metaobjects.qdoc

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page metaobjects.html
    \title The Meta-Object System
    \brief An overview of Qt's meta-object system and introspection capabilities.
    \ingroup explanations-basics
    \ingroup qt-basic-concepts
    \keyword meta-object
    \keyword Meta-Object System
 
    Qt's meta-object system provides the signals and slots mechanism for
    inter-object communication, run-time type information, and the dynamic
    property system.
 
    The meta-object system is based on three things:
 
    \list 1
    \li The \l QObject class provides a base class for objects that can
       take advantage of the meta-object system.
    \li The Q_OBJECT macro inside the private section of the class
       declaration is used to enable meta-object features, such as
       dynamic properties, signals, and slots.
    \li The \l{moc}{Meta-Object Compiler} (\c moc) supplies each
       QObject subclass with the necessary code to implement
       meta-object features.
    \endlist
 
    The \c moc tool 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 each of those classes. This generated source file is
    either \c{#include}'d into the class's source file or, more
    usually, compiled and linked with the class's implementation.
 
    In addition to providing the \l{signals and slots} mechanism for
    communication between objects (the main reason for introducing
    the system), the meta-object code provides the following
    additional features:
 
    \list
    \li QObject::metaObject() returns the associated
       \l{QMetaObject}{meta-object} for the class.
    \li QMetaObject::className() returns the class name as a
       string at run-time, without requiring native run-time type information
       (RTTI) support through the C++ compiler.
    \li QObject::inherits() function returns whether an object is an
       instance of a class that inherits a specified class within the
       QObject inheritance tree.
    \li QObject::tr() translates strings for
       \l{Internationalization with Qt}{internationalization}.
    \li QObject::setProperty() and QObject::property()
       dynamically set and get properties by name.
    \li QMetaObject::newInstance() constructs a new instance of the class.
    \endlist
 
    \target qobjectcast
    It is also possible to perform dynamic casts using qobject_cast()
    on QObject classes. The qobject_cast() function behaves similarly
    to the standard C++ \c dynamic_cast(), 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 \nullptr
    if the object's type is incompatible.
 
    For example, let's assume \c MyWidget inherits from QWidget and
    is declared with the Q_OBJECT macro:
 
    \snippet qtcast/qtcast.cpp 0
 
    The \c obj variable, of type \c{QObject *}, actually refers to a
    \c MyWidget object, so we can cast it appropriately:
 
    \snippet qtcast/qtcast.cpp 1
 
    The cast from QObject to QWidget is successful, because the
    object is actually a \c MyWidget, which is a subclass of QWidget.
    Since we know that \c obj is a \c MyWidget, we can also cast it to
    \c{MyWidget *}:
 
    \snippet qtcast/qtcast.cpp 2
 
    The cast to \c MyWidget is successful because qobject_cast()
    makes no distinction between built-in Qt types and custom types.
 
    \snippet qtcast/qtcast.cpp 3
    \snippet qtcast/qtcast.cpp 4
 
    The cast to QLabel, 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:
 
    \snippet qtcast/qtcast.cpp 5
    \snippet qtcast/qtcast.cpp 6
 
    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 QMetaObject::className() will not return
    the actual name of your class, but the class name of this
    ancestor.
 
    Therefore, we strongly recommend that all subclasses of QObject
    use the Q_OBJECT macro regardless of whether or not they
    actually use signals, slots, and properties.
 
    \sa QMetaObject, {Qt's Property System}, {Signals and Slots}
*/