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<td >Qt 5.9</td><td ><a href="qtcore-index.html">Qt Core</a></td><td >The Meta-Object System</td></tr></table><table class="buildversion"><tr>
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<h1 class="title">The Meta-Object System</h1>
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<p>Qt's meta-object system provides the signals and slots mechanism for inter-object communication, run-time type information, and the dynamic property system.</p>
<p>The meta-object system is based on three things:</p>
<ol class="1" type="1"><li>The <a href="qobject.html">QObject</a> class provides a base class for objects that can take advantage of the meta-object system.</li>
<li>The <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> macro inside the private section of the class declaration is used to enable meta-object features, such as dynamic properties, signals, and slots.</li>
<li>The Meta-Object Compiler (<code>moc</code>) supplies each <a href="qobject.html">QObject</a> subclass with the necessary code to implement meta-object features.</li>
</ol>
<p>The <code>moc</code> tool reads a C++ source file. If it finds one or more class declarations that contain the <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> macro, it produces another C++ source file which contains the meta-object code for each of those classes. This generated source file is either <code>#include</code>'d into the class's source file or, more usually, compiled and linked with the class's implementation.</p>
<p>In addition to providing the <a href="signalsandslots.html">signals and slots</a> mechanism for communication between objects (the main reason for introducing the system), the meta-object code provides the following additional features:</p>
<ul>
<li><a href="qobject.html#metaObject">QObject::metaObject</a>() returns the associated <a href="qmetaobject.html">meta-object</a> for the class.</li>
<li><a href="qmetaobject.html#className">QMetaObject::className</a>() returns the class name as a string at run-time, without requiring native run-time type information (RTTI) support through the C++ compiler.</li>
<li><a href="qobject.html#inherits">QObject::inherits</a>() function returns whether an object is an instance of a class that inherits a specified class within the <a href="qobject.html">QObject</a> inheritance tree.</li>
<li><a href="qobject.html#tr">QObject::tr</a>() and QObject::trUtf8() translate strings for internationalization.</li>
<li><a href="qobject.html#setProperty">QObject::setProperty</a>() and <a href="qobject.html#property">QObject::property</a>() dynamically set and get properties by name.</li>
<li><a href="qmetaobject.html#newInstance">QMetaObject::newInstance</a>() constructs a new instance of the class.</li>
</ul>
<a name="qobjectcast"></a><p>It is also possible to perform dynamic casts using <a href="qobject.html#qobject_cast">qobject_cast</a>() on <a href="qobject.html">QObject</a> classes. The <a href="qobject.html#qobject_cast">qobject_cast</a>() function behaves similarly to the standard C++ <code>dynamic_cast()</code>, with the advantages that it doesn't require RTTI support and it works across dynamic library boundaries. It attempts to cast its argument to the pointer type specified in angle-brackets, returning a non-zero pointer if the object is of the correct type (determined at run-time), or 0 if the object's type is incompatible.</p>
<p>For example, let's assume <code>MyWidget</code> inherits from <a href="../qtwidgets/qwidget.html">QWidget</a> and is declared with the <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> macro:</p>
<pre class="cpp">
<span class="type"><a href="qobject.html">QObject</a></span> <span class="operator">*</span>obj <span class="operator">=</span> <span class="keyword">new</span> MyWidget;
</pre>
<p>The <code>obj</code> variable, of type <code>QObject *</code>, actually refers to a <code>MyWidget</code> object, so we can cast it appropriately:</p>
<pre class="cpp">
<span class="type"><a href="../qtwidgets/qwidget.html">QWidget</a></span> <span class="operator">*</span>widget <span class="operator">=</span> qobject_cast<span class="operator"><</span><span class="type"><a href="../qtwidgets/qwidget.html">QWidget</a></span> <span class="operator">*</span><span class="operator">></span>(obj);
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<p>The cast from <a href="qobject.html">QObject</a> to <a href="../qtwidgets/qwidget.html">QWidget</a> is successful, because the object is actually a <code>MyWidget</code>, which is a subclass of <a href="../qtwidgets/qwidget.html">QWidget</a>. Since we know that <code>obj</code> is a <code>MyWidget</code>, we can also cast it to <code>MyWidget *</code>:</p>
<pre class="cpp">
MyWidget <span class="operator">*</span>myWidget <span class="operator">=</span> qobject_cast<span class="operator"><</span>MyWidget <span class="operator">*</span><span class="operator">></span>(obj);
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<p>The cast to <code>MyWidget</code> is successful because <a href="qobject.html#qobject_cast">qobject_cast</a>() makes no distinction between built-in Qt types and custom types.</p>
<pre class="cpp">
<span class="type"><a href="../qtwidgets/qlabel.html">QLabel</a></span> <span class="operator">*</span>label <span class="operator">=</span> qobject_cast<span class="operator"><</span><span class="type"><a href="../qtwidgets/qlabel.html">QLabel</a></span> <span class="operator">*</span><span class="operator">></span>(obj);
<span class="comment">// label is 0</span>
</pre>
<p>The cast to <a href="../qtwidgets/qlabel.html">QLabel</a>, on the other hand, fails. The pointer is then set to 0. This makes it possible to handle objects of different types differently at run-time, based on the type:</p>
<pre class="cpp">
<span class="keyword">if</span> (<span class="type"><a href="../qtwidgets/qlabel.html">QLabel</a></span> <span class="operator">*</span>label <span class="operator">=</span> qobject_cast<span class="operator"><</span><span class="type"><a href="../qtwidgets/qlabel.html">QLabel</a></span> <span class="operator">*</span><span class="operator">></span>(obj)) {
label<span class="operator">-</span><span class="operator">></span>setText(tr(<span class="string">"Ping"</span>));
} <span class="keyword">else</span> <span class="keyword">if</span> (<span class="type"><a href="../qtwidgets/qpushbutton.html">QPushButton</a></span> <span class="operator">*</span>button <span class="operator">=</span> qobject_cast<span class="operator"><</span><span class="type"><a href="../qtwidgets/qpushbutton.html">QPushButton</a></span> <span class="operator">*</span><span class="operator">></span>(obj)) {
button<span class="operator">-</span><span class="operator">></span>setText(tr(<span class="string">"Pong!"</span>));
}
</pre>
<p>While it is possible to use <a href="qobject.html">QObject</a> as a base class without the <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> macro and without meta-object code, neither signals and slots nor the other features described here will be available if the <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> macro is not used. From the meta-object system's point of view, a <a href="qobject.html">QObject</a> subclass without meta code is equivalent to its closest ancestor with meta-object code. This means for example, that <a href="qmetaobject.html#className">QMetaObject::className</a>() will not return the actual name of your class, but the class name of this ancestor.</p>
<p>Therefore, we strongly recommend that all subclasses of <a href="qobject.html">QObject</a> use the <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> macro regardless of whether or not they actually use signals, slots, and properties.</p>
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<p><b>See also </b><a href="qmetaobject.html">QMetaObject</a>, <a href="properties.html">Qt's Property System</a>, and <a href="signalsandslots.html">Signals and Slots</a>.</p>
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