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<td >Qt 5.12</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>
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<h3><a name="toc">Contents</a></h3>
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<li class="level1"><a href="#public-functions">Public Functions</a></li>
<li class="level1"><a href="#details">Detailed Description</a></li>
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<h1 class="title">QSemaphoreReleaser Class</h1>
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<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>
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<div class="table"><table class="alignedsummary">
<tr><td class="memItemLeft rightAlign topAlign"> Header:</td><td class="memItemRight bottomAlign">   <span class="preprocessor">#include &lt;QSemaphoreReleaser&gt;</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 reentrant.</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 &amp;<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 &amp;&amp;<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 &amp;<i>other</i>)</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> QSemaphoreReleaser &amp;</td><td class="memItemRight bottomAlign"><b><a href="qsemaphorereleaser.html#operator-eq">operator=</a></b>(QSemaphoreReleaser &amp;&amp;<i>other</i>)</td></tr>
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<a name="details"></a>
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<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>
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<p><b>See also </b><a href="qmutexlocker.html">QMutexLocker</a>.</p>
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<h2>Member Function Documentation</h2>
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<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>
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<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> &amp;<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>
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<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>-&gt;release(<i>n</i>) in the destructor.</p>
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<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> &amp;&amp;<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>
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<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>
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<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()-&gt;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>
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<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>
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<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> &amp;<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>
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<h3 class="fn" id="operator-eq"><a name="operator-eq"></a><span class="type"><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></span> &amp;QSemaphoreReleaser::<span class="name">operator=</span>(<span class="type"><a href="qsemaphorereleaser.html#QSemaphoreReleaser">QSemaphoreReleaser</a></span> &amp;&amp;<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= -->
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