1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
|
/****************************************************************************
**
** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** GNU Free Documentation License
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file.
**
** Other Usage
** Alternatively, this file may be used in accordance with the terms
** and conditions contained in a signed written agreement between you
** and Nokia.
**
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page metaobjects.html
\title The Meta-Object System
\brief An overview of Qt's meta-object system and introspection capabilities.
\ingroup qt-basic-concepts
\keyword meta-object
\target 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
\o The \l QObject class provides a base class for objects that can
take advantage of the meta-object system.
\o 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.
\o 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
\o QObject::metaObject() returns the associated
\l{QMetaObject}{meta-object} for the class.
\o 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.
\o QObject::inherits() function returns whether an object is an
instance of a class that inherits a specified class within the
QObject inheritance tree.
\o QObject::tr() and QObject::trUtf8() translate strings for
\l{Internationalization with Qt}{internationalization}.
\o QObject::setProperty() and QObject::property()
dynamically set and get properties by name.
\o 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 0
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 doc/src/snippets/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 doc/src/snippets/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 doc/src/snippets/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 doc/src/snippets/qtcast/qtcast.cpp 3
\snippet doc/src/snippets/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 doc/src/snippets/qtcast/qtcast.cpp 5
\snippet doc/src/snippets/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}
*/
|
