iterator/doc/zip_iterator.html

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<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" />
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<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&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="last reference external" href="mailto:thomas&#64;styleadvisor.com">thomas&#64;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" />
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<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&lt;typename IteratorTuple&gt;
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&lt;typename OtherIteratorTuple&gt;
  zip_iterator(
        const zip_iterator&lt;OtherIteratorTuple&gt;&amp; other
      , typename enable_if_convertible&lt;
              OtherIteratorTuple
            , IteratorTuple&gt;::type* = 0     // exposition only
  );

  const IteratorTuple&amp; get_iterator_tuple() const;

private:
  IteratorTuple m_iterator_tuple;     // exposition only
};

template&lt;typename IteratorTuple&gt;
zip_iterator&lt;IteratorTuple&gt;
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&lt;double&gt;::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-&gt;get&lt;0&gt;() = 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&lt;IteratorTuple1&gt;</span></tt> is interoperable with
<tt class="docutils literal"><span class="pre">zip_iterator&lt;IteratorTuple2&gt;</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&lt;typename OtherIteratorTuple&gt;
zip_iterator(
      const zip_iterator&lt;OtherIteratorTuple&gt;&amp; other
    , typename enable_if_convertible&lt;
            OtherIteratorTuple
          , IteratorTuple&gt;::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&amp;</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&amp;</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&amp;</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&lt;typename IteratorTuple&gt;
zip_iterator&lt;IteratorTuple&gt;
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&lt;IteratorTuple&gt;</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&lt;typename IteratorTuple&gt;
zip_iterator&lt;IteratorTuple&gt;
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&lt;IteratorTuple&gt;</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&lt;double&gt;::const_iterator beg1 = vect_of_doubles.begin();
std::vector&lt;double&gt;::const_iterator end1 = vect_of_doubles.end();
std::vector&lt;int&gt;::const_iterator beg2 = vect_of_ints.begin();
std::vector&lt;int&gt;::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&lt;const boost::tuple&lt;const double&amp;, const int&amp;&gt;&amp;, void&gt;
{
  void operator()(const boost::tuple&lt;const double&amp;, const int&amp;&gt;&amp; t) const
  {
    m_f0(t.get&lt;0&gt;());
    m_f1(t.get&lt;1&gt;());
  }

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&lt;
  std::vector&lt;double&gt;::const_iterator,
  std::vector&lt;double&gt;::const_iterator
  &gt; the_iterator_tuple;

typedef boost::zip_iterator&lt;
  the_iterator_tuple
  &gt; the_zip_iterator;

typedef boost::transform_iterator&lt;
  tuple_multiplies&lt;double&gt;,
  the_zip_iterator
  &gt; the_transform_iterator;

the_transform_iterator it_begin(
  the_zip_iterator(
    the_iterator_tuple(
      vect_1.begin(),
      vect_2.begin()
      )
    ),
  tuple_multiplies&lt;double&gt;()
  );

the_transform_iterator it_end(
  the_zip_iterator(
    the_iterator_tuple(
      vect_1.end(),
      vect_2.end()
      )
    ),
  tuple_multiplies&lt;double&gt;()
  );
</pre>
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