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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Qt Toolkit - Meta Object System</title><style type="text/css"><!--
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<h1 align=center> Meta Object System</h1><br clear="all">
The Meta Object System in Qt is responsible for the signal/slot
mechanism for communication between objects, runtime type information
and the dynamic property system.
<p>
It is based on three things:
<ul>
<li> the <a href="qobject.html">QObject</a> class,
<li> the "Q_OBJECT" macro inside the private section of the class
declaration and
<li> the <a href="moc.html">Meta Object Compiler (moc)</a>.
</ul>
<p>
The <b>moc</b> 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 this
class. This generated source file is either #included into the class'
source file or compiled and linked with the class implementation.
<p>
Apart from the possibility to use <a
href="signalsandslots.html">signals and slots</a> for communication
between objects - the main reason for introducing the system - the
meta object code implements certain other features in QObject:
<p>
<ul>
<li> a function <a href="qobject.html#fa7716">className()</a> that
returns the class name as string during runtime, without requiring
native runtime type information (RTTI) support through the C++
compiler.
<li> a function <a href="qobject.html#beb5d8">inherits()</a> that
returns whether an object is instance of a class that inherits a
specified class within the QObject inheritance tree.
<li> a function <a href="qobject.html#2418a9">tr()</a> for scoped string
translation as used for <a href="i18n.html"> internationalization</a>.
<li> two functions <a href="qobject.html#72ec3f">setProperty()</a>
and <a href="qobject.html#a110be">property()</a> to dynamically set
and get <a href="properties.html">object properties</a> by name.
<li> a function <a href="qobject.html#ef2243">metaObject()</a> 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 Q_OBJECT
macro and without meta object code, neither signals and slots nor the
other features described here will be available. 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. That means
for example that className() will not return the actual name of your
class, but the class name of this ancestor. We strongly recommend to
use the macro in <em>all</em> subclasses of QObject regardless whether they
actually use signals, slots and properties or not.
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<td>Copyright 2001 Trolltech<td><a href="http://www.trolltech.com/trademarks.html">Trademarks</a>
<td align="right"><div align="right">Qt version 2.3.2</div>
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