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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Meta Object System</h1>
<p> <!-- index meta object --><a name="meta-object"></a><!-- index Q_OBJECT --><a name="Q_OBJECT"></a>
<p> Qt's Meta Object System provides the signals and slots mechanism for
inter-object communication, runtime type information, and the dynamic
property system.
<p> The Meta Object System is based on three things:
<ol type=1>
<li> the <a href="qobject.html">QObject</a> class;
<li> the Q_OBJECT macro inside the private section of the class
declaration;
<li> the <a href="moc.html">Meta Object Compiler (moc)</a>.
</ol>
<p> The <em><a href="moc.html#moc">moc</a></em> reads a C++ source file. If it finds one or more class
declarations that contain the Q_OBJECT macro, it produces another C++
source file which contains the meta object code for the classes that
contain the Q_OBJECT macro. This generated source file is either
#included into the class's source file or compiled and linked with the
class's implementation.
<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
additional features in QObject:
<p> <ul>
<p> <li> the <a href="qobject.html#className">className()</a> function that
returns the class name as a string at runtime, without requiring
native runtime type information (RTTI) support through the C++
compiler.
<p> <li> the <a href="qobject.html#inherits">inherits()</a> function that
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.
<p> <li> the <a href="qobject.html#tr">tr()</a> and
<a href="qobject.html#trUtf8">trUtf8()</a> functions for string
translation as used for <a href="i18n.html#internationalization">internationalization</a>.
<p> <li> the <a href="qobject.html#setProperty">setProperty()</a> and <a href="qobject.html#property">property()</a> functions for dynamically
setting and getting <a href="properties.html">object properties</a> by
name.
<p> <li> the <a href="qobject.html#metaObject">metaObject()</a> function that
returns the associated <a href="qmetaobject.html">meta object</a> for the
class.
<p> </ul>
<p> While it is possible to use QObject as a base class without the
Q_OBJECT macro and without meta object code, neither signals and slots
nor the other features described here will be available if the
Q_OBJECT macro is not used. From the meta object system's point of
view, a QObject subclass without meta code is equivalent to its
closest ancestor with meta object code. This means for example, that
className() will not return the actual name of your class, but the
class name of this ancestor. We <em>strongly</em> recommend that all
subclasses of <a href="qobject.html">QObject</a> use the Q_OBJECT macro regardless of whether
they actually use signals, slots and properties or not.
<p>
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