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<td >Qt 5.9</td><td ><a href="qtconcurrent-index.html">Qt Concurrent</a></td><td >Concurrent Run</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right">Qt 5.9.4 Reference Documentation</td>
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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#running-a-function-in-a-separate-thread">Running a Function in a Separate Thread</a></li>
<li class="level1"><a href="#passing-arguments-to-the-function">Passing Arguments to the Function</a></li>
<li class="level1"><a href="#returning-values-from-the-function">Returning Values from the Function</a></li>
<li class="level1"><a href="#additional-api-features">Additional API Features</a></li>
<li class="level2"><a href="#using-member-functions">Using Member Functions</a></li>
<li class="level2"><a href="#using-lambda-functions">Using Lambda Functions</a></li>
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<h1 class="title">Concurrent Run</h1>
<span class="subtitle"></span>
<!-- $$$qtconcurrentrun.html-description -->
<div class="descr"> <a name="details"></a>
<p>The QtConcurrent::run() function runs a function in a separate thread. The return value of the function is made available through the <a href="../qtcore/qfuture.html">QFuture</a> API.</p>
<p>This function is a part of the <a href="qtconcurrent-index.html">Qt Concurrent</a> framework.</p>
<a name="running-a-function-in-a-separate-thread"></a>
<h2 id="running-a-function-in-a-separate-thread">Running a Function in a Separate Thread</h2>
<p>To run a function in another thread, use QtConcurrent::run():</p>
<pre class="cpp">
<span class="keyword">extern</span> <span class="type">void</span> aFunction();
<span class="type"><a href="../qtcore/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-module.html">QtConcurrent</a></span><span class="operator">::</span>run(aFunction);
</pre>
<p>This will run <i>aFunction</i> in a separate thread obtained from the default <a href="../qtcore/qthreadpool.html">QThreadPool</a>. You can use the <a href="../qtcore/qfuture.html">QFuture</a> and <a href="../qtcore/qfuturewatcher.html">QFutureWatcher</a> classes to monitor the status of the function.</p>
<p>To use a dedicated thread pool, you can pass the <a href="../qtcore/qthreadpool.html">QThreadPool</a> as the first argument:</p>
<pre class="cpp">
<span class="keyword">extern</span> <span class="type">void</span> aFunction();
<span class="type"><a href="../qtcore/qthreadpool.html">QThreadPool</a></span> pool;
<span class="type"><a href="../qtcore/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-module.html">QtConcurrent</a></span><span class="operator">::</span>run(<span class="operator">&</span>pool<span class="operator">,</span> aFunction);
</pre>
<a name="passing-arguments-to-the-function"></a>
<h2 id="passing-arguments-to-the-function">Passing Arguments to the Function</h2>
<p>Passing arguments to the function is done by adding them to the QtConcurrent::run() call immediately after the function name. For example:</p>
<pre class="cpp">
<span class="keyword">extern</span> <span class="type">void</span> aFunctionWithArguments(<span class="type">int</span> arg1<span class="operator">,</span> <span class="type">double</span> arg2<span class="operator">,</span> <span class="keyword">const</span> <span class="type"><a href="../qtcore/qstring.html">QString</a></span> <span class="operator">&</span>string);
<span class="type">int</span> integer <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type">double</span> floatingPoint <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="../qtcore/qstring.html">QString</a></span> string <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="../qtcore/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-module.html">QtConcurrent</a></span><span class="operator">::</span>run(aFunctionWithArguments<span class="operator">,</span> integer<span class="operator">,</span> floatingPoint<span class="operator">,</span> string);
</pre>
<p>A copy of each argument is made at the point where QtConcurrent::run() is called, and these values are passed to the thread when it begins executing the function. Changes made to the arguments after calling QtConcurrent::run() are <i>not</i> visible to the thread.</p>
<a name="returning-values-from-the-function"></a>
<h2 id="returning-values-from-the-function">Returning Values from the Function</h2>
<p>Any return value from the function is available via <a href="../qtcore/qfuture.html">QFuture</a>:</p>
<pre class="cpp">
<span class="keyword">extern</span> <span class="type"><a href="../qtcore/qstring.html">QString</a></span> functionReturningAString();
<span class="type"><a href="../qtcore/qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="../qtcore/qstring.html">QString</a></span><span class="operator">></span> future <span class="operator">=</span> <span class="type"><a href="qtconcurrent-module.html">QtConcurrent</a></span><span class="operator">::</span>run(functionReturningAString);
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
<span class="type"><a href="../qtcore/qstring.html">QString</a></span> result <span class="operator">=</span> future<span class="operator">.</span>result();
</pre>
<p>As documented above, passing arguments is done like this:</p>
<pre class="cpp">
<span class="keyword">extern</span> <span class="type"><a href="../qtcore/qstring.html">QString</a></span> someFunction(<span class="keyword">const</span> <span class="type"><a href="../qtcore/qbytearray.html">QByteArray</a></span> <span class="operator">&</span>input);
<span class="type"><a href="../qtcore/qbytearray.html">QByteArray</a></span> bytearray <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="../qtcore/qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="../qtcore/qstring.html">QString</a></span><span class="operator">></span> future <span class="operator">=</span> <span class="type"><a href="qtconcurrent-module.html">QtConcurrent</a></span><span class="operator">::</span>run(someFunction<span class="operator">,</span> bytearray);
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
<span class="type"><a href="../qtcore/qstring.html">QString</a></span> result <span class="operator">=</span> future<span class="operator">.</span>result();
</pre>
<p>Note that the <a href="../qtcore/qfuture.html#result">QFuture::result</a>() function blocks and waits for the result to become available. Use <a href="../qtcore/qfuturewatcher.html">QFutureWatcher</a> to get notification when the function has finished execution and the result is available.</p>
<a name="additional-api-features"></a>
<h2 id="additional-api-features">Additional API Features</h2>
<a name="using-member-functions"></a>
<h3 >Using Member Functions</h3>
<p>QtConcurrent::run() also accepts pointers to member functions. The first argument must be either a const reference or a pointer to an instance of the class. Passing by const reference is useful when calling const member functions; passing by pointer is useful for calling non-const member functions that modify the instance.</p>
<p>For example, calling <a href="../qtcore/qbytearray.html#split">QByteArray::split</a>() (a const member function) in a separate thread is done like this:</p>
<pre class="cpp">
<span class="comment">// call 'QList<QByteArray> QByteArray::split(char sep) const' in a separate thread</span>
<span class="type"><a href="../qtcore/qbytearray.html">QByteArray</a></span> bytearray <span class="operator">=</span> <span class="string">"hello world"</span>;
<span class="type"><a href="../qtcore/qfuture.html">QFuture</a></span><span class="operator"><</span><span class="type"><a href="../qtcore/qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="../qtcore/qbytearray.html">QByteArray</a></span><span class="operator">></span> <span class="operator">></span> future <span class="operator">=</span> <span class="type"><a href="qtconcurrent-module.html">QtConcurrent</a></span><span class="operator">::</span>run(bytearray<span class="operator">,</span> <span class="operator">&</span><span class="type"><a href="../qtcore/qbytearray.html">QByteArray</a></span><span class="operator">::</span>split<span class="operator">,</span> <span class="char">','</span>);
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
<span class="type"><a href="../qtcore/qlist.html">QList</a></span><span class="operator"><</span><span class="type"><a href="../qtcore/qbytearray.html">QByteArray</a></span><span class="operator">></span> result <span class="operator">=</span> future<span class="operator">.</span>result();
</pre>
<p>Calling a non-const member function is done like this:</p>
<pre class="cpp">
<span class="comment">// call 'void QImage::invertPixels(InvertMode mode)' in a separate thread</span>
<span class="type">QImage</span> image <span class="operator">=</span> <span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>;
<span class="type"><a href="../qtcore/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-module.html">QtConcurrent</a></span><span class="operator">::</span>run(<span class="operator">&</span>image<span class="operator">,</span> <span class="operator">&</span><span class="type">QImage</span><span class="operator">::</span>invertPixels<span class="operator">,</span> <span class="type">QImage</span><span class="operator">::</span>InvertRgba);
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
future<span class="operator">.</span>waitForFinished();
<span class="comment">// At this point, the pixels in 'image' have been inverted</span>
</pre>
<a name="using-lambda-functions"></a>
<h3 >Using Lambda Functions</h3>
<p>Calling a lambda function is done like this:</p>
<pre class="cpp">
<span class="type"><a href="../qtcore/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-module.html">QtConcurrent</a></span><span class="operator">::</span>run(<span class="operator">[</span><span class="operator">=</span><span class="operator">]</span>() {
<span class="comment">// Code in this block will run in another thread</span>
});
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
</pre>
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