QObject Class Reference
[QtCore module]

Dynamic Properties

From Qt 4.2, dynamic properties can be added to and removed from QObject instances at run-time. Dynamic properties do not need to be declared at compile-time, yet they provide the same advantages as static properties and are manipulated using the same API - using property() to read them and setProperty() to write them.

From Qt 4.3, dynamic properties are supported by Qt Designer, and both standard Qt widgets and user-created forms can be given dynamic properties.

{Object Trees and Object Ownership}

See also QMetaObject, QPointer, QObjectCleanupHandler, and Q_DISABLE_COPY().

Property Documentation

objectName : QString

This property holds the name of this object.

You can find an object by name (and type) using findChild(). You can find a set of objects with findChildren().

 qDebug("MyClass::setPrecision(): (%s) invalid precision %f",
        qPrintable(objectName()), newPrecision);

By default, this property contains an empty string.

Access functions:

• QString objectName () const
• void setObjectName ( const QString & name )

See also metaObject() and QMetaObject::className().

Member Function Documentation

Q_INVOKABLE QObject::QObject ( QObject * parent = 0 )

Constructs an object with parent object parent.

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.

See also parent(), findChild(), and findChildren().

QObject::~QObject ()   [virtual]

Destroys the object, deleting all its child objects.

All signals to and from the object are automatically disconnected, and any pending posted events for the object are removed from the event queue. However, it is often safer to use deleteLater() rather than deleting a QObject subclass directly.

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 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 if it exists in a different thread than the one currently executing. Use deleteLater() instead, which will cause the event loop to delete the object after all pending events have been delivered to it.

See also deleteLater().

bool QObject::blockSignals ( bool block )

If block is true, signals emitted by this object are blocked (i.e., emitting a signal will not invoke anything connected to it). If block is false, no such blocking will occur.

The return value is the previous value of signalsBlocked().

Note that the destroyed() signal will be emitted even if the signals for this object have been blocked.

See also signalsBlocked().

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

This event handler can be reimplemented in a subclass to receive child events. The event is passed in the event parameter.

QEvent::ChildAdded and QEvent::ChildRemoved events are sent to objects when children are added or removed. In both cases you can only rely on the child being a QObject, or if isWidgetType() returns true, a QWidget. (This is because, in the ChildAdded case, the child is not yet fully constructed, and in the ChildRemoved case it might have been destructed already).

QEvent::ChildPolished events are sent to widgets when children are polished, or when polished children are added. If you receive a child polished event, the child's construction is usually completed. However, this is not guaranteed, and multiple polish events may be delivered during the execution of a widget's constructor.

For every child widget, you receive one ChildAdded event, zero or more ChildPolished events, and one ChildRemoved event.

The ChildPolished event is omitted if a child is removed immediately after it is added. If a child is polished several times during construction and destruction, you may receive several child polished events for the same child, each time with a different virtual table.

See also event().

const QObjectList & QObject::children () const

Returns a list of child objects. The QObjectList class is defined in the <QObject> header file as the following:

 typedef QList<QObject*> QObjectList;

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 findChild(), findChildren(), parent(), and setParent().

bool QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method, Qt::ConnectionType type = Qt::AutoConnection )   [static]

Creates a connection of the given type from the signal in the sender object to the method in the receiver object. Returns true if the connection succeeds; otherwise returns false.

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

 QLabel *label = new QLabel;
 QScrollBar *scrollBar = new QScrollBar;
 QObject::connect(scrollBar, 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:

 // WRONG
 QObject::connect(scrollBar, SIGNAL(valueChanged(int value)),
                  label, SLOT(setNum(int value)));

A signal can also be connected to another signal:

 class MyWidget : public QWidget
 {
     Q_OBJECT

 public:
     MyWidget();

 signals:
     void buttonClicked();

 private:
     QPushButton *myButton;
 };

 MyWidget::MyWidget()
 {
     myButton = new QPushButton(this);
     connect(myButton, SIGNAL(clicked()),
             this, SIGNAL(buttonClicked()));
 }

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 method, or if their signatures aren't compatible.

For every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call.

The optional type parameter describes the type of connection to establish. In particular, it determines whether a particular signal is delivered to a slot immediately or queued for delivery at a later time. If the signal is queued, the parameters must be of types that are known to Qt's meta-object system, because Qt needs to copy the arguments to store them in an event behind the scenes. If you try to use a queued connection and get the error message

 QObject::connect: Cannot queue arguments of type 'MyType'
 (Make sure 'MyType' is registered using qRegisterMetaType().)

call qRegisterMetaType() to register the data type before you establish the connection.

Note: This function is thread-safe.

See also disconnect(), sender(), and qRegisterMetaType().

bool QObject::connect ( const QObject * sender, const char * signal, const char * method, Qt::ConnectionType type = Qt::AutoConnection ) const

This function overloads connect().

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

Equivalent to connect(sender, signal, this, method, type).

Every connection you make emits a signal, so duplicate connections emit two signals. You can break a connection using disconnect().

Note: This function is thread-safe.

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.

If you want to compare signal with a specific signal, use QLatin1String and the SIGNAL() macro as follows:

 if (QLatin1String(signal) == SIGNAL(valueChanged(int))) {
     // signal is valueChanged(int)
 }

If the signal contains multiple parameters or parameters that contain spaces, call QMetaObject::normalizedSignature() on the result of the SIGNAL() macro.

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 ( QEvent * event )   [virtual protected]

This event handler can be reimplemented in a subclass to receive custom events. Custom events are user-defined events with a type value at least as large as the QEvent::User item of the QEvent::Type enum, and is typically a QEvent subclass. The event is passed in the event parameter.

See also event() and QEvent.

void QObject::deleteLater ()   [slot]

Schedules this object for deletion.

The object will be deleted when control returns to the event loop. If the event loop is not running when this function is called (e.g. deleteLater() is called on an object before QCoreApplication::exec()), the object will be deleted once the event loop is started.

Note that entering and leaving a new event loop (e.g., by opening a modal dialog) will not perform the deferred deletion; for the object to be deleted, the control must return to the event loop from which deleteLater() was called.

Note: It is safe to call this function more than once; when the first deferred deletion event is delivered, any pending events for the object are removed from the event queue.

See also destroyed() and QPointer.

void QObject::destroyed ( QObject * obj = 0 )   [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.

See also deleteLater() and QPointer.

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

Disconnects signal in object sender from method in object receiver. Returns true if the connection is successfully broken; otherwise returns false.

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 method 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 method is 0, it disconnects anything that is connected to receiver. If not, only slots named method will be disconnected, and all other slots are left alone. The method must be 0 if receiver is left out, so you cannot disconnect a specifically-named slot on all objects.

Note: This function is thread-safe.

See also connect().

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

This function overloads disconnect().

Disconnects signal from method of receiver.

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

Note: This function is thread-safe.

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

This function overloads disconnect().

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

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.

See connectNotify() for an example of how to compare signal with a specific signal.

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 ()

Dumps information about signal connections, etc. for this object to the debug output.

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

See also dumpObjectTree().

void QObject::dumpObjectTree ()

Dumps a tree of children to the debug output.

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

See also dumpObjectInfo().

QList<QByteArray> QObject::dynamicPropertyNames () const

Returns the names of all properties that were dynamically added to the object using setProperty().

This function was introduced in Qt 4.2.

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

This virtual function receives events to an object and should return true if the event e was recognized and processed.

The event() function can be reimplemented to customize the behavior of an object.

See also installEventFilter(), timerEvent(), QApplication::sendEvent(), QApplication::postEvent(), and QWidget::event().

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

Filters events if this object has been installed as an event filter for the watched object.

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

Example:

 class MainWindow : public QMainWindow
 {
 public:
     MainWindow();

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

 private:
     QTextEdit *textEdit;
 };

 MainWindow::MainWindow()
 {
     textEdit = new QTextEdit;
     setCentralWidget(textEdit);

     textEdit->installEventFilter(this);
 }

 bool MainWindow::eventFilter(QObject *obj, QEvent *event)
 {
     if (obj == textEdit) {
         if (event->type() == QEvent::KeyPress) {
             QKeyEvent *keyEvent = static_cast<QKeyEvent*>(event);
             qDebug() << "Ate key press" << keyEvent->key();
             return true;
         } else {
             return false;
         }
     } else {
         // pass the event on to the parent class
         return QMainWindow::eventFilter(obj, event);
     }
 }

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().

T QObject::findChild ( const QString & name = QString() ) const

Returns the child of this object that can be cast into type T and that is called name, or 0 if there is no such object. Omitting the name argument causes all object names to be matched. The search is performed recursively.

If there is more than one child matching the search, the most direct ancestor is returned. If there are several direct ancestors, it is undefined which one will be returned. In that case, findChildren() should be used.

This example returns a child QPushButton of parentWidget named "button1":

 QPushButton *button = parentWidget->findChild<QPushButton *>("button1");

This example returns a QListWidget child of parentWidget:

 QListWidget *list = parentWidget->findChild<QListWidget *>();

Warning: This function is not available with MSVC 6. Use qFindChild() instead if you need to support that version of the compiler.

See also findChildren() and qFindChild().

QList<T> QObject::findChildren ( const QString & name = QString() ) const

Returns all children of this object with the given name that can be cast to type T, or an empty list if there are no such objects. Omitting the name argument causes all object names to be matched. The search is performed recursively.

The following example shows how to find a list of child QWidgets of the specified parentWidget named widgetname:

 QList<QWidget *> widgets = parentWidget.findChildren<QWidget *>("widgetname");

This example returns all QPushButtons that are children of parentWidget:

 QList<QPushButton *> allPButtons = parentWidget.findChildren<QPushButton *>();

Warning: This function is not available with MSVC 6. Use qFindChildren() instead if you need to support that version of the compiler.

See also findChild() and qFindChildren().

QList<T> QObject::findChildren ( const QRegExp & regExp ) const

This function overloads findChildren().

Returns the children of this object that can be cast to type T and that have names matching the regular expression regExp, or an empty list if there are no such objects. The search is performed recursively.

Warning: This function is not available with MSVC 6. Use qFindChildren() instead if you need to support that version of the compiler.

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

Returns true if this object is an instance of a class that inherits className or a QObject subclass that inherits className; otherwise returns false.

A class is considered to inherit itself.

Example:

 QTimer *timer = new QTimer;         // QTimer inherits QObject
 timer->inherits("QTimer");          // returns true
 timer->inherits("QObject");         // returns true
 timer->inherits("QAbstractButton"); // returns false

 // QLayout inherits QObject and QLayoutItem
 QLayout *layout = new QLayout;
 layout->inherits("QObject");        // returns true
 layout->inherits("QLayoutItem");    // returns false

(QLayoutItem is not a QObject.)

Consider using qobject_cast<Type *>(object) instead. The method is both faster and safer.

See also metaObject() and qobject_cast().

void QObject::installEventFilter ( 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
 {
     Q_OBJECT
     ...

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

 bool KeyPressEater::eventFilter(QObject *obj, QEvent *event)
 {
     if (event->type() == QEvent::KeyPress) {
         QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
         qDebug("Ate key press %d", keyEvent->key());
         return true;
     } else {
         // standard event processing
         return QObject::eventFilter(obj, event);
     }
 }

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 QShortcut class, for example, uses this technique to intercept shortcut 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.

Note that the filtering object must be in the same thread as this object. If filterObj is in a different thread, this function does nothing. If either filterObj or this object are moved to a different thread after calling this function, the event filter will not be called until both objects have the same thread affinity again (it is not removed).

See also removeEventFilter(), eventFilter(), and event().

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 )

Kills the timer with timer identifier, id.

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

See also timerEvent() and startTimer().

const 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 QObject subclass that contains the Q_OBJECT macro will have a meta-object.

The meta-object information is required by the signal/slot connection mechanism and the property system. The inherits() function also makes use of the meta-object.

If you have no pointer to an actual object instance but still want to access the meta-object of a class, you can use staticMetaObject.

Example:

 QObject *obj = new QPushButton;
 obj->metaObject()->className();             // returns "QPushButton"

 QPushButton::staticMetaObject.className();  // returns "QPushButton"

See also staticMetaObject.

void QObject::moveToThread ( QThread * targetThread )

Changes the thread affinity for this object and its children. The object cannot be moved if it has a parent. Event processing will continue in the targetThread.

To move an object to the main thread, use QApplication::instance() to retrieve a pointer to the current application, and then use QApplication::thread() to retrieve the thread in which the application lives. For example:

 myObject->moveToThread(QApplication::instance()->thread());

If targetThread is zero, all event processing for this object and its children stops.

Note that all active timers for the object will be reset. The timers are first stopped in the current thread and restarted (with the same interval) in the targetThread. As a result, constantly moving an object between threads can postpone timer events indefinitely.

A QEvent::ThreadChange event is sent to this object just before the thread affinity is changed. You can handle this event to perform any special processing. Note that any new events that are posted to this object will be handled in the targetThread.

Warning: This function is not thread-safe; the current thread must be same as the current thread affinity. In other words, this function can only "push" an object from the current thread to another thread, it cannot "pull" an object from any arbitrary thread to the current thread.

See also thread().

QObject * QObject::parent () const

Returns a pointer to the parent object.

See also setParent() and children().

QVariant QObject::property ( const char * name ) const

Returns the value of the object's name property.

If no such property exists, the returned variant is invalid.

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

See also setProperty(), QVariant::isValid(), metaObject(), and dynamicPropertyNames().

int QObject::receivers ( const char * signal ) const   [protected]

Returns the number of receivers connected to the signal.

Since both slots and signals can be used as receivers for signals, and the same connections can be made many times, the number of receivers is the same as the number of connections made from this signal.

When calling this function, you can use the SIGNAL() macro to pass a specific signal:

 if (receivers(SIGNAL(valueChanged(QByteArray))) > 0) {
     QByteArray data;
     get_the_value(&data);       // expensive operation
     emit valueChanged(data);
 }

As the code snippet above illustrates, you can use this function to avoid emitting a signal that nobody listens to.

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.

void QObject::removeEventFilter ( QObject * obj )

Removes an event filter object obj from this object. The request is ignored if such an event filter has not been installed.

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().

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

Returns a pointer to the object that sent the signal, if called in a slot activated by a signal; otherwise it returns 0. The pointer is valid only during the execution of the slot that calls this function from this object's thread context.

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.

Warning: As mentioned above, the return value of this function is not valid when the slot is called via a Qt::DirectConnection from a thread different from this object's thread. Do not use this function in this type of scenario.

See also QSignalMapper.

void QObject::setParent ( QObject * parent )

Makes the object a child of parent.

See also parent() and QWidget::setParent().

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

Sets the value of the object's name property to value.

If the property is defined in the class using Q_PROPERTY then true is returned on success and false otherwise. If the property is not defined using Q_PROPERTY, and therefore not listed in the meta-object, it is added as a dynamic property and false is returned.

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

Dynamic properties can be queried again using property() and can be removed by setting the property value to an invalid QVariant. Changing the value of a dynamic property causes a QDynamicPropertyChangeEvent to be sent to the object.

Note: Dynamic properties starting with "_q_" are reserved for internal purposes.

See also property(), metaObject(), and dynamicPropertyNames().

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 )

Starts a timer and returns a timer identifier, or returns zero if it could not start a timer.

A timer event will occur every interval milliseconds until killTimer() 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);

 protected:
     void timerEvent(QTimerEvent *event);
 };

 MyObject::MyObject(QObject *parent)
     : QObject(parent)
 {
     startTimer(50);     // 50-millisecond timer
     startTimer(1000);   // 1-second timer
     startTimer(60000);  // 1-minute timer
 }

 void MyObject::timerEvent(QTimerEvent *event)
 {
     qDebug() << "Timer ID:" << event->timerId();
 }

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

The QTimer class provides a high-level programming interface with single-shot timers and timer signals instead of events. There is also a QBasicTimer class that is more lightweight than QTimer and less clumsy than using timer IDs directly.

See also timerEvent(), killTimer(), and QTimer::singleShot().

QThread * QObject::thread () const

Returns the thread in which the object lives.

See also moveToThread().

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

This event handler can be reimplemented in a subclass to receive timer events for the object.

QTimer provides a higher-level interface to the timer functionality, and also more general information about timers. The timer event is passed in the event parameter.

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

QString QObject::tr ( const char * sourceText, const char * disambiguation = 0, int n = -1 )   [static]

Returns a translated version of sourceText, optionally based on a disambiguation string and value of n for strings containing plurals; otherwise returns sourceText itself if no appropriate translated string is available.

See the sections below on Disambiguation and Handling Plurals for more information about the optional disambiguation and n parameters.

QObject and its subclasses obtain translated strings from any translator objects that have been installed on the application object; see the QTranslator documentation for details about this mechanism.

A translatable string is referenced by its translation context; this is the name of the QObject subclass whose tr() function is invoked, as in the following example:

 void MainWindow::createMenus()
 {
     fileMenu = menuBar()->addMenu(tr("&File"));
     ...

Here, the context is MainWindow because it is the MainWindow::tr() function that is invoked. Translation contexts can be given explicitly by fully qualifying the call to tr(); for example:

 QString text = QScrollBar::tr("Page up");

This call obtains the translated text for "Page up" from the QScrollBar context.

 //: This name refers to a host name.
 hostNameLabel->setText(tr("Name:"));

 /*: This text refers to a C++ code example. */
 QString example = tr("Example");

 MyWindow::MyWindow()
 {
     QLabel *senderLabel = new QLabel(tr("Name:"));
     QLabel *recipientLabel = new QLabel(tr("Name:", "recipient"));
     ...
 }

 int n = messages.count();
 showMessage(tr("%n message(s) saved", "", n));

 n == 1 ? tr("%n message saved") : tr("%n messages saved")

 label->setText(tr("F\374r \310lise"));

 QPushButton::staticMetaObject.className();  // returns "QPushButton"

 QObject *obj = new QPushButton;
 obj->metaObject()->className();             // returns "QPushButton"

 QObject *obj = new QTimer;          // QTimer inherits QObject

 QTimer *timer = qobject_cast<QTimer *>(obj);
 // timer == (QObject *)obj

 QAbstractButton *button = qobject_cast<QAbstractButton *>(obj);
 // button == 0

 class MyClass : public QObject
 {
     Q_OBJECT
     Q_CLASSINFO("Author", "Pierre Gendron")
     Q_CLASSINFO("URL", "http://www.my-organization.qc.ca")

 public:
     ...
 };

 class MyClass : public QObject
 {

   private:
     Q_DISABLE_COPY(MyClass)
 };

 class MyClass : public QObject
 {

   private:
      MyClass(const MyClass &);
      MyClass &operator=(const MyClass &);
 };

   QWidget w = QWidget();

 class MyClass : public QObject
 {
     Q_OBJECT
     Q_ENUMS(Priority)

 public:
     MyClass(QObject *parent = 0);
     ~MyClass();

     enum Priority { High, Low, VeryHigh, VeryLow };
     void setPriority(Priority priority);
     Priority priority() const;
 };

 Q_FLAGS(Options Alignment)

 class BasicToolsPlugin : public QObject,
                          public BrushInterface,
                          public ShapeInterface,
                          public FilterInterface
 {
     Q_OBJECT
     Q_INTERFACES(BrushInterface ShapeInterface FilterInterface)

 public:
     ...
 };

 class Window : public QWidget
 {
     Q_OBJECT

 public:
     Window();
     void normalMethod();
     Q_INVOKABLE void invokableMethod();
 };

 #include <QObject>

 class Counter : public QObject
 {
     Q_OBJECT

 public:
     Counter() { m_value = 0; }

     int value() const { return m_value; }

 public slots:
     void setValue(int value);

 signals:
     void valueChanged(int newValue);

 private:
     int m_value;
 };

 Q_PROPERTY(type name
            READ getFunction
            [WRITE setFunction]
            [RESET resetFunction]
            [DESIGNABLE bool]
            [SCRIPTABLE bool]
            [STORED bool]
            [USER bool])

 Q_PROPERTY(QString title READ title WRITE setTitle USER true)