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< title > Zip Iterator</ title >
< meta name = "author" content = "David Abrahams, Thomas Becker" />
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< body >
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< div class = "document" id = "zip-iterator" >
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< 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" href = "mailto:dave@boost-consulting.com" > dave@ boost-consulting.com</ a > , < a class = "last reference" href = "mailto:thomas@styleadvisor.com" > thomas@ styleadvisor.com</ a ></ td ></ tr >
< tr >< th class = "docinfo-name" > Organization:</ th >
< td >< a class = "first reference" href = "http://www.boost-consulting.com" > Boost Consulting</ a > , < a class = "last reference" href = "http://www.styleadvisor.com" > Zephyr Associates, Inc.</ a ></ td ></ tr >
< tr >< th class = "docinfo-name" > Date:</ th >
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< td > 2004-11-01</ td ></ tr >
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< tr >< th class = "docinfo-name" > Copyright:</ th >
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< td > Copyright David Abrahams and Thomas Becker 2003.</ td ></ tr >
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</ tbody >
</ table >
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<!-- 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) -->
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< 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
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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 >
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< div class = "contents topic" >
< p class = "topic-title first" >< a id = "table-of-contents" name = "table-of-contents" > Table of Contents</ a ></ p >
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< ul class = "simple" >
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< li >< a class = "reference" href = "#zip-iterator-synopsis" id = "id1" name = "id1" >< tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > synopsis</ a ></ li >
< li >< a class = "reference" href = "#zip-iterator-requirements" id = "id2" name = "id2" >< tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > requirements</ a ></ li >
< li >< a class = "reference" href = "#zip-iterator-models" id = "id3" name = "id3" >< tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > models</ a ></ li >
< li >< a class = "reference" href = "#zip-iterator-operations" id = "id4" name = "id4" >< tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > operations</ a ></ li >
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< li >< a class = "reference" href = "#examples" id = "id5" name = "id5" > Examples</ a ></ li >
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</ ul >
</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id1" id = "zip-iterator-synopsis" name = "zip-iterator-synopsis" >< 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) -->
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< pre class = "literal-block" >
template< typename IteratorTuple>
class zip_iterator
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{
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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
};
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template< typename IteratorTuple>
zip_iterator< IteratorTuple>
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make_zip_iterator(IteratorTuple t);
</ pre >
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< 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 >
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argument.</ p >
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< 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
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iterators, then < tt class = "docutils literal" >< span class = "pre" > iterator_category</ span ></ tt > is convertible to
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< 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 >
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</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id2" id = "zip-iterator-requirements" name = "zip-iterator-requirements" >< tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > requirements</ a ></ h1 >
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< p > All iterator types in the argument < tt class = "docutils literal" >< span class = "pre" > IteratorTuple</ span ></ tt > shall model Readable Iterator.</ p >
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</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id3" id = "zip-iterator-models" name = "zip-iterator-models" >< tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > models</ a ></ h1 >
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< p > The resulting < tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > models Readable Iterator.</ p >
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< p > The fact that the < tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > models only Readable Iterator does not
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prevent you from modifying the values that the individual iterators point
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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
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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
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following line will modify the value which the first member iterator of
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< tt class = "docutils literal" >< span class = "pre" > zip_it</ span ></ tt > currently points to:</ p >
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< 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
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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 >
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models the least refined standard traversal concept in this set.</ p >
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< 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 >
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</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id4" id = "zip-iterator-operations" name = "zip-iterator-operations" >< tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > operations</ a ></ h1 >
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< p > In addition to the operations required by the concepts modeled by
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< 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
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operations.</ p >
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< p >< tt class = "docutils literal" >< span class = "pre" > zip_iterator();</ span ></ tt ></ p >
< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< 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 >
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default constructed.</ td >
</ tr >
</ tbody >
</ table >
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< 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" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< 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 >
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</ 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 >
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< 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 >
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</ tr >
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< 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 >
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</ tr >
</ tbody >
</ table >
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< 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" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< 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 >
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</ tr >
</ tbody >
</ table >
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< 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" >
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< 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
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< tt class = "docutils literal" >< span class = "pre" > m_iterator_tuple</ span ></ tt > .</ td >
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</ tr >
</ tbody >
</ table >
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< 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" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< 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 >
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</ tr >
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< tr class = "field" >< th class = "field-name" > Returns:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > *this</ span ></ tt ></ td >
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</ tr >
</ tbody >
</ table >
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< 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" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< 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 >
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</ tr >
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< tr class = "field" >< th class = "field-name" > Returns:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > *this</ span ></ tt ></ td >
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</ tr >
</ tbody >
</ table >
< pre class = "literal-block" >
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template< typename IteratorTuple>
zip_iterator< IteratorTuple>
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make_zip_iterator(IteratorTuple t);
</ pre >
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< 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 >
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</ tr >
</ tbody >
</ table >
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<!-- 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) -->
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< pre class = "literal-block" >
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template< typename IteratorTuple>
zip_iterator< IteratorTuple>
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make_zip_iterator(IteratorTuple t);
</ pre >
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< 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 >
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</ tr >
</ tbody >
</ table >
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<!-- 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) -->
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</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id5" id = "examples" name = "examples" > Examples</ a ></ h1 >
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< p > There are two main types of applications of the < tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > . The first
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one concerns runtime efficiency: If one has several controlled sequences
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of the same length that must be somehow processed, e.g., with the
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< tt class = "docutils literal" >< span class = "pre" > for_each</ span ></ tt > algorithm, then it is more efficient to perform just
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one parallel-iteration rather than several individual iterations. For an
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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 >
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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 >
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< p > A non-generic implementation of < tt class = "docutils literal" >< span class = "pre" > zip_func</ span ></ tt > could look as follows:</ p >
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< pre class = "literal-block" >
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struct zip_func :
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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 >
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< p > The second important application of the < tt class = "docutils literal" >< span class = "pre" > zip_iterator</ span ></ tt > is as a building block
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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
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< 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 >
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< p > Suppose, for example, that you have two vectors of doubles, say
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< 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
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a controlled sequence containing the products of the elements of
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< 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
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in a third vector, you can use a combining iterator that calculates the
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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
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its two arguments packaged in a tuple. Then the two iterators
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< 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 >
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< 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 >
</ body >
</ html >
