QObject Class Reference

Detailed Description

QObject is the heart of the Qt object model. The central feature in this model is a very powerful mechanism for seamless object communication called signals and slots. You can connect a signal to a slot with connect() and destroy the connection with disconnect(). To avoid never ending 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, the object will automatically do an insertChild() on the parent and thus show up in the parent's children() list. The parent takes ownership of the object 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 the 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 the use of this macro in all subclasses of QObject regardless of whether or not they actually use signals, slots and properties, since failure to do so may lead certain functions to exhibit 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.

See also Object Model.

Member Function Documentation

QObject::QObject ( QObject * parent = 0, const char * name = 0 )

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 it contains.

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 some text that can be used to identify a QObject. It's particularly useful in conjunction with Qt Designer. You can find an object by name (and type) using child(). To find several objects use queryList().

See also parent(), name, child(), and queryList().

QObject::~QObject () [virtual]

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.

Warning: Deleting a QObject while pending events are waiting to be delivered can cause a crash. You must not delete the QObject directly from a thread that is not the GUI thread. Use the QObject::deleteLater() method instead, which will cause the event loop to delete the object after all pending events have been delivered to the object.

void QObject::blockSignals ( bool block )

Emitted signals disappear into hyperspace if signals are blocked. Note that the destroyed() signals will be emitted even if the signals for this object have been blocked.

Examples: rot13/rot13.cpp and simple/main.cpp.

bool QObject::checkConnectArgs ( const char * signal, const QObject * receiver, const char * member ) [virtual protected]

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.

QObject * QObject::child ( const char * objName, const char * inheritsClass = 0, bool recursiveSearch = TRUE )

If recursiveSearch is TRUE (the default), child() performs a depth-first search of the object's children.

If there is no such object, this function returns 0. If there are more than one, the first one found is retured; if you need all of them, use queryList().

void QObject::childEvent ( QChildEvent * ) [virtual protected]

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 a child is removed immediately after it is inserted, the ChildInserted event may be suppressed, but the ChildRemoved event will always be sent. In such cases it is possible that there will be a ChildRemoved event without a corresponding ChildInserted event.

If you change state based on ChildInserted events, call QWidget::constPolish(), or do

        QApplication::sendPostedEvents( this, QEvent::ChildInserted );
    

See also event() and QChildEvent.

Reimplemented in QMainWindow and QSplitter.

const QObjectList * QObject::children () const

Returns a list of child objects, or 0 if this object has no children.

The QObjectList class is defined in the qobjectlist.h header file.

The first child added is the first object in the list and the last child added is the last object in the list, i.e. new children are appended at the end.

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().

const char * QObject::className () const [virtual]

Returns the class name of this object.

This function is generated by the Meta Object Compiler.

Warning: This function will return the wrong name if the class definition lacks the Q_OBJECT macro.

See also name, inherits(), isA(), and isWidgetType().

Example: sql/overview/custom1/main.cpp.

bool QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * member ) [static]

You must use the SIGNAL() and SLOT() macros when specifying the signal and the member, for example:

    QLabel     *label  = new QLabel;
    QScrollBar *scroll = new QScrollBar;
    QObject::connect( scroll, SIGNAL(valueChanged(int)),
                      label,  SLOT(setNum(int)) );
    

This example ensures that the label always displays the current scroll bar value. Note that the signal and slots parameters must not contain any variable names, only the type. E.g. the following would not work and return FALSE: QObject::connect( scroll, SIGNAL(valueChanged(int v)), label, SLOT(setNum(int v)) );

A signal can also be connected to another signal:

    class MyWidget : public QWidget
    {
        Q_OBJECT
    public:
        MyWidget();

    signals:
        void myUsefulSignal();

    private:
        QPushButton *aButton;
    };

    MyWidget::MyWidget()
    {
        aButton = new QPushButton( this );
        connect( aButton, SIGNAL(clicked()), SIGNAL(myUsefulSignal()) );
    }
    

In this example, the MyWidget constructor 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 and signals. Many signals can be connected to one slot.

If a signal is connected to several slots, the slots are activated in an 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 it cannot create the connection, for example, if QObject is unable to verify the existence of either signal or member, or if their signatures aren't compatible.

A signal is emitted for every connection you make, so if you duplicate a connection, two signals will be emitted. You can always break a connection using disconnect().

See also disconnect().

Examples: action/main.cpp, application/main.cpp, extension/main.cpp, iconview/main.cpp, network/archivesearch/main.cpp, and t2/main.cpp.

bool QObject::connect ( const QObject * sender, const char * signal, const char * member ) const

Connects signal from the sender object to this object's member.

Equivalent to: QObject::connect(sender, signal, this, member).

See also disconnect().

void QObject::connectNotify ( const char * signal ) [virtual protected]

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().

void QObject::customEvent ( QCustomEvent * ) [virtual protected]

See also event() and QCustomEvent.

void QObject::deleteLater () [slot]

Instead of an immediate deletion this function schedules a deferred delete event for processing when Qt returns to the main event loop.

Example: table/bigtable/main.cpp.

void QObject::destroyed () [signal]

This signal is emitted when the object is being destroyed.

Note that the signal is emitted by the QObject destructor, so the object's virtual table is already degenerated at this point, and it is not safe to call any functions on the object emitting the signal. This signal can not be blocked.

All the objects's children are destroyed immediately after this signal is emitted.

void QObject::destroyed ( QObject * obj ) [signal]

This signal is emitted immediately before the object obj is destroyed, and can not be blocked.

All the objects's children are destroyed immediately after this signal is emitted.

bool QObject::disconnect ( const QObject * sender, const char * signal, const QObject * receiver, const char * member ) [static]

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 demonstrate.

1. Disconnect everything connected to an object's signals:

          disconnect( myObject, 0, 0, 0 );
          

   equivalent to the non-static overloaded function

          myObject->disconnect();
          

2. Disconnect everything connected to a specific signal:

          disconnect( myObject, SIGNAL(mySignal()), 0, 0 );
          

   equivalent to the non-static overloaded function

          myObject->disconnect( SIGNAL(mySignal()) );
          

3. Disconnect a specific receiver:

          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 in a single call.)

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().

bool QObject::disconnect ( const char * signal = 0, const QObject * receiver = 0, const char * member = 0 )

Disconnects signal from member of receiver.

A signal-slot connection is removed when either of the objects involved are destroyed.

bool QObject::disconnect ( const QObject * receiver, const char * member = 0 )

Disconnects all signals in this object from receiver's member.

A signal-slot connection is removed when either of the objects involved are destroyed.

void QObject::disconnectNotify ( const char * signal ) [virtual protected]

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().

void QObject::dumpObjectInfo ()

This function is useful for debugging, but does nothing if the library has been compiled in release mode (i.e. without debugging information).

void QObject::dumpObjectTree ()

This function is useful for debugging, but does nothing if the library has been compiled in release mode (i.e. without debugging information).

bool QObject::event ( QEvent * e ) [virtual]

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.

bool QObject::eventFilter ( QObject * watched, QEvent * e ) [virtual]

In your reimplementation of this function, if you want to filter the event e, out, i.e. stop it being handled further, return TRUE; otherwise return FALSE.

Example:

    class MyMainWindow : public QMainWindow
    {
    public:
        MyMainWindow( QWidget *parent = 0, const char *name = 0 );

    protected:
        bool eventFilter( QObject *obj, QEvent *ev );

    private:
        QTextEdit *textEdit;
    };

    MyMainWindow::MyMainWindow( QWidget *parent, const char *name )
        : QMainWindow( parent, name )
    {
        textEdit = new QTextEdit( this );
        setCentralWidget( textEdit );
        textEdit->installEventFilter( this );
    }

    bool MyMainWindow::eventFilter( QObject *obj, QEvent *ev )
    {
        if ( obj == textEdit ) {
            if ( e->type() == QEvent::KeyPress ) {
                qDebug( "Ate key press %d", k->key() );
                return TRUE;
            } else {
                return FALSE;
            }
        } else {
            // pass the event on to the parent class
            return QMainWindow::eventFilter( obj, ev );
        }
    }
    

Notice in the example above that unhandled events are passed to the base class's eventFilter() function, since the base class might have reimplemented eventFilter() for its own internal purposes.

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 QAccel, QScrollView, and QSpinBox.

bool QObject::highPriority () const

Returns TRUE if the object is a high-priority object, or FALSE if it is a standard-priority object.

High-priority objects are placed first in QObject's list of children on the assumption that they will be referenced very often.

bool QObject::inherits ( const char * clname ) const

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: table/statistics/statistics.cpp, themes/metal.cpp, and themes/wood.cpp.

void QObject::insertChild ( QObject * obj ) [virtual]

Warning: This function cannot be used to make one widget the child widget of another widget. Child widgets can only be created by setting the parent widget in the constructor or by calling QWidget::reparent().

See also removeChild() and QWidget::reparent().

void QObject::installEventFilter ( const QObject * filterObj )

Installs an event filter filterObj on this object. For example:

    monitoredObj->installEventFilter( filterObj );
    

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 filterObj receives events via its eventFilter() function. The eventFilter() function must return TRUE if the event should be filtered, (i.e. stopped); otherwise it must return FALSE.

If multiple event filters are installed on a single object, the filter that was installed last is activated first.

Here's a KeyPressEater class that eats the key presses of its monitored objects:

    class KeyPressEater : public QObject
    {
        ...
    protected:
        bool eventFilter( QObject *o, QEvent *e );
    };

    bool KeyPressEater::eventFilter( QObject *o, QEvent *e )
    {
        if ( e->type() == QEvent::KeyPress ) {
            // special processing for key press
            QKeyEvent *k = (QKeyEvent *)e;
            qDebug( "Ate key press %d", k->key() );
            return TRUE; // eat event
        } else {
            // standard event processing
            return FALSE;
        }
    }
    

And here's how to install it on two widgets:

        KeyPressEater *keyPressEater = new KeyPressEater( this );
        QPushButton *pushButton = new QPushButton( this );
        QListView *listView = new QListView( this );

        pushButton->installEventFilter( keyPressEater );
        listView->installEventFilter( keyPressEater );
    

The QAccel class, for example, uses this technique to intercept accelerator key presses.

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().

bool QObject::isA ( const char * clname ) const

Example:

    QTimer *t = new QTimer; // QTimer inherits QObject
    t->isA( "QTimer" );     // returns TRUE
    t->isA( "QObject" );    // returns FALSE
  

See also inherits() and metaObject().

bool QObject::isWidgetType () const

Returns TRUE if the object is a widget; otherwise returns FALSE.

Calling this function is equivalent to calling inherits("QWidget"), except that it is much faster.

void QObject::killTimer ( int id )

The timer identifier is returned by startTimer() when a timer event is started.

See also timerEvent(), startTimer(), and killTimers().

void QObject::killTimers ()

Warning: 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. We recommend using a QTimer or perhaps killTimer().

See also timerEvent(), startTimer(), and killTimer().

QMetaObject * QObject::metaObject () const [virtual]

Returns a pointer to the meta object of this object.

A meta object contains information about a class that inherits QObject, e.g. class name, superclass 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.

const char * QObject::name () const

Returns the name of this object. See the "name" property for details.

const char * QObject::name ( const char * defaultName ) const

Returns the name of this object, or defaultName if the object does not have a name.

QCString QObject::normalizeSignalSlot ( const char * signalSlot ) [static protected]

const QObjectList * QObject::objectTrees () [static]

The QObjectList class is defined in the qobjectlist.h header file.

The most recent 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().

QObject * QObject::parent () const

Returns a pointer to the parent object.

See also children().

QVariant QObject::property ( const char * name ) const [virtual]

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().

Example: qutlook/centralwidget.cpp.

QObjectList * QObject::queryList ( const char * inheritsClass = 0, const char * objName = 0, bool regexpMatch = TRUE, bool recursiveSearch = TRUE ) const

If regexpMatch is TRUE (the default), objName is a regular expression that the objects's names must match. The syntax is that of a QRegExp. 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 child() function 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
    

The QObjectList class is defined in the qobjectlist.h header file.

Warning: Delete the list 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.

void QObject::removeChild ( QObject * obj ) [virtual]

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().

void QObject::removeEventFilter ( const QObject * obj )

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().

const QObject * QObject::sender () [protected]

The pointer returned by this function becomes invalid if the sender is destroyed, or if the slot is disconnected from the sender's signal.

Warning: This function violates the object-oriented principle of modularity. However, getting access to the sender might be useful when many signals are connected to a single slot. The sender is undefined if the slot is called as a normal C++ function.

void QObject::setName ( const char * name ) [virtual]

bool QObject::setProperty ( const char * name, const QVariant & value ) [virtual]

Returns TRUE if the operation was successful; otherwise returns FALSE.

Information about all available properties is provided through the metaObject().

See also property(), metaObject(), QMetaObject::propertyNames(), and QMetaObject::property().

Example: qutlook/centralwidget.cpp.

bool QObject::signalsBlocked () const

Returns TRUE if signals are blocked; otherwise returns FALSE.

Signals are not blocked by default.

See also blockSignals().

int QObject::startTimer ( int interval )

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
    {
        Q_OBJECT
    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 is possible). Note that QTimer's accuracy depends on the underlying operating system and hardware. Most platforms support an accuracy of 20ms; some provide more. If Qt is unable to deliver the requested number of timer clicks, it will silently discard some.

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().

void QObject::timerEvent ( QTimerEvent * ) [virtual protected]

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: biff/biff.cpp, dclock/dclock.cpp, forever/forever.cpp, grapher/grapher.cpp, qmag/qmag.cpp, and xform/xform.cpp.

QString QObject::tr ( const char * sourceText, const char * comment ) [static]

Returns a translated version of sourceText, or sourceText itself if there is no appropriate translated version. The translation context is QObject with comment (0 by default). All QObject subclasses using the Q_OBJECT macro automatically have a reimplementation of this function with the subclass name as context.

Warning: This method is reentrant only if all translators are installed before calling this method. Installing or removing translators while performing translations is not supported. Doing so will probably result in crashes or other undesirable behavior.

See also trUtf8(), QApplication::translate(), and Internationalization with Qt.

Example: network/networkprotocol/view.cpp.

QString QObject::trUtf8 ( const char * sourceText, const char * comment ) [static]

Returns a translated version of sourceText, or QString::fromUtf8(sourceText) if there is no appropriate version. It is otherwise identical to tr(sourceText, comment).

Warning: This method is reentrant only if all translators are installed before calling this method. Installing or removing translators while performing translations is not supported. Doing so will probably result in crashes or other undesirable behavior.

See also tr() and QApplication::translate().

Property Documentation

QCString name

This property holds the name of this object.

You can find an object by name (and type) using child(). You can find a set of objects with 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 name() function 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) invalid precision %f",
                name(), newPrecision );
    

See also className(), child(), and queryList().

Set this property's value with setName() and get this property's value with name().

Related Functions

void * qt_find_obj_child ( QObject * parent, const char * type, const char * 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" );
    

This file is part of the Qt toolkit. Copyright © 1995-2003 Trolltech. All Rights Reserved.