<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en_US" lang="en_US">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<!-- qtconcurrentfilter.cpp -->
<title>Qt 4.8: <QtConcurrentFilter> - Concurrent Filter and Filter-Reduce</title>
<link rel="stylesheet" type="text/css" href="style/style.css" />
<script src="scripts/jquery.js" type="text/javascript"></script>
<script src="scripts/functions.js" type="text/javascript"></script>
<link rel="stylesheet" type="text/css" href="style/superfish.css" />
<link rel="stylesheet" type="text/css" href="style/narrow.css" />
<!--[if IE]>
<meta name="MSSmartTagsPreventParsing" content="true">
<meta http-equiv="imagetoolbar" content="no">
<![endif]-->
<!--[if lt IE 7]>
<link rel="stylesheet" type="text/css" href="style/style_ie6.css">
<![endif]-->
<!--[if IE 7]>
<link rel="stylesheet" type="text/css" href="style/style_ie7.css">
<![endif]-->
<!--[if IE 8]>
<link rel="stylesheet" type="text/css" href="style/style_ie8.css">
<![endif]-->
<script src="scripts/superfish.js" type="text/javascript"></script>
<script src="scripts/narrow.js" type="text/javascript"></script>
</head>
<body class="" onload="CheckEmptyAndLoadList();">
<div class="header" id="qtdocheader">
<div class="content">
<div id="nav-logo">
<a href="index.html">Home</a></div>
<a href="index.html" class="qtref"><span>Qt Reference Documentation</span></a>
<div id="narrowsearch"></div>
<div id="nav-topright">
<ul>
<li class="nav-topright-home"><a href="http://qt.digia.com/">Qt HOME</a></li>
<li class="nav-topright-dev"><a href="http://qt-project.org/">DEV</a></li>
<li class="nav-topright-doc nav-topright-doc-active"><a href="http://qt-project.org/doc/">
DOC</a></li>
<li class="nav-topright-blog"><a href="http://blog.qt.digia.com/">BLOG</a></li>
</ul>
</div>
<div id="shortCut">
<ul>
<li class="shortCut-topleft-inactive"><span><a href="index.html">Qt 4.8</a></span></li>
<li class="shortCut-topleft-active"><a href="http://qt-project.org/doc/">ALL VERSIONS </a></li>
</ul>
</div>
<ul class="sf-menu" id="narrowmenu">
<li><a href="#">API Lookup</a>
<ul>
<li><a href="classes.html">Class index</a></li>
<li><a href="functions.html">Function index</a></li>
<li><a href="modules.html">Modules</a></li>
<li><a href="namespaces.html">Namespaces</a></li>
<li><a href="qtglobal.html">Global Declarations</a></li>
<li><a href="qdeclarativeelements.html">QML elements</a></li>
</ul>
</li>
<li><a href="#">Qt Topics</a>
<ul>
<li><a href="qt-basic-concepts.html">Programming with Qt</a></li>
<li><a href="qtquick.html">Device UIs & Qt Quick</a></li>
<li><a href="qt-gui-concepts.html">UI Design with Qt</a></li>
<li><a href="supported-platforms.html">Supported Platforms</a></li>
<li><a href="technology-apis.html">Qt and Key Technologies</a></li>
<li><a href="best-practices.html">How-To's and Best Practices</a></li>
</ul>
</li>
<li><a href="#">Examples</a>
<ul>
<li><a href="all-examples.html">Examples</a></li>
<li><a href="tutorials.html">Tutorials</a></li>
<li><a href="demos.html">Demos</a></li>
<li><a href="qdeclarativeexamples.html">QML Examples</a></li>
</ul>
</li>
</ul>
</div>
</div>
<div class="wrapper">
<div class="hd">
<span></span>
</div>
<div class="bd group">
<div class="sidebar">
<div class="searchlabel">
Search index:</div>
<div class="search" id="sidebarsearch">
<form id="qtdocsearch" action="" onsubmit="return false;">
<fieldset>
<input type="text" name="searchstring" id="pageType" value="" />
<div id="resultdialog">
<a href="#" id="resultclose">Close</a>
<p id="resultlinks" class="all"><a href="#" id="showallresults">All</a> | <a href="#" id="showapiresults">API</a> | <a href="#" id="showarticleresults">Articles</a> | <a href="#" id="showexampleresults">Examples</a></p>
<p id="searchcount" class="all"><span id="resultcount"></span><span id="apicount"></span><span id="articlecount"></span><span id="examplecount"></span> results:</p>
<ul id="resultlist" class="all">
</ul>
</div>
</fieldset>
</form>
</div>
<div class="box first bottombar" id="lookup">
<h2 title="API Lookup"><span></span>
API Lookup</h2>
<div id="list001" class="list">
<ul id="ul001" >
<li class="defaultLink"><a href="classes.html">Class index</a></li>
<li class="defaultLink"><a href="functions.html">Function index</a></li>
<li class="defaultLink"><a href="modules.html">Modules</a></li>
<li class="defaultLink"><a href="namespaces.html">Namespaces</a></li>
<li class="defaultLink"><a href="qtglobal.html">Global Declarations</a></li>
<li class="defaultLink"><a href="qdeclarativeelements.html">QML elements</a></li>
</ul>
</div>
</div>
<div class="box bottombar" id="topics">
<h2 title="Qt Topics"><span></span>
Qt Topics</h2>
<div id="list002" class="list">
<ul id="ul002" >
<li class="defaultLink"><a href="qt-basic-concepts.html">Programming with Qt</a></li>
<li class="defaultLink"><a href="qtquick.html">Device UIs & Qt Quick</a></li>
<li class="defaultLink"><a href="qt-gui-concepts.html">UI Design with Qt</a></li>
<li class="defaultLink"><a href="supported-platforms.html">Supported Platforms</a></li>
<li class="defaultLink"><a href="technology-apis.html">Qt and Key Technologies</a></li>
<li class="defaultLink"><a href="best-practices.html">How-To's and Best Practices</a></li>
</ul>
</div>
</div>
<div class="box" id="examples">
<h2 title="Examples"><span></span>
Examples</h2>
<div id="list003" class="list">
<ul id="ul003">
<li class="defaultLink"><a href="all-examples.html">Examples</a></li>
<li class="defaultLink"><a href="tutorials.html">Tutorials</a></li>
<li class="defaultLink"><a href="demos.html">Demos</a></li>
<li class="defaultLink"><a href="qdeclarativeexamples.html">QML Examples</a></li>
</ul>
</div>
</div>
</div>
<div class="wrap">
<div class="toolbar">
<div class="breadcrumb toolblock">
<ul>
<li class="first"><a href="index.html">Home</a></li>
<!-- Breadcrumbs go here -->
</ul>
</div>
<div class="toolbuttons toolblock">
<ul>
<li id="smallA" class="t_button">A</li>
<li id="medA" class="t_button active">A</li>
<li id="bigA" class="t_button">A</li>
<li id="print" class="t_button"><a href="javascript:this.print();">
<span>Print</span></a></li>
</ul>
</div>
</div>
<div class="content mainContent">
<div class="toc">
<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#concurrent-filter">Concurrent Filter</a></li>
<li class="level1"><a href="#concurrent-filter-reduce">Concurrent Filter-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>
<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>
</ul>
</div>
<h1 class="title"><QtConcurrentFilter> - Concurrent Filter and Filter-Reduce</h1>
<span class="subtitle"></span>
<!-- $$$<QtConcurrentFilter>-brief -->
<p>The <QtConcurrentFilter> header provides concurrent Filter and Filter-Reduce. <a href="#details">More...</a></p>
<!-- @@@<QtConcurrentFilter> -->
<ul>
</ul>
<a name="Functionsx"></a>
<h2>Functions</h2>
<table class="alignedsummary">
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<void> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentfilter.html#filter">filter</a></b> ( Sequence & <i>sequence</i>, FilterFunction <i>filterFunction</i> )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<T> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentfilter.html#filtered">filtered</a></b> ( const Sequence & <i>sequence</i>, FilterFunction <i>filterFunction</i> )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<T> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentfilter.html#filtered-2">filtered</a></b> ( ConstIterator <i>begin</i>, ConstIterator <i>end</i>, FilterFunction <i>filterFunction</i> )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<T> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentfilter.html#filteredReduced">filteredReduced</a></b> ( const Sequence & <i>sequence</i>, FilterFunction <i>filterFunction</i>, ReduceFunction <i>reduceFunction</i>, QtConcurrent::ReduceOptions <i>reduceOptions</i> = UnorderedReduce | SequentialReduce )</td></tr>
<tr><td class="memItemLeft topAlign rightAlign"> QFuture<T> </td><td class="memItemRight bottomAlign"><b><a href="qtconcurrentfilter.html#filteredReduced-2">filteredReduced</a></b> ( ConstIterator <i>begin</i>, ConstIterator <i>end</i>, FilterFunction <i>filterFunction</i>, ReduceFunction <i>reduceFunction</i>, QtConcurrent::ReduceOptions <i>reduceOptions</i> = UnorderedReduce | SequentialReduce )</td></tr>
</table>
<!-- $$$<QtConcurrentFilter>-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="qtconcurrentfilter.html#filter">QtConcurrent::filter</a>(), <a href="qtconcurrentfilter.html#filtered">QtConcurrent::filtered</a>() and <a href="qtconcurrentfilter.html#filteredReduced">QtConcurrent::filteredReduced</a>() functions filter items in a sequence such as a <a href="qlist.html">QList</a> or a <a href="qvector.html">QVector</a> in parallel. <a href="qtconcurrentfilter.html#filter">QtConcurrent::filter</a>() modifies a sequence in-place, <a href="qtconcurrentfilter.html#filtered">QtConcurrent::filtered</a>() returns a new sequence containing the filtered content, and <a href="qtconcurrentfilter.html#filteredReduced">QtConcurrent::filteredReduced</a>() returns a single result.</p>
<p>Each of the above functions have 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="qstringlist.html">QStringList</a></span> strings <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="qstringlist.html">QStringList</a></span> lowerCaseStrings <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>blockingFiltered(strings<span class="operator">,</span> allLowerCase);
<span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>blockingFilter(strings<span class="operator">,</span> allLowerCase);
<span class="type"><a href="qset.html">QSet</a></span><span class="operator"><</span><span class="type"><a href="qstring.html">QString</a></span><span class="operator">></span> dictionary <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>blockingFilteredReduced(strings<span class="operator">,</span> allLowerCase<span class="operator">,</span> addToDictionary);</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="qstringlist.html">QStringList</a> and <a href="qset.html">QSet</a><<a href="qstring.html">QString</a>>).</p>
<a name="concurrent-filter"></a>
<h2>Concurrent Filter</h2>
<p><a href="qtconcurrentfilter.html#filtered">QtConcurrent::filtered</a>() takes an input sequence and a filter function. This filter function is then called for each item in the sequence, and a new sequence containing the filtered values is returned.</p>
<p>The filter function must be of the form:</p>
<pre class="cpp"> <span class="type">bool</span> function(<span class="keyword">const</span> T <span class="operator">&</span>t);</pre>
<p>T must match the type stored in the sequence. The function returns true if the item should be kept, false if it should be discarded.</p>
<p>This example shows how to keep strings that are all lower-case from a <a href="qstringlist.html">QStringList</a>:</p>
<pre class="cpp"> <span class="type">bool</span> allLowerCase(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>string)
{
<span class="keyword">return</span> string<span class="operator">.</span>lowered() <span class="operator">=</span><span class="operator">=</span> string;
}
<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="qstring.html">QString</a></span><span class="operator">></span> lowerCaseStrings <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>filtered(strings<span class="operator">,</span> allLowerCase);</pre>
<p>The results of the filter 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="qtconcurrentfilter.html#filter">QtConcurrent::filter</a>():</p>
<pre class="cpp"> <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> future <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>filter(strings<span class="operator">,</span> allLowerCase);</pre>
<p>Since the sequence is modified in place, <a href="qtconcurrentfilter.html#filter">QtConcurrent::filter</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 filter.</p>
<a name="concurrent-filter-reduce"></a>
<h2>Concurrent Filter-Reduce</h2>
<p><a href="qtconcurrentfilter.html#filteredReduced">QtConcurrent::filteredReduced</a>() is similar to <a href="qtconcurrentfilter.html#filtered">QtConcurrent::filtered</a>(), but instead of returing a sequence with the filtered 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 type of items being filtered. Note that the return value and return type of the reduce function are not used.</p>
<p>Call <a href="qtconcurrentfilter.html#filteredReduced">QtConcurrent::filteredReduced</a>() like this:</p>
<pre class="cpp"> <span class="type">void</span> addToDictionary(<span class="type"><a href="qset.html">QSet</a></span><span class="operator"><</span><span class="type"><a href="qstring.html">QString</a></span><span class="operator">></span> <span class="operator">&</span>dictionary<span class="operator">,</span> <span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>string)
{
dictionary<span class="operator">.</span>insert(string);
}
<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"><a href="qstring.html">QString</a></span><span class="operator">></span> <span class="operator">></span> dictionary <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>filteredReduced(strings<span class="operator">,</span> allLowerCase<span class="operator">,</span> addToDictionary);</pre>
<p>The reduce function will be called once for each result kept by the filter function, and should merge the <i>intermediate</i> into the <i>result</i> variable. <a href="qtconcurrentfilter.html#filteredReduced">QtConcurrent::filteredReduced</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.</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="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="qstring.html">QString</a></span><span class="operator">></span> lowerCaseStrings <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>filtered(strings<span class="operator">.</span>constBegin()<span class="operator">,</span> strings<span class="operator">.</span>constEnd()<span class="operator">,</span> allLowerCase);
<span class="comment">// filter 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>filter(strings<span class="operator">.</span>begin()<span class="operator">,</span> strings<span class="operator">.</span>end()<span class="operator">,</span> allLowerCase);
<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"><a href="qstring.html">QString</a></span><span class="operator">></span> <span class="operator">></span> dictionary <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>filteredReduced(strings<span class="operator">.</span>constBegin()<span class="operator">,</span> strings<span class="operator">.</span>constEnd()<span class="operator">,</span> allLowerCase<span class="operator">,</span> addToDictionary);</pre>
<a name="using-member-functions"></a>
<h3>Using Member Functions</h3>
<p><a href="qtconcurrentfilter.html#filter">QtConcurrent::filter</a>(), <a href="qtconcurrentfilter.html#filtered">QtConcurrent::filtered</a>(), and <a href="qtconcurrentfilter.html#filteredReduced">QtConcurrent::filteredReduced</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">// keep only images with an alpha channel</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> alphaImages <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>filter(strings<span class="operator">,</span> <span class="operator">&</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">::</span>hasAlphaChannel);
<span class="comment">// keep only gray scale 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> grayscaleImages <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>filtered(images<span class="operator">,</span> <span class="operator">&</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">::</span>isGrayscale);
<span class="comment">// create a set of all printable characters</span>
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qchar.html">QChar</a></span><span class="operator">></span> characters <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"><a href="qchar.html">QChar</a></span><span class="operator">></span> <span class="operator">></span> set <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>filteredReduced(characters<span class="operator">,</span> <span class="operator">&</span><span class="type"><a href="qchar.html">QChar</a></span><span class="operator">::</span>isPrint<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"><a href="qchar.html">QChar</a></span><span class="operator">></span><span class="operator">::</span>insert);</pre>
<p>Note that when using <a href="qtconcurrentfilter.html#filteredReduced">QtConcurrent::filteredReduced</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::filteredReduced()</span>
<span class="comment">// create a dictionary of all lower cased strings</span>
<span class="keyword">extern</span> <span class="type">bool</span> allLowerCase(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>string);
<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> averageWordLength <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>filteredReduced(strings<span class="operator">,</span> allLowerCase<span class="operator">,</span> <span class="type"><a href="qset.html">QSet</a></span><span class="operator"><</span><span class="type"><a href="qstring.html">QString</a></span><span class="operator">></span><span class="operator">::</span>insert);
<span class="comment">// create a collage of all gray scale images</span>
<span class="keyword">extern</span> <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>grayscaleImage);
<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>filteredReduced(images<span class="operator">,</span> <span class="operator">&</span><span class="type"><a href="qimage.html">QImage</a></span><span class="operator">::</span>isGrayscale<span class="operator">,</span> addToCollage);</pre>
<a name="using-function-objects"></a>
<h3>Using Function Objects</h3>
<p><a href="qtconcurrentfilter.html#filter">QtConcurrent::filter</a>(), <a href="qtconcurrentfilter.html#filtered">QtConcurrent::filtered</a>(), and <a href="qtconcurrentfilter.html#filteredReduced">QtConcurrent::filteredReduced</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> StartsWith
{
StartsWith(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>string)
: m_string(string) { }
<span class="keyword">typedef</span> <span class="type">bool</span> result_type;
<span class="type">bool</span> <span class="keyword">operator</span>()(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>testString)
{
<span class="keyword">return</span> testString<span class="operator">.</span>startsWith(m_string);
}
<span class="type"><a href="qstring.html">QString</a></span> m_string;
};
<span class="type"><a href="qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="qstring.html">QString</a></span><span class="operator">></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="qstring.html">QString</a></span><span class="operator">></span> fooString <span class="operator">=</span> <span class="type"><a href="qtconcurrent.html">QtConcurrent</a></span><span class="operator">::</span>filtered(images<span class="operator">,</span> StartsWith(QLatin1String(<span class="string">"Foo"</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 filter function 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 use <a href="qstring.html#contains">QString::contains</a>():</p>
<pre class="cpp"> <span class="type">bool</span> <span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>contains(<span class="keyword">const</span> <span class="type"><a href="qregexp.html">QRegExp</a></span> <span class="operator">&</span>regexp) <span class="keyword">const</span>;</pre>
<p><a href="qstring.html#contains">QString::contains</a>() takes 2 arguments (including the "this" pointer) and can't be used with <a href="qtconcurrentfilter.html#filtered">QtConcurrent::filtered</a>() directly, because <a href="qtconcurrentfilter.html#filtered">QtConcurrent::filtered</a>() expects a function that takes one argument. To use <a href="qstring.html#contains">QString::contains</a>() with <a href="qtconcurrentfilter.html#filtered">QtConcurrent::filtered</a>() we have to provide a value for the <i>regexp</i> argument:</p>
<pre class="cpp"> boost<span class="operator">::</span>bind(<span class="operator">&</span><span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>contains<span class="operator">,</span> <span class="type"><a href="qregexp.html">QRegExp</a></span>(<span class="string">"^\\S+$"</span>)); <span class="comment">// matches strings without whitespace</span></pre>
<p>The return value from boost::bind() is a function object (functor) with the following signature:</p>
<pre class="cpp"> <span class="type">bool</span> contains(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>string)</pre>
<p>This matches what <a href="qtconcurrentfilter.html#filtered">QtConcurrent::filtered</a>() expects, and the complete example becomes:</p>
<pre class="cpp"> <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>;
boost<span class="operator">::</span>bind(<span class="keyword">static_cast</span><span class="operator"><</span><span class="type">bool</span>(<span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span><span class="operator">*</span>)(<span class="keyword">const</span> <span class="type"><a href="qregexp.html">QRegExp</a></span><span class="operator">&</span>)<span class="operator">></span>( <span class="operator">&</span><span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>contains )<span class="operator">,</span> <span class="type"><a href="qregexp.html">QRegExp</a></span>(<span class="string">"..."</span> ));</pre>
</div>
<!-- @@@<QtConcurrentFilter> -->
<h2>Function Documentation</h2>
<!-- $$$filter[overload1]$$$filterSequence&FilterFunction -->
<h3 class="fn"><a name="filter"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">void</span>> QtConcurrent::<span class="name">filter</span> ( <span class="type">Sequence</span> & <i>sequence</i>, <span class="type">FilterFunction</span> <i>filterFunction</i> )</h3>
<p>Calls <i>filterFunction</i> once for each item in <i>sequence</i>. If <i>filterFunction</i> returns true, the item is kept in <i>sequence</i>; otherwise, the item is removed from <i>sequence</i>.</p>
<!-- @@@filter -->
<!-- $$$filtered[overload1]$$$filteredconstSequence&FilterFunction -->
<h3 class="fn"><a name="filtered"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">T</span>> QtConcurrent::<span class="name">filtered</span> ( const <span class="type">Sequence</span> & <i>sequence</i>, <span class="type">FilterFunction</span> <i>filterFunction</i> )</h3>
<p>Calls <i>filterFunction</i> once for each item in <i>sequence</i> and returns a new Sequence of kept items. If <i>filterFunction</i> returns true, a copy of the item is put in the new Sequence. Otherwise, the item will <i>not</i> appear in the new Sequence.</p>
<!-- @@@filtered -->
<!-- $$$filtered$$$filteredConstIteratorConstIteratorFilterFunction -->
<h3 class="fn"><a name="filtered-2"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">T</span>> QtConcurrent::<span class="name">filtered</span> ( <span class="type">ConstIterator</span> <i>begin</i>, <span class="type">ConstIterator</span> <i>end</i>, <span class="type">FilterFunction</span> <i>filterFunction</i> )</h3>
<p>Calls <i>filterFunction</i> once for each item from <i>begin</i> to <i>end</i> and returns a new Sequence of kept items. If <i>filterFunction</i> returns true, a copy of the item is put in the new Sequence. Otherwise, the item will <i>not</i> appear in the new Sequence.</p>
<!-- @@@filtered -->
<!-- $$$filteredReduced[overload1]$$$filteredReducedconstSequence&FilterFunctionReduceFunctionQtConcurrent::ReduceOptions -->
<h3 class="fn"><a name="filteredReduced"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">T</span>> QtConcurrent::<span class="name">filteredReduced</span> ( const <span class="type">Sequence</span> & <i>sequence</i>, <span class="type">FilterFunction</span> <i>filterFunction</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>filterFunction</i> once for each item in <i>sequence</i>. If <i>filterFunction</i> returns true for an item, that item is then passed to <i>reduceFunction</i>. In other words, the return value is the result of <i>reduceFunction</i> for each item where <i>filterFunction</i> returns true.</p>
<p>Note that while <i>filterFunction</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 if <i>reduceOptions</i> is <a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::UnorderedReduce</a>. If <i>reduceOptions</i> is <a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::OrderedReduce</a>, <i>reduceFunction</i> is called in the order of the original sequence.</p>
<!-- @@@filteredReduced -->
<!-- $$$filteredReduced$$$filteredReducedConstIteratorConstIteratorFilterFunctionReduceFunctionQtConcurrent::ReduceOptions -->
<h3 class="fn"><a name="filteredReduced-2"></a><span class="type"><a href="qfuture.html">QFuture</a></span><<span class="type">T</span>> QtConcurrent::<span class="name">filteredReduced</span> ( <span class="type">ConstIterator</span> <i>begin</i>, <span class="type">ConstIterator</span> <i>end</i>, <span class="type">FilterFunction</span> <i>filterFunction</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>filterFunction</i> once for each item from <i>begin</i> to <i>end</i>. If <i>filterFunction</i> returns true for an item, that item is then passed to <i>reduceFunction</i>. In other words, the return value is the result of <i>reduceFunction</i> for each item where <i>filterFunction</i> returns true.</p>
<p>Note that while <i>filterFunction</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 if <i>reduceOptions</i> is <a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::UnorderedReduce</a>. If <i>reduceOptions</i> is <a href="qtconcurrent.html#ReduceOption-enum">QtConcurrent::OrderedReduce</a>, the <i>reduceFunction</i> is called in the order of the original sequence.</p>
<!-- @@@filteredReduced -->
</div>
</div>
</div>
<div class="ft">
<span></span>
</div>
</div>
<div class="footer">
<p>
<acronym title="Copyright">©</acronym> 2015 The Qt Company Ltd.
Documentation contributions included herein are the copyrights of
their respective owners.</p>
<br />
<p>
The documentation provided herein is licensed under the terms of the
<a href="http://www.gnu.org/licenses/fdl.html">GNU Free Documentation
License version 1.3</a> as published by the Free Software Foundation.</p>
<p>
Documentation sources may be obtained from <a href="http://www.qt-project.org">
www.qt-project.org</a>.</p>
<br />
<p>
Qt and respective logos are trademarks of The Qt Company Ltd
in Finland and/or other countries worldwide. All other trademarks are property
of their respective owners. <a title="Privacy Policy"
href="http://en.gitorious.org/privacy_policy/">Privacy Policy</a></p>
</div>
<script src="scripts/functions.js" type="text/javascript"></script>
</body>
</html>
