<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html> <html lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <!-- qsemaphore.cpp --> <title>QSemaphoreReleaser Class | Qt Core 5.11</title> <link rel="stylesheet" type="text/css" href="style/offline-simple.css" /> <script type="text/javascript"> document.getElementsByTagName("link").item(0).setAttribute("href", "style/offline.css"); // loading style sheet breaks anchors that were jumped to before // so force jumping to anchor again setTimeout(function() { var anchor = location.hash; // need to jump to different anchor first (e.g. none) location.hash = "#"; setTimeout(function() { location.hash = anchor; }, 0); }, 0); </script> </head> <body> <div class="header" id="qtdocheader"> <div class="main"> <div class="main-rounded"> <div class="navigationbar"> <table><tr> <td ><a href="../qtdoc/index.html">Qt 5.11</a></td><td ><a href="qtcore-index.html">Qt Core</a></td><td ><a href="qtcore-module.html">C++ Classes</a></td><td >QSemaphoreReleaser</td></tr></table><table class="buildversion"><tr> <td id="buildversion" width="100%" align="right">Qt 5.11.2 Reference Documentation</td> </tr></table> </div> </div> <div class="content"> <div class="line"> <div class="content mainContent"> <div class="sidebar"> <div class="toc"> <h3><a name="toc">Contents</a></h3> <ul> <li class="level1"><a href="#public-functions">Public Functions</a></li> <li class="level1"><a href="#details">Detailed Description</a></li> </ul> </div> <div class="sidebar-content" id="sidebar-content"></div></div> <h1 class="title">QSemaphoreReleaser Class</h1> <!-- $$$QSemaphoreReleaser-brief --> <p>The <a href="qsemaphorereleaser.html">QSemaphoreReleaser</a> class provides exception-safe deferral of a <a href="qsemaphore.html#release">QSemaphore::release</a>() call. <a href="#details">More...</a></p> <!-- @@@QSemaphoreReleaser --> <div class="table"><table class="alignedsummary"> <tr><td class="memItemLeft rightAlign topAlign"> Header:</td><td class="memItemRight bottomAlign"> <span class="preprocessor">#include <QSemaphoreReleaser></span> </td></tr><tr><td class="memItemLeft rightAlign topAlign"> qmake:</td><td class="memItemRight bottomAlign"> QT += core</td></tr><tr><td class="memItemLeft rightAlign topAlign"> Since:</td><td class="memItemRight bottomAlign"> Qt 5.10</td></tr></table></div><ul> <li><a href="qsemaphorereleaser-members.html">List of all members, including inherited members</a></li> </ul> <p><b>Note:</b> All functions in this class are <a href="../qtdoc/threads-reentrancy.html">reentrant</a>.</p> <a name="public-functions"></a> <h2 id="public-functions">Public Functions</h2> <div class="table"><table class="alignedsummary"> <tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></b>()</td></tr> <tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qsemaphorereleaser.html#QSemaphoreReleaser-1">QSemaphoreReleaser</a></b>(QSemaphore &<i>sem</i>, int <i>n</i> = 1)</td></tr> <tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qsemaphorereleaser.html#QSemaphoreReleaser-2">QSemaphoreReleaser</a></b>(QSemaphore *<i>sem</i>, int <i>n</i> = 1)</td></tr> <tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qsemaphorereleaser.html#QSemaphoreReleaser-3">QSemaphoreReleaser</a></b>(QSemaphoreReleaser &&<i>other</i>)</td></tr> <tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qsemaphorereleaser.html#dtor.QSemaphoreReleaser">~QSemaphoreReleaser</a></b>()</td></tr> <tr><td class="memItemLeft rightAlign topAlign"> QSemaphore *</td><td class="memItemRight bottomAlign"><b><a href="qsemaphorereleaser.html#cancel">cancel</a></b>()</td></tr> <tr><td class="memItemLeft rightAlign topAlign"> QSemaphore *</td><td class="memItemRight bottomAlign"><b><a href="qsemaphorereleaser.html#semaphore">semaphore</a></b>() const</td></tr> <tr><td class="memItemLeft rightAlign topAlign"> void </td><td class="memItemRight bottomAlign"><b><a href="qsemaphorereleaser.html#swap">swap</a></b>(QSemaphoreReleaser &<i>other</i>)</td></tr> <tr><td class="memItemLeft rightAlign topAlign"> QSemaphoreReleaser &</td><td class="memItemRight bottomAlign"><b><a href="qsemaphorereleaser.html#operator-eq">operator=</a></b>(QSemaphoreReleaser &&<i>other</i>)</td></tr> </table></div> <a name="details"></a> <!-- $$$QSemaphoreReleaser-description --> <div class="descr"> <h2 id="details">Detailed Description</h2> <p>The <a href="qsemaphorereleaser.html">QSemaphoreReleaser</a> class provides exception-safe deferral of a <a href="qsemaphore.html#release">QSemaphore::release</a>() call.</p> <p><a href="qsemaphorereleaser.html">QSemaphoreReleaser</a> can be used wherever you would otherwise use <a href="qsemaphore.html#release">QSemaphore::release</a>(). Constructing a <a href="qsemaphorereleaser.html">QSemaphoreReleaser</a> defers the release() call on the semaphore until the <a href="qsemaphorereleaser.html">QSemaphoreReleaser</a> is destroyed (see <a href="http://en.cppreference.com/w/cpp/language/raii">RAII pattern</a>).</p> <p>You can use this to reliably release a semaphore to avoid dead-lock in the face of exceptions or early returns:</p> <pre class="cpp"> <span class="comment">// ... do something that may throw or return early</span> sem<span class="operator">.</span>release(); </pre> <p>If an early return is taken or an exception is thrown before the <code>sem.release()</code> call is reached, the semaphore is not released, possibly preventing the thread waiting in the corresponding <code>sem.acquire()</code> call from ever continuing execution.</p> <p>When using RAII instead:</p> <pre class="cpp"> <span class="keyword">const</span> <span class="type"><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></span> releaser(sem); <span class="comment">// ... do something that may throw or early return</span> <span class="comment">// implicitly calls sem.release() here and at every other return in between</span> </pre> <p>this can no longer happen, because the compiler will make sure that the <a href="qsemaphorereleaser.html">QSemaphoreReleaser</a> destructor is always called, and therefore the semaphore is always released.</p> <p><a href="qsemaphorereleaser.html">QSemaphoreReleaser</a> is move-enabled and can therefore be returned from functions to transfer responsibility for releasing a semaphore out of a function or a scope:</p> <pre class="cpp"> { <span class="comment">// some scope</span> <span class="type"><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></span> releaser; <span class="comment">// does nothing</span> <span class="comment">// ...</span> <span class="keyword">if</span> (someCondition) { releaser <span class="operator">=</span> <span class="type"><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></span>(sem); <span class="comment">// ...</span> } <span class="comment">// ...</span> } <span class="comment">// conditionally calls sem.release(), depending on someCondition</span> </pre> <p>A <a href="qsemaphorereleaser.html">QSemaphoreReleaser</a> can be canceled by a call to <a href="qsemaphorereleaser.html#cancel">cancel</a>(). A canceled semaphore releaser will no longer call <a href="qsemaphore.html#release">QSemaphore::release</a>() in its destructor.</p> </div> <p><b>See also </b><a href="qmutexlocker.html">QMutexLocker</a>.</p> <!-- @@@QSemaphoreReleaser --> <div class="func"> <h2>Member Function Documentation</h2> <!-- $$$QSemaphoreReleaser[overload1]$$$QSemaphoreReleaser --> <h3 class="fn" id="QSemaphoreReleaser"><a name="QSemaphoreReleaser"></a>QSemaphoreReleaser::<span class="name">QSemaphoreReleaser</span>()</h3> <p>Default constructor. Creates a <a href="qsemaphorereleaser.html">QSemaphoreReleaser</a> that does nothing.</p> <!-- @@@QSemaphoreReleaser --> <!-- $$$QSemaphoreReleaser$$$QSemaphoreReleaserQSemaphore&int --> <h3 class="fn" id="QSemaphoreReleaser-1"><a name="QSemaphoreReleaser-1"></a>QSemaphoreReleaser::<span class="name">QSemaphoreReleaser</span>(<span class="type"><a href="qsemaphore.html">QSemaphore</a></span> &<i>sem</i>, <span class="type">int</span> <i>n</i> = 1)</h3> <p>Constructor. Stores the arguments and calls <i>sem</i>.release(<i>n</i>) in the destructor.</p> <!-- @@@QSemaphoreReleaser --> <!-- $$$QSemaphoreReleaser$$$QSemaphoreReleaserQSemaphore*int --> <h3 class="fn" id="QSemaphoreReleaser-2"><a name="QSemaphoreReleaser-2"></a>QSemaphoreReleaser::<span class="name">QSemaphoreReleaser</span>(<span class="type"><a href="qsemaphore.html">QSemaphore</a></span> *<i>sem</i>, <span class="type">int</span> <i>n</i> = 1)</h3> <p>Constructor. Stores the arguments and calls <i>sem</i>->release(<i>n</i>) in the destructor.</p> <!-- @@@QSemaphoreReleaser --> <!-- $$$QSemaphoreReleaser$$$QSemaphoreReleaserQSemaphoreReleaser&& --> <h3 class="fn" id="QSemaphoreReleaser-3"><a name="QSemaphoreReleaser-3"></a>QSemaphoreReleaser::<span class="name">QSemaphoreReleaser</span>(<span class="type"><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></span> &&<i>other</i>)</h3> <p>Move constructor. Takes over responsibility to call <a href="qsemaphore.html#release">QSemaphore::release</a>() from <i>other</i>, which in turn is canceled.</p> <p><b>See also </b><a href="qsemaphorereleaser.html#cancel">cancel</a>().</p> <!-- @@@QSemaphoreReleaser --> <!-- $$$~QSemaphoreReleaser[overload1]$$$~QSemaphoreReleaser --> <h3 class="fn" id="dtor.QSemaphoreReleaser"><a name="dtor.QSemaphoreReleaser"></a>QSemaphoreReleaser::<span class="name">~QSemaphoreReleaser</span>()</h3> <p>Unless canceled, calls <a href="qsemaphore.html#release">QSemaphore::release</a>() with the arguments provided to the constructor, or by the last move assignment.</p> <!-- @@@~QSemaphoreReleaser --> <!-- $$$cancel[overload1]$$$cancel --> <h3 class="fn" id="cancel"><a name="cancel"></a><span class="type"><a href="qsemaphore.html">QSemaphore</a></span> *QSemaphoreReleaser::<span class="name">cancel</span>()</h3> <p>Cancels this <a href="qsemaphorereleaser.html">QSemaphoreReleaser</a> such that the destructor will no longer call <code>semaphore()->release()</code>. Returns the value of <a href="qsemaphorereleaser.html#semaphore">semaphore</a>() before this call. After this call, <a href="qsemaphorereleaser.html#semaphore">semaphore</a>() will return <code>nullptr</code>.</p> <p>To enable again, assign a new <a href="qsemaphorereleaser.html">QSemaphoreReleaser</a>:</p> <pre class="cpp"> releaser<span class="operator">.</span>cancel(); <span class="comment">// avoid releasing old semaphore()</span> releaser <span class="operator">=</span> <span class="type"><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></span>(sem<span class="operator">,</span> <span class="number">42</span>); <span class="comment">// now will call sem.release(42) when 'releaser' is destroyed</span> </pre> <!-- @@@cancel --> <!-- $$$semaphore[overload1]$$$semaphore --> <h3 class="fn" id="semaphore"><a name="semaphore"></a><span class="type"><a href="qsemaphore.html">QSemaphore</a></span> *QSemaphoreReleaser::<span class="name">semaphore</span>() const</h3> <p>Returns a pointer to the <a href="qsemaphore.html">QSemaphore</a> object provided to the constructor, or by the last move assignment, if any. Otherwise, returns <code>nullptr</code>.</p> <!-- @@@semaphore --> <!-- $$$swap[overload1]$$$swapQSemaphoreReleaser& --> <h3 class="fn" id="swap"><a name="swap"></a><span class="type">void</span> QSemaphoreReleaser::<span class="name">swap</span>(<span class="type"><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></span> &<i>other</i>)</h3> <p>Exchanges the responsibilites of <code>*this</code> and <i>other</i>.</p> <p>Unlike move assignment, neither of the two objects ever releases its semaphore, if any, as a consequence of swapping.</p> <p>Therefore this function is very fast and never fails.</p> <!-- @@@swap --> <!-- $$$operator=[overload1]$$$operator=QSemaphoreReleaser&& --> <h3 class="fn" id="operator-eq"><a name="operator-eq"></a><span class="type"><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></span> &QSemaphoreReleaser::<span class="name">operator=</span>(<span class="type"><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></span> &&<i>other</i>)</h3> <p>Move assignment operator. Takes over responsibility to call <a href="qsemaphore.html#release">QSemaphore::release</a>() from <i>other</i>, which in turn is canceled.</p> <p>If this semaphore releaser had the responsibility to call some <a href="qsemaphore.html#release">QSemaphore::release</a>() itself, it performs the call before taking over from <i>other</i>.</p> <p><b>See also </b><a href="qsemaphorereleaser.html#cancel">cancel</a>().</p> <!-- @@@operator= --> </div> </div> </div> </div> </div> </div> <div class="footer"> <p> <acronym title="Copyright">©</acronym> 2018 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners.<br/> 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.<br/> Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. 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