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The QObject class is the base class of all Qt objects. More...
#include <qobject.h>
Inherits Qt.
Inherited by QLayout, QAccel, QAccessibleObject, QAction, QApplication, QDataPump, QWidget, QCanvas, QStyle, QClipboard, QCopChannel, QDns, QDragObject, QEditorFactory, QFileIconProvider, QNetworkProtocol, QServerSocket, QWSKeyboardHandler, QNetworkOperation, QNPInstance, QProcess, QSessionManager, QSignal, QSignalMapper, QSocket, QSocketNotifier, QSound, QSqlDatabase, QSqlDriver, QSqlForm, QStyleSheet, QTimer, QToolTipGroup, QTranslator, QUrlOperator, QValidator and QWSMouseHandler.
QObject is the heart of the Qt object model. The central feature in this model is a very powerful mechanism for seamless object communication dubbed signals and slots. With connect(), you can connect a signal to a slot and destroy the connection with disconnect(). To avoid neverending notification loops you can temporarily block signals with blockSignals(). The protected functions connectNotify() and disconnectNotify() make it possible to track connections.
QObjects organize themselves in object trees. When you create a QObject with another object as parent, it will automatically do an insertChild() on the parent and thus show up in the parent's children() list. The parent receives object ownership, i.e., it will automatically delete its children in its destructor. You can look for an object by name and optionally type using child() or queryList(), and get the list of tree roots using objectTrees().
Every object has an object name() and can report its className() and whether it inherits() another class in the QObject inheritance hierarchy.
When an object is deleted, it emits a destroyed() signal. You can catch this signal to avoid dangling references to QObjects. The QGuardedPtr class provides an elegant way to use this feature.
QObjects can receive events through event() and filter events of other objects. See installEventFilter() and eventFilter() for details. A convenience handler childEvent() can be reimplemented to catch child events.
Last but not least, QObject provides the basic timer support in Qt; see QTimer for high-level support for timers.
Notice that the Q_OBJECT macro is mandatory for any object that implements signals, slots or properties. You also need to run the moc program (Meta Object Compiler) on the source file. We strongly recommend to use the macro in all subclasses of QObject regardless whether or not they actually use signals, slots and properties. Otherwise certain functions can show undefined behaviour.
All Qt widgets inherit QObject. The convenience function isWidgetType() returns whether an object is actually a widget. It is much faster than inherits( "QWidget" ).
Some QObject functions, e.g. children(), objectTrees() and queryList() return a QObjectList. A QObjectList is a QPtrList of QObjects. QObjectLists support the same operations as QPtrLists and have an iterator class, QObjectListIt.
The parent of an object may be viewed as the object's owner. For instance, a dialog box is the parent of the "ok" and "cancel" buttons inside it.
The destructor of a parent object destroys all child objects.
Setting parent to 0 constructs an object with no parent. If the object is a widget, it will become a top-level window.
The object name is a text that can be used to identify this QObject. It's particularly useful in conjunction with the Qt Designer. You can find an object by name (and type) using child(), and more than one using queryList().
See also parent(), name, child() and queryList().
All signals to and from the object are automatically disconnected.
Warning: All child objects are deleted. If any of these objects are on the stack or global, sooner or later your program will crash. We do not recommend holding pointers to child objects from outside the parent. If you still do, the QObject::destroyed() signal gives you an opportunity to detect when an object is destroyed.
Emitted signals disappear into hyperspace if signals are blocked.
Example: rot13/rot13.cpp.
Returns TRUE if the signal and the member arguments are compatible; otherwise returns FALSE. (The receiver argument is currently ignored.)
Warning: We recommend that you use the default implementation and do not reimplement this function.
If recursiveSearch is TRUE (the default), child() searches nth-generation as well as first-generation children.
If there is no such object, this function returns 0. If there are more than one, the first one in depth-first is retured; if you need all of them, use queryList() instead.
Child events are sent to objects when children are inserted or removed.
Note that events with QEvent::type() QEvent::ChildInserted are posted (with QApplication::postEvent()) to make sure that the child's construction is completed before this function is called.
If you change state based on ChildInserted events, call
QWidget::constPolish(), or do
QApplication::sendPostedEvents( this, QEvent::ChildInserted );
in functions that depend on the state. One notable example is
QWidget::sizeHint().
See also event() and QChildEvent.
Reimplemented in QMainWindow and QSplitter.
The QObjectList class is defined in the qobjectlist.h header file.
The latest child added is the first object in the list and the first child added is the last object in the list.
Note that the list order changes when QWidget children are raised or lowered. A widget that is raised becomes the last object in the list, and a widget that is lowered becomes the first object in the list.
See also child(), queryList(), parent(), insertChild() and removeChild().
Returns the class name of this object.
This function is generated by the Meta Object Compiler.
Warning: This function will return a wrong name if the class definition lacks the Q_OBJECT macro.
See also name, inherits(), isA() and isWidgetType().
Example: sql/overview/custom1/main.cpp.
You must use the SIGNAL() and SLOT() macros when specifying the signal and the member, like this:
QLabel *label = new QLabel; QScrollBar *scroll = new QScrollBar; QObject::connect( scroll, SIGNAL(valueChanged(int)), label, SLOT(setNum(int)) );
(This example makes the label always display the current scroll bar value.)
A signal can also be connected to another signal:
class MyWidget : public QWidget { public: MyWidget(); ... signals: void myUsefulSignal(); ... private: ... QPushButton *aButton; }; MyWidget::MyWidget() { aButton = new QPushButton( this ); connect( aButton, SIGNAL(clicked()), SIGNAL(myUsefulSignal()) ); }
The MyWidget constructor thus relays a signal from a private member variable, and makes it available under a name that relates to MyWidget.
A signal can be connected to many slots/signals. Many signals can be connected to one slot.
If a signal is connected to several slots, the slots are activated in arbitrary order when the signal is emitted.
The function returns TRUE if it successfully connects the signal to the slot. It will return FALSE if QObject is unable to verify the existence of either signal or member, or if their signatures aren't compatible.
See also disconnect().
Examples: action/actiongroup/editor.cpp, action/main.cpp, action/toggleaction/toggleaction.cpp, application/main.cpp, iconview/main.cpp, mdi/main.cpp and t2/main.cpp.
Connects signal from the sender object to member in this object.
Equivalent to: QObject::connect(sender, signal, this, member)
.
See also disconnect().
This virtual function is called when something has been connected to signal in this object.
Warning: This function violates the object-oriented principle of modularity. However, it might be useful when you need to perform expensive initialization only if something is connected to a signal.
See also connect() and disconnectNotify().
See also event() and QCustomEvent.
This function does not cause an immediate destruction - rather, it schedules a deferred delete event for processing when Qt returns to the main event loop.
This signal is emitted immediately before the object is destroyed.
All the objects's children are destroyed immediately after this signal is emitted.
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
This signal is emitted immediately before the object obj is destroyed.
All the objects's children are destroyed immediately after this signal is emitted.
A signal-slot connection is removed when either of the objects involved are destroyed.
disconnect() is typically used in three ways, as the following examples show.
disconnect( myObject, 0, 0, 0 );equivalent to the non-static overloaded function
myObject->disconnect();
disconnect( myObject, SIGNAL(mySignal()), 0, 0 );equivalent to the non-static overloaded function
myObject->disconnect( SIGNAL(mySignal()) );
disconnect( myObject, 0, myReceiver, 0 );equivalent to the non-static overloaded function
myObject->disconnect( myReceiver );
0 may be used as a wildcard, meaning "any signal", "any receiving object", or "any slot in the receiving object", respectively.
The sender may never be 0. (You cannot disconnect signals from more than one object.)
If signal is 0, it disconnects receiver and member from any signal. If not, only the specified signal is disconnected.
If receiver is 0, it disconnects anything connected to signal. If not, slots in objects other than receiver are not disconnected.
If member is 0, it disconnects anything that is connected to receiver. If not, only slots named member will be disconnected, and all other slots are left alone. The member must be 0 if receiver is left out, so you cannot disconnect a specifically-named slot on all objects.
See also connect().
Disconnects signal from member of receiver.
A signal-slot connection is removed when either of the objects involved are destroyed.
Disconnects all signals in this object from member of receiver.
A signal-slot connection is removed when either of the objects involved are destroyed.
This virtual function is called when something has been disconnected from signal in this object.
Warning: This function violates the object-oriented principle of modularity. However, it might be useful for optimizing access to expensive resources.
See also disconnect() and connectNotify().
This function is useful for debugging. This function does nothing if the library has been compiled in release mode (i.e., without debugging information).
This function is useful for debugging. This function does nothing if the library has been compiled in release mode (i.e., without debugging information).
The event() function can be reimplemented to customize the behavior of an object.
See also installEventFilter(), timerEvent(), QApplication::sendEvent(), QApplication::postEvent() and QWidget::event().
Reimplemented in QWidget.
The reimplementation of this virtual function should return TRUE if the event e should be stopped, or FALSE if the event should be dispatched normally.
Warning: If you delete the receiver object in this function, be sure to return TRUE. Otherwise, Qt will forward the event to the deleted object and the program might crash.
See also installEventFilter().
Reimplemented in QLayout, QAccel, QScrollView, QSGIStyle and QSpinBox.
High-priority objects are placed first in QObject's list of children on the assumption that they will be referenced very often.
A class is considered to inherit itself.
Example:
QTimer *t = new QTimer; // QTimer inherits QObject t->inherits( "QTimer" ); // returns TRUE t->inherits( "QObject" ); // returns TRUE t->inherits( "QButton" ); // returns FALSE // QScrollBar inherits QWidget and QRangeControl QScrollBar *s = new QScrollBar( 0 ); s->inherits( "QWidget" ); // returns TRUE s->inherits( "QRangeControl" ); // returns FALSE
(QRangeControl is not a QObject.)
See also isA() and metaObject().
Examples: themes/metal.cpp and themes/wood.cpp.
Warning: This function cannot be used to make a widget a child widget of another. Child widgets can be created only by setting the parent widget in the constructor or by calling QWidget::reparent().
See also removeChild() and QWidget::reparent().
An event filter is an object that receives all events that are sent to this object. The filter can either stop the event or forward it to this object. The event filter obj receives events via its eventFilter() function. The eventFilter() function must return TRUE if the event should be stopped, or FALSE if the event should be dispatched normally.
If multiple event filters are installed for a single object, the filter that was installed last is activated first.
Example:
#include <qwidget.h> class MyWidget : public QWidget { public: MyWidget::MyWidget( QWidget *parent=0, const char *name=0 ); protected: bool eventFilter( QObject *, QEvent * ); }; MyWidget::MyWidget( QWidget *parent, const char *name ) : QWidget( parent, name ) { if ( parent ) // has a parent widget parent->installEventFilter( this ); // then install filter } bool MyWidget::eventFilter( QObject *o, QEvent *e ) { if ( e->type() == QEvent::KeyPress ) { // key press QKeyEvent *k = (QKeyEvent*)e; qDebug( "Ate key press %d", k->key() ); return TRUE; // eat event } return QWidget::eventFilter( o, e ); // standard event processing }
The QAccel class, for example, uses this technique.
Warning: If you delete the receiver object in your eventFilter() function, be sure to return TRUE. If you return FALSE, Qt sends the event to the deleted object and the program will crash.
See also removeEventFilter(), eventFilter() and event().
Example:
QTimer *t = new QTimer; // QTimer inherits QObject t->isA( "QTimer" ); // returns TRUE t->isA( "QObject" ); // returns FALSE
See also inherits() and metaObject().
Calling this function is equivalent to calling inherits("QWidget"), except that it is much faster.
The timer identifier is returned by startTimer() when a timer event is started.
See also timerEvent(), startTimer() and killTimers().
Note that using this function can cause hard-to-find bugs: It kills timers started by sub- and superclasses as well as those started by you, which is often not what you want. Therefore, we recommend using a QTimer or perhaps killTimer().
See also timerEvent(), startTimer() and killTimer().
A meta object contains information about a class that inherits QObject: class name, super class name, properties, signals and slots. Every class that contains the Q_OBJECT macro will also have a meta object.
The meta object information is required by the signal/slot connection mechanism and the property system. The functions isA() and inherits() also make use of the meta object.
Returns the name of this object. See the "name" property for details.
Returns the name of this object, or defaultName if the object does not have a name.
The QObjectList class is defined in the qobjcoll.h header file.
The latest root object created is the first object in the list and the first root object added is the last object in the list.
See also children(), parent(), insertChild() and removeChild().
See also children().
If no such property exists, the returned variant is invalid.
Information about all available properties are provided through the metaObject().
See also setProperty(), QVariant::isValid(), metaObject(), QMetaObject::propertyNames() and QMetaObject::property().
If regexpMatch is TRUE (the default), objName is a regexp that the objects's names must match. If regexpMatch is FALSE, objName is a string and object names must match it exactly.
Note that inheritsClass uses single inheritance from QObject, the way inherits() does. According to inherits(), QMenuBar inherits QWidget but not QMenuData. This does not quite match reality, but is the best that can be done on the wide variety of compilers Qt supports.
Finally, if recursiveSearch is TRUE (the default), queryList() searches nth-generation as well as first-generation children.
If all this seems a bit complex for your needs, the simpler function child() may be what you want.
This somewhat contrived example disables all the buttons in this window:
QObjectList * l = topLevelWidget()->queryList( "QButton" ); QObjectListIt it( *l ); // iterate over the buttons QObject * obj; while ( (obj=it.current()) != 0 ) { // for each found object... ++it; ((QButton*)obj)->setEnabled( FALSE ); } delete l; // delete the list, not the objects
Warning: Delete the list away as soon you have finished using it. The list contains pointers that may become invalid at almost any time without notice - as soon as the user closes a window you may have dangling pointers, for example.
See also child(), children(), parent(), inherits(), name and QRegExp.
Warning: This function will not remove a child widget from the screen. It will only remove it from the parent widget's list of children.
See also insertChild() and QWidget::reparent().
All event filters for this object are automatically removed when this object is destroyed.
It is always safe to remove an event filter, even during event filter activation (i.e., from the eventFilter() function).
See also installEventFilter(), eventFilter() and event().
Warning: This function will return something apparently correct in other cases as well. However, its value may change during any function call, depending on what signal-slot connections are activated during that call. In Qt 3.0 the value will change more often than in 2.x.
Warning: This function violates the object-oriented principle of modularity, However, getting access to the sender might be practical when many signals are connected to a single slot. The sender is undefined if the slot is called as a normal C++ function.
Returns TRUE if the operation was successful, FALSE otherwise.
Information about all available properties is provided through the metaObject().
See also property(), metaObject(), QMetaObject::propertyNames() and QMetaObject::property().
Signals are not blocked by default.
See also blockSignals().
A timer event will occur every interval milliseconds until killTimer() or killTimers() is called. If interval is 0, then the timer event occurs once every time there are no more window system events to process.
The virtual timerEvent() function is called with the QTimerEvent event parameter class when a timer event occurs. Reimplement this function to get timer events.
If multiple timers are running, the QTimerEvent::timerId() can be used to find out which timer was activated.
Example:
class MyObject : public QObject { public: MyObject( QObject *parent=0, const char *name=0 ); protected: void timerEvent( QTimerEvent * ); }; MyObject::MyObject( QObject *parent, const char *name ) : QObject( parent, name ) { startTimer( 50 ); // 50-millisecond timer startTimer( 1000 ); // 1-second timer startTimer( 60000 ); // 1-minute timer } void MyObject::timerEvent( QTimerEvent *e ) { qDebug( "timer event, id=%d", e->timerId() ); }
There is practically no upper limit for the interval value (more than one year). The accuracy depends on the underlying operating system. Windows 95 has 55-millisecond (18.2 times per second) accuracy; other systems that we have tested (UNIX X11, Windows NT, and OS/2) can handle 1-millisecond intervals.
The QTimer class provides a high-level programming interface with one-shot timers and timer signals instead of events.
See also timerEvent(), killTimer() and killTimers().
QTimer provides a higher-level interface to the timer functionality, and also more general information about timers.
See also startTimer(), killTimer(), killTimers() and event().
Examples: dclock/dclock.cpp, forever/forever.cpp, grapher/grapher.cpp, qmag/qmag.cpp and xform/xform.cpp.
Returns a translated version of sourceText, or sourceText itself if there is no appropriate translated version. The translation context is QObject with comment (null by default). All QObject subclasses using the Q_OBJECT macro automatically have a reimplementation of this function with the subclass name as context.
See also trUtf8(), QApplication::translate() and Internationalization with Qt.
Example: network/networkprotocol/view.cpp.
Returns a translated version of sourceText, or QString::fromUtf8(sourceText) if there is no appropriate version. It is otherwise identical to tr(sourceText, comment).
See also tr() and QApplication::translate().
This property holds the name of this object.
You can find an object by name (and type) using child(), and more than one using queryList().
The object name is set by the constructor or by the setName() function. The object name is not very useful in the current version of Qt, but will become increasingly important in the future.
If the object does not have a name, the getter function name() returns "unnamed", so printf() (used in qDebug()) will not be asked to output a null pointer. If you want a null pointer to be returned for unnamed objects, you can call name( 0 ).
qDebug( "MyClass::setPrecision(): (%s) unable to set precision to %f", name(), newPrecision );
See also className(), child() and queryList().
Set this property's value with setName() and get this property's value with name().
Returns a pointer to the object named name that inherits type and with a given parent.
Returns 0 if there is no such child.
QListBox *c = (QListBox *) qt_find_obj_child( myWidget, "QListBox", "my list box" ); if ( c ) c->insertItem( "another string" );
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Copyright © 2001 Trolltech | Trademarks | Qt version 3.0.0-beta2
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