 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" href="function_output_iterator.html"><tt class="docutils literal"><span class="pre">function_output_iterator</span></tt></a> (<a class="reference" 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" href="indirect_iterator.html"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt></a> (<a class="reference" 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" href="permutation_iterator.html"><tt class="docutils literal"><span class="pre">permutation_iterator</span></tt></a> (<a class="reference" 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" href="reverse_iterator.html"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt></a> (<a class="reference" 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" 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" 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" href="transform_iterator.html"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt></a> (<a class="reference" 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" href="zip_iterator.html"><tt class="docutils literal"><span class="pre">zip_iterator</span></tt></a> (<a class="reference" 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">
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<h1><a class="toc-backref" href="#id25" id="iterator-utilities" name="iterator-utilities">Iterator Utilities</a></h1>
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<div class="section">
|
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<h2><a class="toc-backref" href="#id26" id="traits" name="traits">Traits</a></h2>
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<ul class="simple">
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|
<li><a class="reference" href="pointee.html"><tt class="docutils literal"><span class="pre">pointee.hpp</span></tt></a> (<a class="reference" 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>
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<li><a class="reference" href="iterator_traits.html"><tt class="docutils literal"><span class="pre">iterator_traits.hpp</span></tt></a> (<a class="reference" href="iterator_traits.pdf">PDF</a>): Provides <a class="reference" 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>
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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">
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<h2><a class="toc-backref" href="#id27" id="testing-and-concept-checking" name="testing-and-concept-checking">Testing and Concept Checking</a></h2>
|
|
<ul class="simple">
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|
<li><a class="reference" href="iterator_concepts.html"><tt class="docutils literal"><span class="pre">iterator_concepts.hpp</span></tt></a> (<a class="reference" href="iterator_concepts.pdf">PDF</a>): Concept checking classes for the new iterator concepts.</li>
|
|
<li><a class="reference" href="iterator_archetypes.html"><tt class="docutils literal"><span class="pre">iterator_archetypes.hpp</span></tt></a> (<a class="reference" href="iterator_archetypes.pdf">PDF</a>): Concept archetype classes for the new iterators concepts.</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="section">
|
|
<h1><a class="toc-backref" href="#id28" id="upgrading-from-the-old-boost-iterator-adaptor-library" name="upgrading-from-the-old-boost-iterator-adaptor-library">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" href="../../../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" name="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>
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</div>
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<div class="section">
|
|
<h1><a class="toc-backref" href="#id29" id="history" name="history">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">
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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="#id21" name="cop95">[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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|
<!-- LocalWords: Abrahams Siek Witt const bool Sutter's WG int UL LI href Lvalue
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LocalWords: ForwardTraversalIterator BidirectionalTraversalIterator lvalue
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LocalWords: RandomAccessTraversalIterator dereferenceable Incrementable tmp
|
|
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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