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<h1 class="topictitle1">parallel_do Template Function</h1>
<div>
<div class="section"><h2 class="sectiontitle">Summary</h2>
Template function that processes work items in
parallel.
</div>
<div class="section"><h2 class="sectiontitle">Header</h2>
<p>
<pre> #include "tbb/parallel_do.h"</pre>
</p>
</div>
<div class="section"><h2 class="sectiontitle">Syntax</h2>
<pre>template<typename InputIterator, typename Body>
void parallel_do( InputIterator first, InputIterator last,
Body body[, task_group_context& group] );
</pre>
</div>
<div class="section"><h2 class="sectiontitle">Description</h2>
<p>A
<samp class="codeph">parallel_do(<em>first,last,body</em>)</samp> applies a function
object body over the half-open interval [<em>first,last</em>). Items may be
processed in parallel. Additional work items can be added by body if it has a
second argument of type<samp class="codeph"> parallel_do_feeder</samp>. The function
terminates when<samp class="codeph"> body(x)</samp> returns for all items x that were in
the input sequence or added to it by method
<samp class="codeph">parallel_do_feeder::add</samp>.
</p>
<p>The requirements for input iterators are specified in Section 24.1 of
the ISO C++ standard. The table below shows the requirements on type Body.
</p>
<div class="tablenoborder"><table cellpadding="4" summary="" width="100%" frame="hsides" border="1" rules="all"><caption><span class="tablecap">parallel_do Requirements for Body B and its
Argument Type T</span></caption>
<thead align="left">
<tr>
<th class="cellrowborder" valign="top" id="d2552e96">
<p>Pseudo-Signature
</p>
</th>
<th class="row-nocellborder" valign="top" id="d2552e102">
<p>Semantics
</p>
</th>
</tr>
</thead>
<tbody>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d2552e96 ">
<pre>B::operator()(
<em>cv-qualifiers</em> T& item,
parallel_do_feeder<T>& feeder
) const
OR
B::operator()(<em>cv-qualifiers</em> T&
item ) const
</pre>
</td>
<td class="row-nocellborder" valign="top" headers="d2552e102 ">
<p>Process item. Template<samp class="codeph"> parallel_do</samp> may
concurrently invoke operator() for the same
<samp class="codeph">this</samp> but different
<samp class="codeph">item</samp>.
</p>
<p> The signature with feeder permits additional work items to be
added.
</p>
</td>
</tr>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d2552e96 ">
<p><samp class="codeph"> T( const T& )
</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" headers="d2552e102 ">
<p> Copy a work item.
</p>
</td>
</tr>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d2552e96 ">
<p><samp class="codeph"> ~T::T()</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" headers="d2552e102 ">
<p> Destroy a work item.
</p>
</td>
</tr>
</tbody>
</table>
</div>
<p>For example, a unary function object, as defined in Section 20.3 of
the C++ standard, models the requirements for B.
</p>
<div class="Note"><h3 class="NoteTipHead">
Caution</h3>
<p>Defining both the one-argument and two-argument forms of
<samp class="codeph">operator()</samp> is not permitted.
</p>
</div>
<div class="Note"><h3 class="NoteTipHead">
Note</h3>
<p>The parallelism in
<samp class="codeph">parallel_do</samp> is not scalable if all of the items come
from an input stream that does not have random access. To achieve scaling, do
one of the following:
</p>
<ul type="disc">
<li>
<p>Use random access iterators to specify the input stream.
</p>
</li>
<li>
<p>Design your algorithm such that the body often adds more than
one piece of work.
</p>
</li>
<li>
<p>Use<samp class="codeph"> parallel_for</samp> instead.
</p>
</li>
</ul>
</div>
<p>To achieve speedup, the grainsize of
<samp class="codeph">B::operator()</samp> needs to be on the order of at least
~100,000 clock cycles. Otherwise, the internal overheads of
<samp class="codeph">parallel_do</samp> swamp the useful work.
</p>
<p>The algorithm can be passed a
<samp class="codeph">task_group_context</samp> object so that its tasks are
executed in this group. By default the algorithm is executed in a bound group
of its own.
</p>
<p><strong>Example</strong>
</p>
<p>The following code sketches a body with the
two-argument form of
<samp class="codeph">operator(</samp>).
</p>
<pre>struct MyBody {
void operator()(item_t item,
parallel_do_feeder<item_t>& feeder ) {
for each new piece of work implied by item do {
item_t new_item = initializer;
feeder.add(new_item);
}
}
};
</pre>
</div>
</div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../../reference/algorithms.htm">Algorithms</a></div>
</div>
<div>
<ul class="ullinks">
<li class="ulchildlink"><a href="../../reference/algorithms/parallel_do_func/parallel_do_feeder_cls.htm">parallel_do_feeder<item> class</a><br>
</li>
</ul>
</div>
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