<?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>Zip Iterator</title> <meta name="author" content="David Abrahams, Thomas Becker" /> <meta name="organization" content="Boost Consulting, Zephyr Associates, Inc." /> <meta name="date" content="2006-09-11" /> <meta name="copyright" content="Copyright David Abrahams and Thomas Becker 2003." /> <link rel="stylesheet" href="../../../rst.css" type="text/css" /> </head> <body> <div class="document" id="zip-iterator"> <h1 class="title">Zip 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, Thomas Becker</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="last reference external" href="mailto:thomas@styleadvisor.com">thomas@styleadvisor.com</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>, <a class="last reference external" href="http://www.styleadvisor.com">Zephyr Associates, Inc.</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 and Thomas Becker 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) --> The zip iterator provides the ability to parallel-iterate over several controlled sequences simultaneously. A zip iterator is constructed from a tuple of iterators. Moving the zip iterator moves all the iterators in parallel. Dereferencing the zip iterator returns a tuple that contains the results of dereferencing the individual iterators.</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="#zip-iterator-synopsis" id="id1"><tt class="docutils literal"><span class="pre">zip_iterator</span></tt> synopsis</a></li> <li><a class="reference internal" href="#zip-iterator-requirements" id="id2"><tt class="docutils literal"><span class="pre">zip_iterator</span></tt> requirements</a></li> <li><a class="reference internal" href="#zip-iterator-models" id="id3"><tt class="docutils literal"><span class="pre">zip_iterator</span></tt> models</a></li> <li><a class="reference internal" href="#zip-iterator-operations" id="id4"><tt class="docutils literal"><span class="pre">zip_iterator</span></tt> operations</a></li> <li><a class="reference internal" href="#examples" id="id5">Examples</a></li> </ul> </div> <div class="section" id="zip-iterator-synopsis"> <h1><a class="toc-backref" href="#id1"><tt class="docutils literal"><span class="pre">zip_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<typename IteratorTuple> class zip_iterator { public: typedef /* see below */ reference; typedef reference value_type; typedef value_type* pointer; typedef /* see below */ difference_type; typedef /* see below */ iterator_category; zip_iterator(); zip_iterator(IteratorTuple iterator_tuple); template<typename OtherIteratorTuple> zip_iterator( const zip_iterator<OtherIteratorTuple>& other , typename enable_if_convertible< OtherIteratorTuple , IteratorTuple>::type* = 0 // exposition only ); const IteratorTuple& get_iterator_tuple() const; private: IteratorTuple m_iterator_tuple; // exposition only }; template<typename IteratorTuple> zip_iterator<IteratorTuple> make_zip_iterator(IteratorTuple t); </pre> <p>The <tt class="docutils literal"><span class="pre">reference</span></tt> member of <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> is the type of the tuple made of the reference types of the iterator types in the <tt class="docutils literal"><span class="pre">IteratorTuple</span></tt> argument.</p> <p>The <tt class="docutils literal"><span class="pre">difference_type</span></tt> member of <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> is the <tt class="docutils literal"><span class="pre">difference_type</span></tt> of the first of the iterator types in the <tt class="docutils literal"><span class="pre">IteratorTuple</span></tt> argument.</p> <p>The <tt class="docutils literal"><span class="pre">iterator_category</span></tt> member of <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> is convertible to the minimum of the traversal categories of the iterator types in the <tt class="docutils literal"><span class="pre">IteratorTuple</span></tt> argument. For example, if the <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> holds only vector iterators, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to <tt class="docutils literal"><span class="pre">boost::random_access_traversal_tag</span></tt>. If you add a list iterator, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> will be convertible to <tt class="docutils literal"><span class="pre">boost::bidirectional_traversal_tag</span></tt>, but no longer to <tt class="docutils literal"><span class="pre">boost::random_access_traversal_tag</span></tt>.</p> </div> <div class="section" id="zip-iterator-requirements"> <h1><a class="toc-backref" href="#id2"><tt class="docutils literal"><span class="pre">zip_iterator</span></tt> requirements</a></h1> <p>All iterator types in the argument <tt class="docutils literal"><span class="pre">IteratorTuple</span></tt> shall model Readable Iterator.</p> </div> <div class="section" id="zip-iterator-models"> <h1><a class="toc-backref" href="#id3"><tt class="docutils literal"><span class="pre">zip_iterator</span></tt> models</a></h1> <p>The resulting <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> models Readable Iterator.</p> <p>The fact that the <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> models only Readable Iterator does not prevent you from modifying the values that the individual iterators point to. The tuple returned by the <tt class="docutils literal"><span class="pre">zip_iterator</span></tt>'s <tt class="docutils literal"><span class="pre">operator*</span></tt> is a tuple constructed from the reference types of the individual iterators, not their value types. For example, if <tt class="docutils literal"><span class="pre">zip_it</span></tt> is a <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> whose first member iterator is an <tt class="docutils literal"><span class="pre">std::vector<double>::iterator</span></tt>, then the following line will modify the value which the first member iterator of <tt class="docutils literal"><span class="pre">zip_it</span></tt> currently points to:</p> <pre class="literal-block"> zip_it->get<0>() = 42.0; </pre> <p>Consider the set of standard traversal concepts obtained by taking the most refined standard traversal concept modeled by each individual iterator type in the <tt class="docutils literal"><span class="pre">IteratorTuple</span></tt> argument.The <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> models the least refined standard traversal concept in this set.</p> <p><tt class="docutils literal"><span class="pre">zip_iterator<IteratorTuple1></span></tt> is interoperable with <tt class="docutils literal"><span class="pre">zip_iterator<IteratorTuple2></span></tt> if and only if <tt class="docutils literal"><span class="pre">IteratorTuple1</span></tt> is interoperable with <tt class="docutils literal"><span class="pre">IteratorTuple2</span></tt>.</p> </div> <div class="section" id="zip-iterator-operations"> <h1><a class="toc-backref" href="#id4"><tt class="docutils literal"><span class="pre">zip_iterator</span></tt> operations</a></h1> <p>In addition to the operations required by the concepts modeled by <tt class="docutils literal"><span class="pre">zip_iterator</span></tt>, <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> provides the following operations.</p> <p><tt class="docutils literal"><span class="pre">zip_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">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_iterator_tuple</span></tt> default constructed.</td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">zip_iterator(IteratorTuple</span> <span class="pre">iterator_tuple);</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">An instance of <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_iterator_tuple</span></tt> initialized to <tt class="docutils literal"><span class="pre">iterator_tuple</span></tt>.</td> </tr> </tbody> </table> <pre class="literal-block"> template<typename OtherIteratorTuple> zip_iterator( const zip_iterator<OtherIteratorTuple>& other , typename enable_if_convertible< OtherIteratorTuple , IteratorTuple>::type* = 0 // exposition only ); </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">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> that is a copy of <tt class="docutils literal"><span class="pre">other</span></tt>.</td> </tr> <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">OtherIteratorTuple</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">IteratorTuple</span></tt>.</td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">const</span> <span class="pre">IteratorTuple&</span> <span class="pre">get_iterator_tuple()</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_tuple</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">A tuple consisting of the results of dereferencing all iterators in <tt class="docutils literal"><span class="pre">m_iterator_tuple</span></tt>.</td> </tr> </tbody> </table> <p><tt class="docutils literal"><span class="pre">zip_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">Increments each iterator in <tt class="docutils literal"><span class="pre">m_iterator_tuple</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">zip_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">Decrements each iterator in <tt class="docutils literal"><span class="pre">m_iterator_tuple</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> <pre class="literal-block"> template<typename IteratorTuple> zip_iterator<IteratorTuple> make_zip_iterator(IteratorTuple t); </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">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">zip_iterator<IteratorTuple></span></tt> with <tt class="docutils literal"><span class="pre">m_iterator_tuple</span></tt> initialized to <tt class="docutils literal"><span class="pre">t</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) --> <pre class="literal-block"> template<typename IteratorTuple> zip_iterator<IteratorTuple> make_zip_iterator(IteratorTuple t); </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">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">zip_iterator<IteratorTuple></span></tt> with <tt class="docutils literal"><span class="pre">m_iterator_tuple</span></tt> initialized to <tt class="docutils literal"><span class="pre">t</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="examples"> <h1><a class="toc-backref" href="#id5">Examples</a></h1> <p>There are two main types of applications of the <tt class="docutils literal"><span class="pre">zip_iterator</span></tt>. The first one concerns runtime efficiency: If one has several controlled sequences of the same length that must be somehow processed, e.g., with the <tt class="docutils literal"><span class="pre">for_each</span></tt> algorithm, then it is more efficient to perform just one parallel-iteration rather than several individual iterations. For an example, assume that <tt class="docutils literal"><span class="pre">vect_of_doubles</span></tt> and <tt class="docutils literal"><span class="pre">vect_of_ints</span></tt> are two vectors of equal length containing doubles and ints, respectively, and consider the following two iterations:</p> <pre class="literal-block"> std::vector<double>::const_iterator beg1 = vect_of_doubles.begin(); std::vector<double>::const_iterator end1 = vect_of_doubles.end(); std::vector<int>::const_iterator beg2 = vect_of_ints.begin(); std::vector<int>::const_iterator end2 = vect_of_ints.end(); std::for_each(beg1, end1, func_0()); std::for_each(beg2, end2, func_1()); </pre> <p>These two iterations can now be replaced with a single one as follows:</p> <pre class="literal-block"> std::for_each( boost::make_zip_iterator( boost::make_tuple(beg1, beg2) ), boost::make_zip_iterator( boost::make_tuple(end1, end2) ), zip_func() ); </pre> <p>A non-generic implementation of <tt class="docutils literal"><span class="pre">zip_func</span></tt> could look as follows:</p> <pre class="literal-block"> struct zip_func : public std::unary_function<const boost::tuple<const double&, const int&>&, void> { void operator()(const boost::tuple<const double&, const int&>& t) const { m_f0(t.get<0>()); m_f1(t.get<1>()); } private: func_0 m_f0; func_1 m_f1; }; </pre> <p>The second important application of the <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> is as a building block to make combining iterators. A combining iterator is an iterator that parallel-iterates over several controlled sequences and, upon dereferencing, returns the result of applying a functor to the values of the sequences at the respective positions. This can now be achieved by using the <tt class="docutils literal"><span class="pre">zip_iterator</span></tt> in conjunction with the <tt class="docutils literal"><span class="pre">transform_iterator</span></tt>.</p> <p>Suppose, for example, that you have two vectors of doubles, say <tt class="docutils literal"><span class="pre">vect_1</span></tt> and <tt class="docutils literal"><span class="pre">vect_2</span></tt>, and you need to expose to a client a controlled sequence containing the products of the elements of <tt class="docutils literal"><span class="pre">vect_1</span></tt> and <tt class="docutils literal"><span class="pre">vect_2</span></tt>. Rather than placing these products in a third vector, you can use a combining iterator that calculates the products on the fly. Let us assume that <tt class="docutils literal"><span class="pre">tuple_multiplies</span></tt> is a functor that works like <tt class="docutils literal"><span class="pre">std::multiplies</span></tt>, except that it takes its two arguments packaged in a tuple. Then the two iterators <tt class="docutils literal"><span class="pre">it_begin</span></tt> and <tt class="docutils literal"><span class="pre">it_end</span></tt> defined below delimit a controlled sequence containing the products of the elements of <tt class="docutils literal"><span class="pre">vect_1</span></tt> and <tt class="docutils literal"><span class="pre">vect_2</span></tt>:</p> <pre class="literal-block"> typedef boost::tuple< std::vector<double>::const_iterator, std::vector<double>::const_iterator > the_iterator_tuple; typedef boost::zip_iterator< the_iterator_tuple > the_zip_iterator; typedef boost::transform_iterator< tuple_multiplies<double>, the_zip_iterator > the_transform_iterator; the_transform_iterator it_begin( the_zip_iterator( the_iterator_tuple( vect_1.begin(), vect_2.begin() ) ), tuple_multiplies<double>() ); the_transform_iterator it_end( the_zip_iterator( the_iterator_tuple( vect_1.end(), vect_2.end() ) ), tuple_multiplies<double>() ); </pre> </div> </div> <div class="footer"> <hr class="footer" /> <a class="reference external" href="zip_iterator.rst">View document source</a>. 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