<?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>Indirect Iterator</title> <meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" /> <meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" /> <meta name="date" content="2006-09-11" /> <meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." /> <link rel="stylesheet" href="../../../rst.css" type="text/css" /> </head> <body> <div class="document" id="indirect-iterator"> <h1 class="title">Indirect Iterator</h1> <table class="docinfo" frame="void" rules="none"> <col class="docinfo-name" /> <col class="docinfo-content" /> <tbody valign="top"> <tr><th class="docinfo-name">Author:</th> <td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr> <tr><th class="docinfo-name">Contact:</th> <td><a class="first 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="last reference external" href="mailto:witt@ive.uni-hannover.de">witt@ive.uni-hannover.de</a></td></tr> <tr><th class="docinfo-name">Organization:</th> <td><a class="first 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>, University of Hanover <a class="last reference external" href="http://www.ive.uni-hannover.de">Institute for Transport Railway Operation and Construction</a></td></tr> <tr><th class="docinfo-name">Date:</th> <td>2006-09-11</td></tr> <tr><th class="docinfo-name">Copyright:</th> <td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr> </tbody> </table> <!-- 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) --> <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"><!-- Copyright David Abrahams 2006. 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) --> <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> adapts an iterator by applying an <em>extra</em> dereference inside of <tt class="docutils literal"><span class="pre">operator*()</span></tt>. For example, this iterator adaptor makes it possible to view a container of pointers (e.g. <tt class="docutils literal"><span class="pre">list<foo*></span></tt>) as if it were a container of the pointed-to type (e.g. <tt class="docutils literal"><span class="pre">list<foo></span></tt>). <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> depends on two auxiliary traits, <tt class="docutils literal"><span class="pre">pointee</span></tt> and <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>, to provide support for underlying iterators whose <tt class="docutils literal"><span class="pre">value_type</span></tt> is not an iterator.</td> </tr> </tbody> </table> <div class="contents topic" id="table-of-contents"> <p class="topic-title first">Table of Contents</p> <ul class="simple"> <li><a class="reference internal" href="#indirect-iterator-synopsis" id="id2"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> synopsis</a></li> <li><a class="reference internal" href="#indirect-iterator-requirements" id="id3"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> requirements</a></li> <li><a class="reference internal" href="#indirect-iterator-models" id="id4"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models</a></li> <li><a class="reference internal" href="#indirect-iterator-operations" id="id5"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> operations</a></li> <li><a class="reference internal" href="#example" id="id6">Example</a></li> </ul> </div> <div class="section" id="indirect-iterator-synopsis"> <h1><a class="toc-backref" href="#id2"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> synopsis</a></h1> <!-- Copyright David Abrahams 2006. 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) --> <pre class="literal-block"> template < class Iterator , class Value = use_default , class CategoryOrTraversal = use_default , class Reference = use_default , class Difference = use_default > class indirect_iterator { public: typedef /* see below */ value_type; typedef /* see below */ reference; typedef /* see below */ pointer; typedef /* see below */ difference_type; typedef /* see below */ iterator_category; indirect_iterator(); indirect_iterator(Iterator x); template < class Iterator2, class Value2, class Category2 , class Reference2, class Difference2 > indirect_iterator( indirect_iterator< Iterator2, Value2, Category2, Reference2, Difference2 > const& y , typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition ); Iterator const& base() const; reference operator*() const; indirect_iterator& operator++(); indirect_iterator& operator--(); private: Iterator m_iterator; // exposition }; </pre> <p>The member types of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> are defined according to the following pseudo-code, where <tt class="docutils literal"><span class="pre">V</span></tt> is <tt class="docutils literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt></p> <pre class="literal-block"> if (Value is use_default) then typedef remove_const<pointee<V>::type>::type value_type; else typedef remove_const<Value>::type value_type; if (Reference is use_default) then if (Value is use_default) then typedef indirect_reference<V>::type reference; else typedef Value& reference; else typedef Reference reference; if (Value is use_default) then typedef pointee<V>::type* pointer; else typedef Value* pointer; if (Difference is use_default) typedef iterator_traits<Iterator>::difference_type difference_type; else typedef Difference difference_type; if (CategoryOrTraversal is use_default) typedef <em>iterator-category</em> ( iterator_traversal<Iterator>::type,``reference``,``value_type`` ) iterator_category; else typedef <em>iterator-category</em> ( CategoryOrTraversal,``reference``,``value_type`` ) iterator_category; </pre> </div> <div class="section" id="indirect-iterator-requirements"> <h1><a class="toc-backref" href="#id3"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> requirements</a></h1> <p>The expression <tt class="docutils literal"><span class="pre">*v</span></tt>, where <tt class="docutils literal"><span class="pre">v</span></tt> is an object of <tt class="docutils literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt>, shall be valid expression and convertible to <tt class="docutils literal"><span class="pre">reference</span></tt>. <tt class="docutils literal"><span class="pre">Iterator</span></tt> shall model the traversal concept indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>. <tt class="docutils literal"><span class="pre">Value</span></tt>, <tt class="docutils literal"><span class="pre">Reference</span></tt>, and <tt class="docutils literal"><span class="pre">Difference</span></tt> shall be chosen so that <tt class="docutils literal"><span class="pre">value_type</span></tt>, <tt class="docutils literal"><span class="pre">reference</span></tt>, and <tt class="docutils literal"><span class="pre">difference_type</span></tt> meet the requirements indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p> <p>[Note: there are further requirements on the <tt class="docutils literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt> if the <tt class="docutils literal"><span class="pre">Value</span></tt> parameter is not <tt class="docutils literal"><span class="pre">use_default</span></tt>, as implied by the algorithm for deducing the default for the <tt class="docutils literal"><span class="pre">value_type</span></tt> member.]</p> </div> <div class="section" id="indirect-iterator-models"> <h1><a class="toc-backref" href="#id4"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models</a></h1> <p>In addition to the concepts indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt> and by <tt class="docutils literal"><span class="pre">iterator_traversal<indirect_iterator>::type</span></tt>, a specialization of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models the following concepts, Where <tt class="docutils literal"><span class="pre">v</span></tt> is an object of <tt class="docutils literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt>:</p> <blockquote> <ul class="simple"> <li>Readable Iterator if <tt class="docutils literal"><span class="pre">reference(*v)</span></tt> is convertible to <tt class="docutils literal"><span class="pre">value_type</span></tt>.</li> <li>Writable Iterator if <tt class="docutils literal"><span class="pre">reference(*v)</span> <span class="pre">=</span> <span class="pre">t</span></tt> is a valid expression (where <tt class="docutils literal"><span class="pre">t</span></tt> is an object of type <tt class="docutils literal"><span class="pre">indirect_iterator::value_type</span></tt>)</li> <li>Lvalue Iterator if <tt class="docutils literal"><span class="pre">reference</span></tt> is a reference type.</li> </ul> </blockquote> <p><tt class="docutils literal"><span class="pre">indirect_iterator<X,V1,C1,R1,D1></span></tt> is interoperable with <tt class="docutils literal"><span class="pre">indirect_iterator<Y,V2,C2,R2,D2></span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p> </div> <div class="section" id="indirect-iterator-operations"> <h1><a class="toc-backref" href="#id5"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> operations</a></h1> <p>In addition to the operations required by the concepts described above, specializations of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> provide the following operations.</p> <p><tt class="docutils literal"><span class="pre">indirect_iterator();</span></tt></p> <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">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td> </tr> <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> with a default-constructed <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">indirect_iterator(Iterator</span> <span class="pre">x);</span></tt></p> <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">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">x</span></tt>.</td> </tr> </tbody> </table> <pre class="literal-block"> template < class Iterator2, class Value2, unsigned Access, class Traversal , class Reference2, class Difference2 > indirect_iterator( indirect_iterator< Iterator2, Value2, Access, Traversal, Reference2, Difference2 > const& y , typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition ); </pre> <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">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator2</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td> </tr> <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> whose <tt class="docutils literal"><span class="pre">m_iterator</span></tt> subobject is constructed from <tt class="docutils literal"><span class="pre">y.base()</span></tt>.</td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p> <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">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p> <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">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">**m_iterator</span></tt></td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">indirect_iterator&</span> <span class="pre">operator++();</span></tt></p> <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">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator</span></tt></td> </tr> <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">indirect_iterator&</span> <span class="pre">operator--();</span></tt></p> <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">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator</span></tt></td> </tr> <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td> </tr> </tbody> </table> <!-- Copyright David Abrahams 2006. 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) --> </div> <div class="section" id="example"> <h1><a class="toc-backref" href="#id6">Example</a></h1> <p>This example prints an array of characters, using <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> to access the array of characters through an array of pointers. Next <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> is used with the <tt class="docutils literal"><span class="pre">transform</span></tt> algorithm to copy the characters (incremented by one) to another array. A constant indirect iterator is used for the source and a mutable indirect iterator is used for the destination. The last part of the example prints the original array of characters, but this time using the <tt class="docutils literal"><span class="pre">make_indirect_iterator</span></tt> helper function.</p> <pre class="literal-block"> char characters[] = "abcdefg"; const int N = sizeof(characters)/sizeof(char) - 1; // -1 since characters has a null char char* pointers_to_chars[N]; // at the end. for (int i = 0; i < N; ++i) pointers_to_chars[i] = &characters[i]; // Example of using indirect_iterator boost::indirect_iterator<char**, char> indirect_first(pointers_to_chars), indirect_last(pointers_to_chars + N); std::copy(indirect_first, indirect_last, std::ostream_iterator<char>(std::cout, ",")); std::cout << std::endl; // Example of making mutable and constant indirect iterators char mutable_characters[N]; char* pointers_to_mutable_chars[N]; for (int j = 0; j < N; ++j) pointers_to_mutable_chars[j] = &mutable_characters[j]; boost::indirect_iterator<char* const*> mutable_indirect_first(pointers_to_mutable_chars), mutable_indirect_last(pointers_to_mutable_chars + N); boost::indirect_iterator<char* const*, char const> const_indirect_first(pointers_to_chars), const_indirect_last(pointers_to_chars + N); std::transform(const_indirect_first, const_indirect_last, mutable_indirect_first, std::bind1st(std::plus<char>(), 1)); std::copy(mutable_indirect_first, mutable_indirect_last, std::ostream_iterator<char>(std::cout, ",")); std::cout << std::endl; // Example of using make_indirect_iterator() std::copy(boost::make_indirect_iterator(pointers_to_chars), boost::make_indirect_iterator(pointers_to_chars + N), std::ostream_iterator<char>(std::cout, ",")); std::cout << std::endl; </pre> <p>The output is:</p> <pre class="literal-block"> a,b,c,d,e,f,g, b,c,d,e,f,g,h, a,b,c,d,e,f,g, </pre> <p>The source code for this example can be found <a class="reference external" href="../example/indirect_iterator_example.cpp">here</a>.</p> </div> </div> <div class="footer"> <hr class="footer" /> <a class="reference external" href="indirect_iterator.rst">View document source</a>. 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