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<title>Qt 4.8: <QtConcurrentMap> - Concurrent Map and Map-Reduce</title>
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<li class="level1"><a href="#concurrent-map">Concurrent Map</a></li>
<li class="level1"><a href="#concurrent-map-reduce">Concurrent Map-Reduce</a></li>
<li class="level1"><a href="#additional-api-features">Additional API Features</a></li>
<li class="level2"><a href="#using-iterators-instead-of-sequence">Using Iterators instead of Sequence</a></li>
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<li class="level2"><a href="#using-member-functions">Using Member Functions</a></li>
<li class="level2"><a href="#using-function-objects">Using Function Objects</a></li>
<li class="level2"><a href="#using-bound-function-arguments">Using Bound Function Arguments</a></li>
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<h1 class="title"><QtConcurrentMap> - Concurrent Map and Map-Reduce</h1>
<span class="subtitle"></span>
<!-- $$$<QtConcurrentMap>-brief -->
<p>The <QtConcurrentMap> header provides concurrent Map and MapReduce. <a href="#details">More...</a></p>
<!-- @@@<QtConcurrentMap> -->
<ul>
</ul>
<a name="Functionsx"></a>
<h2>Functions</h2>
<table class="alignedsummary">
<tr><td class="memItemLeft topAlign rightAlign"> T </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentmap.html#blockingMappedReduced">blockingMappedReduced</a></b> ( const Sequence & <i>sequence</i>, MapFunction <i>mapFunction</i>, ReduceFunction <i>reduceFunction</i>, QtConcurrent::ReduceOptions <i>reduceOptions</i> = UnorderedReduce | SequentialReduce )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> T </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentmap.html#blockingMappedReduced-2">blockingMappedReduced</a></b> ( ConstIterator <i>begin</i>, ConstIterator <i>end</i>, MapFunction <i>mapFunction</i>, ReduceFunction <i>reduceFunction</i>, QtConcurrent::ReduceOptions <i>reduceOptions</i> = UnorderedReduce | SequentialReduce )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<void> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentmap.html#map">map</a></b> ( Sequence & <i>sequence</i>, MapFunction <i>function</i> )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<void> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentmap.html#map-2">map</a></b> ( Iterator <i>begin</i>, Iterator <i>end</i>, MapFunction <i>function</i> )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<T> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentmap.html#mapped">mapped</a></b> ( const Sequence & <i>sequence</i>, MapFunction <i>function</i> )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<T> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentmap.html#mapped-2">mapped</a></b> ( ConstIterator <i>begin</i>, ConstIterator <i>end</i>, MapFunction <i>function</i> )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<T> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentmap.html#mappedReduced">mappedReduced</a></b> ( const Sequence & <i>sequence</i>, MapFunction <i>mapFunction</i>, ReduceFunction <i>reduceFunction</i>, QtConcurrent::ReduceOptions <i>reduceOptions</i> = UnorderedReduce | SequentialReduce )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<T> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentmap.html#mappedReduced-2">mappedReduced</a></b> ( ConstIterator <i>begin</i>, ConstIterator <i>end</i>, MapFunction <i>mapFunction</i>, ReduceFunction <i>reduceFunction</i>, QtConcurrent::ReduceOptions <i>reduceOptions</i> = UnorderedReduce | SequentialReduce )</td></tr>
</table>
<!-- $$$<QtConcurrentMap>-description -->
<div class="descr"> <a name="details"></a>
<p>These functions are a part of the <a href="threads-qtconcurrent.html">Qt Concurrent</a> framework.</p>
<p>The <a href="qtconcurrentmap.html#map">QtConcurrent::map</a>(), <a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>() and <a href="qtconcurrentmap.html#mappedReduced">QtConcurrent::mappedReduced</a>() functions run computations in parallel on the items in a sequence such as a <a href="qlist.html">QList</a> or a <a href="qvector.html">QVector</a>. <a href="qtconcurrentmap.html#map">QtConcurrent::map</a>() modifies a sequence in-place, <a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>() returns a new sequence containing the modified content, and <a href="qtconcurrentmap.html#mappedReduced">QtConcurrent::mappedReduced</a>() returns a single result.</p>
<p>Each of the above functions has a blocking variant that returns the final result instead of a <a href="qfuture.html">QFuture</a>. You use them in the same way as the asynchronous variants.</p>
<pre class="cpp"> <span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> images <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="comment">// each call blocks until the entire operation is finished</span>
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> future <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>blockingMapped(images<span class="operator">,</span> scaled);
<span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>blockingMap(images<span class="operator">,</span> scale);
<span class="type"><a href="qimage.html">QImage</a></span> collage <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>blockingMappedReduced(images<span class="operator">,</span> scaled<span class="operator">,</span> addToCollage);</pre>
<p>Note that the result types above are not <a href="qfuture.html">QFuture</a> objects, but real result types (in this case, <a href="qlist.html">QList</a><<a href="qimage.html">QImage</a>> and <a href="qimage.html">QImage</a>).</p>
<a name="concurrent-map"></a>
<h2>Concurrent Map</h2>
<p><a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>() takes an input sequence and a map function. This map function is then called for each item in the sequence, and a new sequence containing the return values from the map function is returned.</p>
<p>The map function must be of the form:</p>
<pre class="cpp"> U function(<span class="keyword">const</span> T <span class="operator">&</span>t);</pre>
<p>T and U can be any type (and they can even be the same type), but T must match the type stored in the sequence. The function returns the modified or <i>mapped</i> content.</p>
<p>This example shows how to apply a scale function to all the items in a sequence:</p>
<pre class="cpp"> <span class="type"><a href="qimage.html">QImage</a></span> scaled(<span class="keyword">const</span> <span class="type"><a href="qimage.html">QImage</a></span> <span class="operator">&</span>image)
{
<span class="keyword">return</span> image<span class="operator">.</span>scaled(<span class="number">100</span><span class="operator">,</span> <span class="number">100</span>);
}
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> images <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> thumbnails <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>mapped(images<span class="operator">,</span> scaled);</pre>
<p>The results of the map are made available through <a href="qfuture.html">QFuture</a>. See the <a href="qfuture.html">QFuture</a> and <a href="qfuturewatcher.html">QFutureWatcher</a> documentation for more information on how to use <a href="qfuture.html">QFuture</a> in your applications.</p>
<p>If you want to modify a sequence in-place, use <a href="qtconcurrentmap.html#map">QtConcurrent::map</a>(). The map function must then be of the form:</p>
<pre class="cpp"> U function(T <span class="operator">&</span>t);</pre>
<p>Note that the return value and return type of the map function are not used.</p>
<p>Using <a href="qtconcurrentmap.html#map">QtConcurrent::map</a>() is similar to using <a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>():</p>
<pre class="cpp"> <span class="type">void</span> scale(<span class="type"><a href="qimage.html">QImage</a></span> <span class="operator">&</span>image)
{
image <span class="operator">=</span> image<span class="operator">.</span>scaled(<span class="number">100</span><span class="operator">,</span> <span class="number">100</span>);
}
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> images <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type">void</span><span class="operator">></span> future <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>map(images<span class="operator">,</span> scale);</pre>
<p>Since the sequence is modified in place, <a href="qtconcurrentmap.html#map">QtConcurrent::map</a>() does not return any results via <a href="qfuture.html">QFuture</a>. However, you can still use <a href="qfuture.html">QFuture</a> and <a href="qfuturewatcher.html">QFutureWatcher</a> to monitor the status of the map.</p>
<a name="concurrent-map-reduce"></a>
<h2>Concurrent Map-Reduce</h2>
<p><a href="qtconcurrentmap.html#mappedReduced">QtConcurrent::mappedReduced</a>() is similar to <a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>(), but instead of returning a sequence with the new results, the results are combined into a single value using a reduce function.</p>
<p>The reduce function must be of the form:</p>
<pre class="cpp"> V function(T <span class="operator">&</span>result<span class="operator">,</span> <span class="keyword">const</span> U <span class="operator">&</span>intermediate)</pre>
<p>T is the type of the final result, U is the return type of the map function. Note that the return value and return type of the reduce function are not used.</p>
<p>Call <a href="qtconcurrentmap.html#mappedReduced">QtConcurrent::mappedReduced</a>() like this:</p>
<pre class="cpp"> <span class="type">void</span> addToCollage(<span class="type"><a href="qimage.html">QImage</a></span> <span class="operator">&</span>collage<span class="operator">,</span> <span class="keyword">const</span> <span class="type"><a href="qimage.html">QImage</a></span> <span class="operator">&</span>thumbnail)
{
<span class="type"><a href="qpainter.html">QPainter</a></span> p(<span class="operator">&</span>collage);
<span class="keyword">static</span> <span class="type"><a href="qpoint.html">QPoint</a></span> offset <span class="operator">=</span> <span class="type"><a href="qpoint.html">QPoint</a></span>(<span class="number">0</span><span class="operator">,</span> <span class="number">0</span>);
p<span class="operator">.</span>drawImage(offset<span class="operator">,</span> thumbnail);
offset <span class="operator">+</span><span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
}
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> images <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> collage <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>mappedReduced(images<span class="operator">,</span> scaled<span class="operator">,</span> addToCollage);</pre>
<p>The reduce function will be called once for each result returned by the map function, and should merge the <i>intermediate</i> into the <i>result</i> variable. <a href="qtconcurrentmap.html#mappedReduced">QtConcurrent::mappedReduced</a>() guarantees that only one thread will call reduce at a time, so using a mutex to lock the result variable is not necessary. The <a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::ReduceOptions</a> enum provides a way to control the order in which the reduction is done. If <a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::UnorderedReduce</a> is used (the default), the order is undefined, while <a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::OrderedReduce</a> ensures that the reduction is done in the order of the original sequence.</p>
<a name="additional-api-features"></a>
<h2>Additional API Features</h2>
<a name="using-iterators-instead-of-sequence"></a>
<h3>Using Iterators instead of Sequence</h3>
<p>Each of the above functions has a variant that takes an iterator range instead of a sequence. You use them in the same way as the sequence variants:</p>
<pre class="cpp"> <span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> images <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> thumbnails <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>mapped(images<span class="operator">.</span>constBegin()<span class="operator">,</span> images<span class="operator">.</span>constEnd()<span class="operator">,</span> scaled);
<span class="comment">// map in-place only works on non-const iterators</span>
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type">void</span><span class="operator">></span> future <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>map(images<span class="operator">.</span>begin()<span class="operator">,</span> images<span class="operator">.</span>end()<span class="operator">,</span> scale);
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> collage <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>mappedReduced(images<span class="operator">.</span>constBegin()<span class="operator">,</span> images<span class="operator">.</span>constEnd()<span class="operator">,</span> scaled<span class="operator">,</span> addToCollage);</pre>
<a name="blocking-variants"></a>
<h3>Blocking Variants</h3>
<p>Each of the above functions has a blocking variant that returns the final result instead of a <a href="qfuture.html">QFuture</a>. You use them in the same way as the asynchronous variants.</p>
<pre class="cpp"> <span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> images <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="comment">// each call blocks until the entire operation is finished</span>
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> future <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>blockingMapped(images<span class="operator">,</span> scaled);
<span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>blockingMap(images<span class="operator">,</span> scale);
<span class="type"><a href="qimage.html">QImage</a></span> collage <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>blockingMappedReduced(images<span class="operator">,</span> scaled<span class="operator">,</span> addToCollage);</pre>
<p>Note that the result types above are not <a href="qfuture.html">QFuture</a> objects, but real result types (in this case, <a href="qlist.html">QList</a><<a href="qimage.html">QImage</a>> and <a href="qimage.html">QImage</a>).</p>
<a name="using-member-functions"></a>
<h3>Using Member Functions</h3>
<p><a href="qtconcurrentmap.html#map">QtConcurrent::map</a>(), <a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>(), and <a href="qtconcurrentmap.html#mappedReduced">QtConcurrent::mappedReduced</a>() accept pointers to member functions. The member function class type must match the type stored in the sequence:</p>
<pre class="cpp"> <span class="comment">// squeeze all strings in a QStringList</span>
<span class="type"><a href="qstringlist.html">QStringList</a></span> strings <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type">void</span><span class="operator">></span> squeezedStrings <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>map(strings<span class="operator">,</span> <span class="operator">&</span><span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>squeeze);
<span class="comment">// swap the rgb values of all pixels on a list of images</span>
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> images <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> bgrImages <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>mapped(images<span class="operator">,</span> <span class="operator">&</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">::</span>rgbSwapped);
<span class="comment">// create a set of the lengths of all strings in a list</span>
<span class="type"><a href="qstringlist.html">QStringList</a></span> strings <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="qset.html">QSet</a></span><span class="operator"><</span><span class="type">int</span><span class="operator">></span> <span class="operator">></span> wordLengths <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>mappedReduced(string<span class="operator">,</span> <span class="operator">&</span><span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>length<span class="operator">,</span> <span class="operator">&</span><span class="type"><a href="qset.html">QSet</a></span><span class="operator"><</span><span class="type">int</span><span class="operator">></span><span class="operator">::</span>insert);</pre>
<p>Note that when using <a href="qtconcurrentmap.html#mappedReduced">QtConcurrent::mappedReduced</a>(), you can mix the use of normal and member functions freely:</p>
<pre class="cpp"> <span class="comment">// can mix normal functions and member functions with QtConcurrent::mappedReduced()</span>
<span class="comment">// compute the average length of a list of strings</span>
<span class="keyword">extern</span> <span class="type">void</span> computeAverage(<span class="type">int</span> <span class="operator">&</span>average<span class="operator">,</span> <span class="type">int</span> length);
<span class="type"><a href="qstringlist.html">QStringList</a></span> strings <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type">int</span><span class="operator">></span> averageWordLength <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>mappedReduced(strings<span class="operator">,</span> <span class="operator">&</span><span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>length<span class="operator">,</span> computeAverage);
<span class="comment">// create a set of the color distribution of all images in a list</span>
<span class="keyword">extern</span> <span class="type">int</span> colorDistribution(<span class="keyword">const</span> <span class="type"><a href="qimage.html">QImage</a></span> <span class="operator">&</span>string);
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> images <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="qset.html">QSet</a></span><span class="operator"><</span><span class="type">int</span><span class="operator">></span> <span class="operator">></span> totalColorDistribution <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>mappedReduced(images<span class="operator">,</span> colorDistribution<span class="operator">,</span> <span class="type"><a href="qset.html">QSet</a></span><span class="operator"><</span><span class="type">int</span><span class="operator">></span><span class="operator">::</span>insert);</pre>
<a name="using-function-objects"></a>
<h3>Using Function Objects</h3>
<p><a href="qtconcurrentmap.html#map">QtConcurrent::map</a>(), <a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>(), and <a href="qtconcurrentmap.html#mappedReduced">QtConcurrent::mappedReduced</a>() accept function objects, which can be used to add state to a function call. The result_type typedef must define the result type of the function call operator:</p>
<pre class="cpp"> <span class="keyword">struct</span> Scaled
{
Scaled(<span class="type">int</span> size)
: m_size(size) { }
<span class="keyword">typedef</span> <span class="type"><a href="qimage.html">QImage</a></span> result_type;
<span class="type"><a href="qimage.html">QImage</a></span> <span class="keyword">operator</span>()(<span class="keyword">const</span> <span class="type"><a href="qimage.html">QImage</a></span> <span class="operator">&</span>image)
{
<span class="keyword">return</span> image<span class="operator">.</span>scaled(m_size<span class="operator">,</span> m_size);
}
<span class="type">int</span> m_size;
};
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> images <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> thumbnails <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>mapped(images<span class="operator">,</span> Scaled(<span class="number">100</span>));</pre>
<a name="using-bound-function-arguments"></a>
<h3>Using Bound Function Arguments</h3>
<p>Note that Qt does not provide support for bound functions. This is provided by 3rd party libraries like <a href="http://www.boost.org/libs/bind/bind.html">Boost</a> or <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf">C++ TR1 Library Extensions</a>.</p>
<p>If you want to use a map function that takes more than one argument you can use boost::bind() or std::tr1::bind() to transform it onto a function that takes one argument.</p>
<p>As an example, we'll use <a href="qimage.html#scaledToWidth">QImage::scaledToWidth</a>():</p>
<pre class="cpp"> <span class="type"><a href="qimage.html">QImage</a></span> <span class="type"><a href="qimage.html">QImage</a></span><span class="operator">::</span>scaledToWidth(<span class="type">int</span> width<span class="operator">,</span> <span class="type"><a href="qt.html">Qt</a></span><span class="operator">::</span>TransformationMode) <span class="keyword">const</span>;</pre>
<p>scaledToWidth takes three arguments (including the "this" pointer) and can't be used with <a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>() directly, because <a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>() expects a function that takes one argument. To use <a href="qimage.html#scaledToWidth">QImage::scaledToWidth</a>() with <a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>() we have to provide a value for the <i>width</i> and the <i>transformation mode</i>:</p>
<pre class="cpp"> boost<span class="operator">::</span>bind(<span class="operator">&</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">::</span>scaledToWidth<span class="operator">,</span> <span class="number">100</span><span class="operator">,</span> <span class="type"><a href="qt.html">Qt</a></span><span class="operator">::</span>SmoothTransformation)</pre>
<p>The return value from boost::bind() is a function object (functor) with the following signature:</p>
<pre class="cpp"> <span class="type"><a href="qimage.html">QImage</a></span> scaledToWith(<span class="keyword">const</span> <span class="type"><a href="qimage.html">QImage</a></span> <span class="operator">&</span>image)</pre>
<p>This matches what <a href="qtconcurrentmap.html#mapped">QtConcurrent::mapped</a>() expects, and the complete example becomes:</p>
<pre class="cpp"> <span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> images <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">></span> thumbnails <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>mapped(images<span class="operator">,</span> boost<span class="operator">::</span>bind(<span class="operator">&</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">::</span>scaledToWidth<span class="operator">,</span> <span class="number">100</span> <span class="type"><a href="qt.html">Qt</a></span><span class="operator">::</span>SmoothTransformation));</pre>
</div>
<!-- @@@<QtConcurrentMap> -->
<h2>Function Documentation</h2>
<!-- $$$blockingMappedReduced[overload1]$$$blockingMappedReducedconstSequence&MapFunctionReduceFunctionQtConcurrent::ReduceOptions -->
<h3 class="fn"><a name="blockingMappedReduced"></a><span class="type">T</span> QtConcurrent::<span class="name">blockingMappedReduced</span> ( const <span class="type">Sequence</span> & <i>sequence</i>, <span class="type">MapFunction</span> <i>mapFunction</i>, <span class="type">ReduceFunction</span> <i>reduceFunction</i>, <span class="type"><a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::ReduceOptions</a></span> <i>reduceOptions</i> = UnorderedReduce | SequentialReduce )</h3>
<p>Calls <i>mapFunction</i> once for each item in <i>sequence</i>. The return value of each <i>mapFunction</i> is passed to <i>reduceFunction</i>.</p>
<p>Note that while <i>mapFunction</i> is called concurrently, only one thread at a time will call <i>reduceFunction</i>. The order in which <i>reduceFunction</i> is called is determined by <i>reduceOptions</i>.</p>
<p><b>Note:</b> This function will block until all items in the sequence have been processed.</p>
<p><b>See also </b><a href="qtconcurrentmap.html#mapped">mapped</a>().</p>
<!-- @@@blockingMappedReduced -->
<!-- $$$blockingMappedReduced$$$blockingMappedReducedConstIteratorConstIteratorMapFunctionReduceFunctionQtConcurrent::ReduceOptions -->
<h3 class="fn"><a name="blockingMappedReduced-2"></a><span class="type">T</span> QtConcurrent::<span class="name">blockingMappedReduced</span> ( <span class="type">ConstIterator</span> <i>begin</i>, <span class="type">ConstIterator</span> <i>end</i>, <span class="type">MapFunction</span> <i>mapFunction</i>, <span class="type">ReduceFunction</span> <i>reduceFunction</i>, <span class="type"><a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::ReduceOptions</a></span> <i>reduceOptions</i> = UnorderedReduce | SequentialReduce )</h3>
<p>Calls <i>mapFunction</i> once for each item from <i>begin</i> to <i>end</i>. The return value of each <i>mapFunction</i> is passed to <i>reduceFunction</i>.</p>
<p>Note that while <i>mapFunction</i> is called concurrently, only one thread at a time will call <i>reduceFunction</i>. The order in which <i>reduceFunction</i> is called is undefined.</p>
<p><b>Note:</b> This function will block until the iterator reaches the end of the sequence being processed.</p>
<p><b>See also </b><a href="qtconcurrentmap.html#blockingMappedReduced">blockingMappedReduced</a>().</p>
<!-- @@@blockingMappedReduced -->
<!-- $$$map[overload1]$$$mapSequence&MapFunction -->
<h3 class="fn"><a name="map"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">void</span>> QtConcurrent::<span class="name">map</span> ( <span class="type">Sequence</span> & <i>sequence</i>, <span class="type">MapFunction</span> <i>function</i> )</h3>
<p>Calls <i>function</i> once for each item in <i>sequence</i>. The <i>function</i> is passed a reference to the item, so that any modifications done to the item will appear in <i>sequence</i>.</p>
<!-- @@@map -->
<!-- $$$map$$$mapIteratorIteratorMapFunction -->
<h3 class="fn"><a name="map-2"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">void</span>> QtConcurrent::<span class="name">map</span> ( <span class="type">Iterator</span> <i>begin</i>, <span class="type">Iterator</span> <i>end</i>, <span class="type">MapFunction</span> <i>function</i> )</h3>
<p>Calls <i>function</i> once for each item from <i>begin</i> to <i>end</i>. The <i>function</i> is passed a reference to the item, so that any modifications done to the item will appear in the sequence which the iterators belong to.</p>
<!-- @@@map -->
<!-- $$$mapped[overload1]$$$mappedconstSequence&MapFunction -->
<h3 class="fn"><a name="mapped"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">T</span>> QtConcurrent::<span class="name">mapped</span> ( const <span class="type">Sequence</span> & <i>sequence</i>, <span class="type">MapFunction</span> <i>function</i> )</h3>
<p>Calls <i>function</i> once for each item in <i>sequence</i> and returns a future with each mapped item as a result. You can use <a href="qfuture-const-iterator.html">QFuture::const_iterator</a> or <a href="qfutureiterator.html">QFutureIterator</a> to iterate through the results.</p>
<!-- @@@mapped -->
<!-- $$$mapped$$$mappedConstIteratorConstIteratorMapFunction -->
<h3 class="fn"><a name="mapped-2"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">T</span>> QtConcurrent::<span class="name">mapped</span> ( <span class="type">ConstIterator</span> <i>begin</i>, <span class="type">ConstIterator</span> <i>end</i>, <span class="type">MapFunction</span> <i>function</i> )</h3>
<p>Calls <i>function</i> once for each item from <i>begin</i> to <i>end</i> and returns a future with each mapped item as a result. You can use <a href="qfuture-const-iterator.html">QFuture::const_iterator</a> or <a href="qfutureiterator.html">QFutureIterator</a> to iterate through the results.</p>
<!-- @@@mapped -->
<!-- $$$mappedReduced[overload1]$$$mappedReducedconstSequence&MapFunctionReduceFunctionQtConcurrent::ReduceOptions -->
<h3 class="fn"><a name="mappedReduced"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">T</span>> QtConcurrent::<span class="name">mappedReduced</span> ( const <span class="type">Sequence</span> & <i>sequence</i>, <span class="type">MapFunction</span> <i>mapFunction</i>, <span class="type">ReduceFunction</span> <i>reduceFunction</i>, <span class="type"><a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::ReduceOptions</a></span> <i>reduceOptions</i> = UnorderedReduce | SequentialReduce )</h3>
<p>Calls <i>mapFunction</i> once for each item in <i>sequence</i>. The return value of each <i>mapFunction</i> is passed to <i>reduceFunction</i>.</p>
<p>Note that while <i>mapFunction</i> is called concurrently, only one thread at a time will call <i>reduceFunction</i>. The order in which <i>reduceFunction</i> is called is determined by <i>reduceOptions</i>.</p>
<!-- @@@mappedReduced -->
<!-- $$$mappedReduced$$$mappedReducedConstIteratorConstIteratorMapFunctionReduceFunctionQtConcurrent::ReduceOptions -->
<h3 class="fn"><a name="mappedReduced-2"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">T</span>> QtConcurrent::<span class="name">mappedReduced</span> ( <span class="type">ConstIterator</span> <i>begin</i>, <span class="type">ConstIterator</span> <i>end</i>, <span class="type">MapFunction</span> <i>mapFunction</i>, <span class="type">ReduceFunction</span> <i>reduceFunction</i>, <span class="type"><a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::ReduceOptions</a></span> <i>reduceOptions</i> = UnorderedReduce | SequentialReduce )</h3>
<p>Calls <i>mapFunction</i> once for each item from <i>begin</i> to <i>end</i>. The return value of each <i>mapFunction</i> is passed to <i>reduceFunction</i>.</p>
<p>Note that while <i>mapFunction</i> is called concurrently, only one thread at a time will call <i>reduceFunction</i>. By default, the order in which <i>reduceFunction</i> is called is undefined.</p>
<p><b>Note:</b> <a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::OrderedReduce</a> results in the ordered reduction.</p>
<!-- @@@mappedReduced -->
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