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| 7 | <title>The Boost.Iterator Library Boost</title> |
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| 9 | </head> |
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| 10 | <body> |
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| 11 | <div class="document" id="the-boost-iterator-library-logo"> |
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| 12 | <h1 class="title">The Boost.Iterator Library <a class="reference external" href="../../../index.htm"><img alt="Boost" src="../../../boost.png" /></a></h1> |
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| 13 | |
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| 14 | <!-- Distributed under the Boost --> |
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| 15 | <!-- Software License, Version 1.0. (See accompanying --> |
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| 16 | <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> |
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| 17 | <hr class="docutils" /> |
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| 18 | <table class="docutils field-list" frame="void" rules="none"> |
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| 19 | <col class="field-name" /> |
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| 20 | <col class="field-body" /> |
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| 21 | <tbody valign="top"> |
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| 22 | <tr class="field"><th class="field-name">Authors:</th><td class="field-body">David Abrahams, Jeremy Siek, Thomas Witt</td> |
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| 23 | </tr> |
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| 24 | <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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| 25 | </tr> |
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| 26 | <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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| 27 | Lab</a>, <a class="reference external" href="http://www.styleadvisor.com">Zephyr Associates, Inc.</a></td> |
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| 28 | </tr> |
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| 29 | <tr class="field"><th class="field-name">date:</th><td class="field-body">$Date: 2007/07/06 19:47:15 $</td> |
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| 30 | </tr> |
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| 31 | <tr class="field"><th class="field-name">copyright:</th><td class="field-body">Copyright David Abrahams, Jeremy Siek, Thomas Witt 2003.</td> |
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| 32 | </tr> |
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| 33 | </tbody> |
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| 34 | </table> |
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| 35 | <table class="docutils field-list" frame="void" rules="none"> |
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| 36 | <col class="field-name" /> |
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| 37 | <col class="field-body" /> |
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| 38 | <tbody valign="top"> |
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| 39 | <tr class="field"><th class="field-name">Abstract:</th><td class="field-body">The Boost Iterator Library contains two parts. The first |
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| 40 | is a system of <a class="reference external" href="../../../more/generic_programming.html#concept">concepts</a> which extend the C++ standard |
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| 41 | iterator requirements. The second is a framework of |
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| 42 | components for building iterators based on these |
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| 43 | extended concepts and includes several useful iterator |
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| 44 | adaptors. The extended iterator concepts have been |
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| 45 | carefully designed so that old-style iterators |
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| 46 | can fit in the new concepts and so that new-style |
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| 47 | iterators will be compatible with old-style algorithms, |
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| 48 | though algorithms may need to be updated if they want to |
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| 49 | take full advantage of the new-style iterator |
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| 50 | capabilities. Several components of this library have |
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| 51 | been accepted into the C++ standard technical report. |
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| 52 | The components of the Boost Iterator Library replace the |
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| 53 | older Boost Iterator Adaptor Library.</td> |
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| 54 | </tr> |
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| 55 | </tbody> |
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| 56 | </table> |
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| 57 | <div class="contents topic" id="table-of-contents"> |
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| 58 | <p class="topic-title first"><strong>Table of Contents</strong></p> |
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| 59 | <ul class="simple"> |
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| 60 | <li><a class="reference internal" href="#new-style-iterators" id="id22">New-Style Iterators</a></li> |
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| 61 | <li><a class="reference internal" href="#iterator-facade-and-adaptor" id="id23">Iterator Facade and Adaptor</a></li> |
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| 62 | <li><a class="reference internal" href="#specialized-adaptors" id="id24">Specialized Adaptors</a></li> |
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| 63 | <li><a class="reference internal" href="#iterator-utilities" id="id25">Iterator Utilities</a><ul> |
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| 64 | <li><a class="reference internal" href="#traits" id="id26">Traits</a></li> |
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| 65 | <li><a class="reference internal" href="#testing-and-concept-checking" id="id27">Testing and Concept Checking</a></li> |
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| 66 | </ul> |
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| 67 | </li> |
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| 68 | <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> |
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| 69 | <li><a class="reference internal" href="#history" id="id29">History</a></li> |
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| 70 | </ul> |
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| 71 | </div> |
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| 72 | <hr class="docutils" /> |
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| 73 | <div class="section" id="new-style-iterators"> |
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| 74 | <h1><a class="toc-backref" href="#id22">New-Style Iterators</a></h1> |
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| 75 | <p>The iterator categories defined in C++98 are extremely limiting |
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| 76 | because they bind together two orthogonal concepts: traversal and |
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| 77 | element access. For example, because a random access iterator is |
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| 78 | required to return a reference (and not a proxy) when dereferenced, |
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| 79 | it is impossible to capture the capabilities of |
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| 80 | <tt class="docutils literal"><span class="pre">vector<bool>::iterator</span></tt> using the C++98 categories. This is the |
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| 81 | infamous "<tt class="docutils literal"><span class="pre">vector<bool></span></tt> is not a container, and its iterators |
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| 82 | aren't random access iterators", debacle about which Herb Sutter |
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| 83 | 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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| 84 | 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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| 85 | patching up <tt class="docutils literal"><span class="pre">vector<bool></span></tt>, though: there are lots of other |
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| 86 | iterators already in use which can't be adequately represented by |
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| 87 | the existing concepts. For details about the new iterator |
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| 88 | concepts, see our</p> |
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| 89 | <blockquote> |
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| 90 | <a class="reference external" href="new-iter-concepts.html">Standard Proposal For New-Style Iterators</a> (<a class="reference external" href="new-iter-concepts.pdf">PDF</a>)</blockquote> |
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| 91 | </div> |
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| 92 | <div class="section" id="iterator-facade-and-adaptor"> |
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| 93 | <h1><a class="toc-backref" href="#id23">Iterator Facade and Adaptor</a></h1> |
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| 94 | <p>Writing standard-conforming iterators is tricky, but the need comes |
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| 95 | up often. In order to ease the implementation of new iterators, |
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| 96 | the Boost.Iterator library provides the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> class template, |
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| 97 | which implements many useful defaults and compile-time checks |
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| 98 | designed to help the iterator author ensure that his iterator is |
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| 99 | correct.</p> |
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| 100 | <p>It is also common to define a new iterator that is similar to some |
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| 101 | underlying iterator or iterator-like type, but that modifies some |
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| 102 | aspect of the underlying type's behavior. For that purpose, the |
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| 103 | library supplies the <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> class template, which is specially |
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| 104 | designed to take advantage of as much of the underlying type's |
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| 105 | behavior as possible.</p> |
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| 106 | <p>The documentation for these two classes can be found at the following |
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| 107 | web pages:</p> |
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| 108 | <ul class="simple"> |
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| 109 | <li><a class="reference external" href="iterator_facade.html"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a> (<a class="reference external" href="iterator_facade.pdf">PDF</a>)</li> |
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| 110 | <li><a class="reference external" href="iterator_adaptor.html"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt></a> (<a class="reference external" href="iterator_adaptor.pdf">PDF</a>)</li> |
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| 111 | </ul> |
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| 112 | <p>Both <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> as well as many of the <a class="reference internal" href="#specialized-adaptors">specialized |
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| 113 | adaptors</a> mentioned below have been proposed for standardization, |
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| 114 | and accepted into the first C++ technical report; see our</p> |
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| 115 | <blockquote> |
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| 116 | <a class="reference external" href="facade-and-adaptor.html">Standard Proposal For Iterator Facade and Adaptor</a> (<a class="reference external" href="facade-and-adaptor.pdf">PDF</a>)</blockquote> |
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| 117 | <p>for more details.</p> |
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| 118 | </div> |
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| 119 | <div class="section" id="specialized-adaptors"> |
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| 120 | <h1><a class="toc-backref" href="#id24">Specialized Adaptors</a></h1> |
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| 121 | <p>The iterator library supplies a useful suite of standard-conforming |
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| 122 | 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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| 123 | <ul class="simple"> |
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| 124 | <li><a class="reference external" href="counting_iterator.html"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a> (<a class="reference external" href="counting_iterator.pdf">PDF</a>): an iterator over a sequence of consecutive values. |
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| 125 | Implements a "lazy sequence"</li> |
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| 126 | <li><a class="reference external" href="filter_iterator.html"><tt class="docutils literal"><span class="pre">filter_iterator</span></tt></a> (<a class="reference external" href="filter_iterator.pdf">PDF</a>): an iterator over the subset of elements of some |
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| 127 | sequence which satisfy a given predicate</li> |
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| 128 | <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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| 129 | object; each time an element is written into the dereferenced |
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| 130 | iterator, it is passed as a parameter to the function object.</li> |
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| 131 | <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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| 132 | elements of some sequence.</li> |
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| 133 | <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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| 134 | sequence, rearranged according to some sequence of integer indices.</li> |
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| 135 | <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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| 136 | bidirectional sequence in reverse. Corrects many of the |
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| 137 | shortcomings of C++98's <tt class="docutils literal"><span class="pre">std::reverse_iterator</span></tt>.</li> |
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| 138 | <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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| 139 | 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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| 140 | <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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| 141 | applying some functional transformation to the elements of an |
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| 142 | underlying sequence. This component also replaces the old |
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| 143 | <tt class="docutils literal"><span class="pre">projection_iterator_adaptor</span></tt>.</li> |
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| 144 | <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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| 145 | positions of heterogeneous underlying iterators.</li> |
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| 146 | </ul> |
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| 147 | </div> |
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| 148 | <div class="section" id="iterator-utilities"> |
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| 149 | <h1><a class="toc-backref" href="#id25">Iterator Utilities</a></h1> |
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| 150 | <div class="section" id="traits"> |
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| 151 | <h2><a class="toc-backref" href="#id26">Traits</a></h2> |
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| 152 | <ul class="simple"> |
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| 153 | <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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| 154 | of pointers, smart pointers and iterators in generic code. Used |
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| 155 | in <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt>.</li> |
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| 156 | <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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| 157 | retrieve an iterator's traits. Also corrects for the deficiencies |
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| 158 | of broken implementations of <tt class="docutils literal"><span class="pre">std::iterator_traits</span></tt>.</li> |
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| 159 | </ul> |
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| 160 | <!-- * |interoperable|_ (PDF__): Provides an MPL_\ -compatible metafunction for |
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| 161 | testing iterator interoperability --> |
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| 162 | <!-- comment! __ interoperable.pdf --> |
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| 163 | </div> |
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| 164 | <div class="section" id="testing-and-concept-checking"> |
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| 165 | <h2><a class="toc-backref" href="#id27">Testing and Concept Checking</a></h2> |
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| 166 | <ul class="simple"> |
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| 167 | <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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| 168 | <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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| 169 | </ul> |
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| 170 | </div> |
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| 171 | </div> |
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| 172 | <div class="section" id="upgrading-from-the-old-boost-iterator-adaptor-library"> |
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| 173 | <h1><a class="toc-backref" href="#id28">Upgrading from the old Boost Iterator Adaptor Library</a></h1> |
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| 174 | <p id="upgrading">If you have been using the old Boost Iterator Adaptor library to |
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| 175 | implement iterators, you probably wrote a <tt class="docutils literal"><span class="pre">Policies</span></tt> class which |
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| 176 | captures the core operations of your iterator. In the new library |
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| 177 | design, you'll move those same core operations into the body of the |
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| 178 | iterator class itself. If you were writing a family of iterators, |
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| 179 | you probably wrote a <a class="reference external" href="../../../more/generic_programming.html#type_generator">type generator</a> to build the |
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| 180 | <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> specialization you needed; in the new library |
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| 181 | design you don't need a type generator (though may want to keep it |
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| 182 | around as a compatibility aid for older code) because, due to the |
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| 183 | use of the Curiously Recurring Template Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id21">[Cop95]</a>, |
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| 184 | you can now define the iterator class yourself and acquire |
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| 185 | functionality through inheritance from <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> or |
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| 186 | <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>. As a result, you also get much finer control |
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| 187 | over how your iterator works: you can add additional constructors, |
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| 188 | or even override the iterator functionality provided by the |
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| 189 | library.</p> |
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| 190 | <p>If you're looking for the old <tt class="docutils literal"><span class="pre">projection_iterator</span></tt> component, |
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| 191 | its functionality has been merged into <tt class="docutils literal"><span class="pre">transform_iterator</span></tt>: as |
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| 192 | 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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| 193 | template argument, if explicitly specified) is a true reference |
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| 194 | type, <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> will behave like |
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| 195 | <tt class="docutils literal"><span class="pre">projection_iterator</span></tt> used to.</p> |
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| 196 | </div> |
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| 197 | <div class="section" id="history"> |
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| 198 | <h1><a class="toc-backref" href="#id29">History</a></h1> |
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| 199 | <p>In 2000 Dave Abrahams was writing an iterator for a container of |
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| 200 | pointers, which would access the pointed-to elements when |
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| 201 | dereferenced. Naturally, being a library writer, he decided to |
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| 202 | generalize the idea and the Boost Iterator Adaptor library was born. |
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| 203 | Dave was inspired by some writings of Andrei Alexandrescu and chose a |
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| 204 | policy based design (though he probably didn't capture Andrei's idea |
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| 205 | very well - there was only one policy class for all the iterator's |
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| 206 | orthogonal properties). Soon Jeremy Siek realized he would need the |
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| 207 | library and they worked together to produce a "Boostified" version, |
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| 208 | which was reviewed and accepted into the library. They wrote a paper |
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| 209 | and made several important revisions of the code.</p> |
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| 210 | <p>Eventually, several shortcomings of the older library began to make |
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| 211 | the need for a rewrite apparent. Dave and Jeremy started working |
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| 212 | at the Santa Cruz C++ committee meeting in 2002, and had quickly |
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| 213 | generated a working prototype. At the urging of Mat Marcus, they |
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| 214 | decided to use the GenVoca/CRTP pattern approach, and moved the |
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| 215 | policies into the iterator class itself. Thomas Witt expressed |
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| 216 | interest and became the voice of strict compile-time checking for |
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| 217 | the project, adding uses of the SFINAE technique to eliminate false |
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| 218 | converting constructors and operators from the overload set. He |
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| 219 | also recognized the need for a separate <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, and |
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| 220 | factored it out of <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt>. Finally, after a |
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| 221 | near-complete rewrite of the prototype, they came up with the |
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| 222 | library you see today.</p> |
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| 223 | <table class="docutils citation" frame="void" id="cop95" rules="none"> |
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| 224 | <colgroup><col class="label" /><col /></colgroup> |
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| 225 | <tbody valign="top"> |
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| 226 | <tr><td class="label"><a class="fn-backref" href="#id21">[Cop95]</a></td><td>[Coplien, 1995] Coplien, J., Curiously Recurring Template |
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| 227 | Patterns, C++ Report, February 1995, pp. 24-27.</td></tr> |
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| 228 | </tbody> |
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| 229 | </table> |
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| 230 | <!-- LocalWords: Abrahams Siek Witt const bool Sutter's WG int UL LI href Lvalue |
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| 231 | LocalWords: ReadableIterator WritableIterator SwappableIterator cv pre iter |
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| 232 | LocalWords: ConstantLvalueIterator MutableLvalueIterator CopyConstructible TR |
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| 233 | LocalWords: ForwardTraversalIterator BidirectionalTraversalIterator lvalue |
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| 234 | LocalWords: RandomAccessTraversalIterator dereferenceable Incrementable tmp |
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| 235 | LocalWords: incrementable xxx min prev inplace png oldeqnew AccessTag struct |
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| 236 | LocalWords: TraversalTag typename lvalues DWA Hmm JGS --> |
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| 237 | </div> |
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| 238 | </div> |
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| 239 | <div class="footer"> |
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| 240 | <hr class="footer" /> |
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| 241 | <a class="reference external" href="index.rst">View document source</a>. |
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| 242 | Generated by <a class="reference external" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference external" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source. |
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| 243 | |
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| 244 | </div> |
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