File: metaobjects.doc

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/****************************************************************************
**
** Explanation of moc and the meta object system
**
** Copyright (C) 1992-2008 Trolltech ASA.  All rights reserved.
**
** This file is part of the Qt GUI Toolkit.
**
** This file may be used under the terms of the GNU General
** Public License versions 2.0 or 3.0 as published by the Free
** Software Foundation and appearing in the files LICENSE.GPL2
** and LICENSE.GPL3 included in the packaging of this file.
** Alternatively you may (at your option) use any later version
** of the GNU General Public License if such license has been
** publicly approved by Trolltech ASA (or its successors, if any)
** and the KDE Free Qt Foundation.
**
** Please review the following information to ensure GNU General
** Public Licensing requirements will be met:
** http://trolltech.com/products/qt/licenses/licensing/opensource/.
** If you are unsure which license is appropriate for your use, please
** review the following information:
** http://trolltech.com/products/qt/licenses/licensing/licensingoverview
** or contact the sales department at sales@trolltech.com.
**
** This file may be used under the terms of the Q Public License as
** defined by Trolltech ASA and appearing in the file LICENSE.QPL
** included in the packaging of this file.  Licensees holding valid Qt
** Commercial licenses may use this file in accordance with the Qt
** Commercial License Agreement provided with the Software.
**
** This file is provided "AS IS" with NO WARRANTY OF ANY KIND,
** INCLUDING THE WARRANTIES OF DESIGN, MERCHANTABILITY AND FITNESS FOR
** A PARTICULAR PURPOSE. Trolltech reserves all rights not granted
** herein.
**
**********************************************************************/

/*!
\page metaobjects.html

\title Meta Object System

\keyword meta object
\keyword Q_OBJECT

Qt's Meta Object System provides the signals and slots mechanism for
inter-object communication, runtime type information, and the dynamic
property system.

The Meta Object System is based on three things:
\list 1
\i the \l QObject class;
\i the Q_OBJECT macro inside the private section of the class
declaration;
\i the \link moc.html Meta Object Compiler (moc)\endlink. 
\endlist

The \e moc 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.

In addition to providing the \link signalsandslots.html signals and
slots \endlink mechanism for communication between objects (the main
reason for introducing the system), the meta object code provides
additional features in QObject:

\list

\i the \link QObject::className() className() \endlink function that
returns the class name as a string at runtime, without requiring
native runtime type information (RTTI) support through the C++
compiler.

\i the \link QObject::inherits() inherits() \endlink function that
returns whether an object is an instance of a class that inherits a
specified class within the QObject inheritance tree.

\i the \link QObject::tr() tr() \endlink and
\link QObject::trUtf8() trUtf8() \endlink functions for string
translation as used for internationalization.

\i the \link QObject::setProperty() setProperty() \endlink and \link
QObject::property() property() \endlink functions for dynamically
setting and getting \link properties.html object properties\endlink by
name.

\i the \link QObject::metaObject() metaObject() \endlink function that
returns the associated \link QMetaObject meta object \endlink for the
class.

\endlist

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 \e strongly recommend that all
subclasses of QObject use the Q_OBJECT macro regardless of whether
they actually use signals, slots and properties or not.

*/