Qt 4.3: QCoreApplication Class Reference

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# QCoreApplication Class Reference
[QtCore module]

The QCoreApplication class provides an event loop for console Qt applications. More...

     #include <QCoreApplication>

Inherits QObject.

Inherited by QApplication.

- List of all members, including inherited members
- Qt 3 support members

### Public Types

- enum Encoding { CodecForTr, UnicodeUTF8, DefaultCodec }
- typedef EventFilter

### Properties

- applicationName : QString
- organizationDomain : QString
- organizationName : QString

- 1 property inherited from QObject

### Public Functions

- QCoreApplication ( int & argc, char ** argv )
- ~QCoreApplication ()
- bool filterEvent ( void * message, long * result )
- virtual bool notify ( QObject * receiver, QEvent * event )
- EventFilter setEventFilter ( EventFilter filter )
- virtual bool winEventFilter ( MSG * msg, long * result )

- 29 public functions inherited from QObject

### Public Slots

- void quit ()

- 1 public slot inherited from QObject

### Signals

- void aboutToQuit ()

- 1 signal inherited from QObject

### Static Public Members

- void addLibraryPath ( const QString & path )
- QString applicationDirPath ()
- QString applicationFilePath ()
- QString applicationName ()
- QStringList arguments ()
- bool closingDown ()
- int exec ()
- void exit ( int returnCode = 0 )
- void flush ()
- bool hasPendingEvents ()
- void installTranslator ( QTranslator * translationFile )
- QCoreApplication * instance ()
- QStringList libraryPaths ()
- QString organizationDomain ()
- QString organizationName ()
- void postEvent ( QObject * receiver, QEvent * event )
- void postEvent ( QObject * receiver, QEvent * event, int priority )
- void processEvents ( QEventLoop::ProcessEventsFlags flags = QEventLoop::AllEvents )
- void processEvents ( QEventLoop::ProcessEventsFlags flags, int maxtime )
- void removeLibraryPath ( const QString & path )
- void removePostedEvents ( QObject * receiver )
- void removePostedEvents ( QObject * receiver, int eventType )
- void removeTranslator ( QTranslator * translationFile )
- bool sendEvent ( QObject * receiver, QEvent * event )
- void sendPostedEvents ( QObject * receiver, int event_type )
- void sendPostedEvents ()
- void setApplicationName ( const QString & application )
- void setAttribute ( Qt::ApplicationAttribute attribute, bool on = true )
- void setLibraryPaths ( const QStringList & paths )
- void setOrganizationDomain ( const QString & orgDomain )
- void setOrganizationName ( const QString & orgName )
- bool startingUp ()
- bool testAttribute ( Qt::ApplicationAttribute attribute )
- QString translate ( const char * context, const char * sourceText, const char * comment, Encoding encoding, int n )
- QString translate ( const char * context, const char * sourceText, const char * comment = 0, Encoding encoding = CodecForTr )

- 5 static public members inherited from QObject

### Related Non-Members

- void qAddPostRoutine ( QtCleanUpFunction ptr )

### Macros

- Q_DECLARE_TR_FUNCTIONS ( context )

### Additional Inherited Members

- 7 protected functions inherited from QObject

## Detailed Description

The QCoreApplication class provides an event loop for console Qt applications.

This class is used by non-GUI applications to provide their event loop. For non-GUI application that uses Qt, there should be exactly one QCoreApplication object. For GUI applications, see QApplication.

QCoreApplication contains the main event loop, where all events from the operating system (e.g., timer and network events) and other sources are processed and dispatched. It also handles the application's initialization and finalization, as well as system-wide and application-wide settings.

The command line arguments which QCoreApplication's constructor should be called with are accessible using arguments(). The event loop is started with a call to exec(). Long running operations can call processEvents() to keep the application responsive.

Some Qt classes, such as QString, can be used without a QCoreApplication object. However, in general, we recommend that you create a QCoreApplication or a QApplication object in your `main()` function as early as possible. The application will enter the event loop when exec() is called. exit() will not return until the event loop exits, e.g., when quit() is called.

An application has an applicationDirPath() and an applicationFilePath(). Translation files can be added or removed using installTranslator() and removeTranslator(). Application strings can be translated using translate(). The QObject::tr() and QObject::trUtf8() functions are implemented in terms of translate().

The class provides a quit() slot and an aboutToQuit() signal.

Several static convenience functions are also provided. The QCoreApplication object is available from instance(). Events can be sent or posted using sendEvent(), postEvent(), and sendPostedEvents(). Pending events can be removed with removePostedEvents() or flushed with flush(). Library paths (see QLibrary) can be retrieved with libraryPaths() and manipulated by setLibraryPaths(), addLibraryPath(), and removeLibraryPath().

See also QApplication, QAbstractEventDispatcher, QEventLoop, Semaphores Example, and Wait Conditions Example.

## Member Type Documentation

### enum QCoreApplication::Encoding

This enum type defines the 8-bit encoding of character string arguments to translate():

Constant | Value | Description
--- | --- | ---
`QCoreApplication::CodecForTr` | `0` | The encoding specified by QTextCodec::codecForTr() (Latin-1 if none has been set).
`QCoreApplication::UnicodeUTF8` | `1` | UTF-8.
`QCoreApplication::DefaultCodec` | `CodecForTr` | (Obsolete) Use CodecForTr instead.

See also QObject::tr(), QObject::trUtf8(), and QString::fromUtf8().

### typedef QCoreApplication::EventFilter

A function with the following signature that can be used as an event filter:

     bool myEventFilter(void *message, long *result);

See also setEventFilter().

## Property Documentation

### applicationName : QString

This property holds the name of this application.

The value is used by the QSettings class when it is constructed using the empty constructor. This saves having to repeat this information each time a QSettings object is created.

Access functions:

- QString applicationName ()
- void setApplicationName ( const QString & application )

See also organizationName and organizationDomain.

### organizationDomain : QString

This property holds the Internet domain of the organization that wrote this application.

The value is used by the QSettings class when it is constructed using the empty constructor. This saves having to repeat this information each time a QSettings object is created.

On Mac, QSettings uses organizationDomain() as the organization if it's not an empty string; otherwise it uses organizationName(). On all other platforms, QSettings uses organizationName() as the organization.

Access functions:

- QString organizationDomain ()
- void setOrganizationDomain ( const QString & orgDomain )

See also organizationName and applicationName.

### organizationName : QString

This property holds the name of the organization that wrote this application.

The value is used by the QSettings class when it is constructed using the empty constructor. This saves having to repeat this information each time a QSettings object is created.

On Mac, QSettings uses organizationDomain() as the organization if it's not an empty string; otherwise it uses organizationName(). On all other platforms, QSettings uses organizationName() as the organization.

Access functions:

- QString organizationName ()
- void setOrganizationName ( const QString & orgName )

See also organizationDomain and applicationName.

## Member Function Documentation

### QCoreApplication::QCoreApplication ( int & argc, char ** argv )

Constructs a Qt kernel application. Kernel applications are applications without a graphical user interface. These type of applications are used at the console or as server processes.

The argc and argv arguments are processed by the application, and made available in a more convenient form by the arguments() function.

Warning: The data pointed to by argc and argv must stay valid for the entire lifetime of the QCoreApplication object.

### QCoreApplication::~QCoreApplication ()

Destroys the QCoreApplication object.

### void QCoreApplication::aboutToQuit () `[signal]`

This signal is emitted when the application is about to quit the main event loop, e.g. when the event loop level drops to zero. This may happen either after a call to quit() from inside the application or when the users shuts down the entire desktop session.

The signal is particularly useful if your application has to do some last-second cleanup. Note that no user interaction is possible in this state.

See also quit().

### void QCoreApplication::addLibraryPath ( const QString & path ) `[static]`

Appends path to the end of the library path list. If path is empty or already in the path list, the path list is not changed.

The default path list consists of a single entry, the installation directory for plugins. The default installation directory for plugins is `INSTALL/plugins`, where `INSTALL` is the directory where Qt was installed.

See also removeLibraryPath(), libraryPaths(), and setLibraryPaths().

### QString QCoreApplication::applicationDirPath () `[static]`

Returns the directory that contains the application executable.

For example, if you have installed Qt in the `C:\Trolltech\Qt` directory, and you run the `regexp` example, this function will return "C:/Trolltech/Qt/examples/tools/regexp".

On Mac OS X this will point to the directory actually containing the executable, which may be inside of an application bundle (if the application is bundled).

Warning: On Unix, this function assumes that argv[0] contains the file name of the executable (which it normally does). It also assumes that the current directory hasn't been changed by the application.

See also applicationFilePath().

### QString QCoreApplication::applicationFilePath () `[static]`

Returns the file path of the application executable.

For example, if you have installed Qt in the `/usr/local/qt` directory, and you run the `regexp` example, this function will return "/usr/local/qt/examples/tools/regexp/regexp".

Warning: On Unix, this function assumes that argv[0] contains the file name of the executable (which it normally does). It also assumes that the current directory hasn't been changed by the application.

See also applicationDirPath().

### QStringList QCoreApplication::arguments () `[static]`

Returns the list of command-line arguments.

arguments().at(0) is the program name, arguments().at(1) is the first argument, and arguments().last() is the last argument.

Calling this function is slow - you should store the result in a variable when parsing the command line.

Warning: On Unix, this list is built from the argc and argv parameters passed to the constructor in the main() function. The string-data in argv is interpreted using QString::fromLocal8Bit(); hence it is not possible to pass i.e. Japanese command line arguments on a system that runs in a latin1 locale. Most modern Unix systems do not have this limitation, as they are Unicode based.

On NT-based Windows, this limitation does not apply either.

This function was introduced in Qt 4.1.

### bool QCoreApplication::closingDown () `[static]`

Returns true if the application objects are being destroyed; otherwise returns false.

See also startingUp().

### int QCoreApplication::exec () `[static]`

Enters the main event loop and waits until exit() is called. Returns the value that was set to exit() (which is 0 if exit() is called via quit()).

It is necessary to call this function to start event handling. The main event loop receives events from the window system and dispatches these to the application widgets.

To make your application perform idle processing (i.e. executing a special function whenever there are no pending events), use a QTimer with 0 timeout. More advanced idle processing schemes can be achieved using processEvents().

See also quit(), exit(), processEvents(), and QApplication::exec().

### void QCoreApplication::exit ( int returnCode = 0 ) `[static]`

Tells the application to exit with a return code.

After this function has been called, the application leaves the main event loop and returns from the call to exec(). The exec() function returns returnCode. If the event loop is not running, this function does nothing.

By convention, a returnCode of 0 means success, and any non-zero value indicates an error.

Note that unlike the C library function of the same name, this function does return to the caller -- it is event processing that stops.

See also quit() and exec().

### bool QCoreApplication::filterEvent ( void * message, long * result )

Sends message through the event filter that was set by setEventFilter(). If no event filter has been set, this function returns false; otherwise, this function returns the result of the event filter function in the result parameter.

See also setEventFilter().

### void QCoreApplication::flush () `[static]`

Flushes the platform specific event queues.

If you are doing graphical changes inside a loop that does not return to the event loop on asynchronous window systems like X11 or double buffered window systems like Mac OS X, and you want to visualize these changes immediately (e.g. Splash Screens), call this function.

See also sendPostedEvents().

### bool QCoreApplication::hasPendingEvents () `[static]`

This function returns true if there are pending events; otherwise returns false. Pending events can be either from the window system or posted events using postEvent().

See also QAbstractEventDispatcher::hasPendingEvents().

### void QCoreApplication::installTranslator ( QTranslator * translationFile ) `[static]`

Adds the translation file translationFile to the list of translation files to be used for translations.

Multiple translation files can be installed. Translations are searched for in the last installed translation file on, back to the first installed translation file. The search stops as soon as a matching translation is found.

See also removeTranslator(), translate(), and QTranslator::load().

### QCoreApplication * QCoreApplication::instance () `[static]`

Returns a pointer to the application's QCoreApplication (or QApplication) instance.

### QStringList QCoreApplication::libraryPaths () `[static]`

Returns a list of paths that the application will search when dynamically loading libraries.

Qt provides default library paths, but they can also be set using a qt.conf file. Paths specified in this file will override default values.

This list will include the installation directory for plugins if it exists (the default installation directory for plugins is `INSTALL/plugins`, where `INSTALL` is the directory where Qt was installed). The directory of the application executable (NOT the working directory) is always added, as well as the colon separated entries of the QT_PLUGIN_PATH environment variable.

If you want to iterate over the list, you can use the foreach pseudo-keyword:

     foreach (QString path, app.libraryPaths())
         do_something(path);

See also setLibraryPaths(), addLibraryPath(), removeLibraryPath(), QLibrary, and How to Create Qt Plugins.

### bool QCoreApplication::notify ( QObject * receiver, QEvent * event ) `[virtual]`

Sends event to receiver: receiver->event(event). Returns the value that is returned from the receiver's event handler.

For certain types of events (e.g. mouse and key events), the event will be propagated to the receiver's parent and so on up to the top-level object if the receiver is not interested in the event (i.e., it returns false).

There are five different ways that events can be processed; reimplementing this virtual function is just one of them. All five approaches are listed below:

1. Reimplementing paintEvent(), mousePressEvent() and so on. This is the commonest, easiest and least powerful way.
2. Reimplementing this function. This is very powerful, providing complete control; but only one subclass can be active at a time.
3. Installing an event filter on QCoreApplication::instance(). Such an event filter is able to process all events for all widgets, so it's just as powerful as reimplementing notify(); furthermore, it's possible to have more than one application-global event filter. Global event filters even see mouse events for disabled widgets.
4. Reimplementing QObject::event() (as QWidget does). If you do this you get Tab key presses, and you get to see the events before any widget-specific event filters.
5. Installing an event filter on the object. Such an event filter gets all the events except Tab and Shift-Tab key presses.

See also QObject::event() and installEventFilter().

### void QCoreApplication::postEvent ( QObject * receiver, QEvent * event ) `[static]`

Adds the event event, with the object receiver as the receiver of the event, to an event queue and returns immediately.

The event must be allocated on the heap since the post event queue will take ownership of the event and delete it once it has been posted. It is not safe to modify or delete the event after it has been posted.

When control returns to the main event loop, all events that are stored in the queue will be sent using the notify() function.

Events are processed in the order posted. For more control over the processing order, use the postEvent() overload below, which takes a priority argument. This function posts all event with a Qt::NormalEventPriority.

Note: This function is thread-safe.

See also sendEvent(), notify(), and sendPostedEvents().

### void QCoreApplication::postEvent ( QObject * receiver, QEvent * event, int priority ) `[static]`

This is an overloaded member function, provided for convenience.

Adds the event event, with the object receiver as the receiver of the event, to an event queue and returns immediately.

The event must be allocated on the heap since the post event queue will take ownership of the event and delete it once it has been posted. It is not safe to modify or delete the event after it has been posted.

When control returns to the main event loop, all events that are stored in the queue will be sent using the notify() function.

Events are sorted in descending priority order, i.e. events with a high priority are queued before events with a lower priority. The priority can be any integer value, i.e. between INT_MAX and INT_MIN, inclusive; see Qt::EventPriority for more details. Events with equal priority will be processed in the order posted.

Note: This function is thread-safe.

This function was introduced in Qt 4.3.

See also sendEvent(), notify(), sendPostedEvents(), and Qt::EventPriority.

### void QCoreApplication::processEvents ( QEventLoop::ProcessEventsFlags flags = QEventLoop::AllEvents ) `[static]`

Processes all pending events for the calling thread according to the specified flags until there are no more events to process.

You can call this function occasionally when your program is busy performing a long operation (e.g. copying a file).

In event you are running a local loop which calls this function continuously, without an event loop, the DeferredDelete events will not be processed. This can affect the behaviour of widgets, e.g. QToolTip, that rely on DeferredDelete events to function properly. An alternative would be to call sendPostedEvents() from within that local loop.

Calling this function processes events only for the calling thread.

Note: This function is thread-safe.

See also exec(), QTimer, QEventLoop::processEvents(), flush(), and sendPostedEvents().

### void QCoreApplication::processEvents ( QEventLoop::ProcessEventsFlags flags, int maxtime ) `[static]`

This is an overloaded member function, provided for convenience.

Processes pending events for the calling thread for maxtime milliseconds or until there are no more events to process, whichever is shorter.

You can call this function occasionally when you program is busy doing a long operation (e.g. copying a file).

Calling this function processes events only for the calling thread.

Note: This function is thread-safe.

See also exec(), QTimer, and QEventLoop::processEvents().

### void QCoreApplication::quit () `[static slot]`

Tells the application to exit with return code 0 (success). Equivalent to calling QCoreApplication::exit(0).

It's common to connect the QApplication::lastWindowClosed() signal to quit(), and you also often connect e.g. QAbstractButton::clicked() or signals in QAction, QMenu, or QMenuBar to it.

Example:

     QPushButton *quitButton = new QPushButton("Quit");
     connect(quitButton, SIGNAL(clicked()), &app, SLOT(quit()));

See also exit(), aboutToQuit(), and QApplication::lastWindowClosed().

### void QCoreApplication::removeLibraryPath ( const QString & path ) `[static]`

Removes path from the library path list. If path is empty or not in the path list, the list is not changed.

See also addLibraryPath(), libraryPaths(), and setLibraryPaths().

### void QCoreApplication::removePostedEvents ( QObject * receiver ) `[static]`

Removes all events posted using postEvent() for receiver.

The events are not dispatched, instead they are removed from the queue. You should never need to call this function. If you do call it, be aware that killing events may cause receiver to break one or more invariants.

Note: This function is thread-safe.

### void QCoreApplication::removePostedEvents ( QObject * receiver, int eventType ) `[static]`

This is an overloaded member function, provided for convenience.

Removes all events of the given eventType that were posted using postEvent() for receiver.

The events are not dispatched, instead they are removed from the queue. You should never need to call this function. If you do call it, be aware that killing events may cause receiver to break one or more invariants.

If receiver is null, the events of eventType are removed for all objects. If eventType is 0, all the events are removed for receiver.

Note: This function is thread-safe.

This function was introduced in Qt 4.3.

### void QCoreApplication::removeTranslator ( QTranslator * translationFile ) `[static]`

Removes the translation file translationFile from the list of translation files used by this application. (It does not delete the translation file from the file system.)

See also installTranslator(), translate(), and QObject::tr().

### bool QCoreApplication::sendEvent ( QObject * receiver, QEvent * event ) `[static]`

Sends event event directly to receiver receiver, using the notify() function. Returns the value that was returned from the event handler.

The event is not deleted when the event has been sent. The normal approach is to create the event on the stack, for example:

     QMouseEvent event(QEvent::MouseButtonPress, pos, 0, 0, 0);
     QApplication::sendEvent(mainWindow, &event);

See also postEvent() and notify().

### void QCoreApplication::sendPostedEvents ( QObject * receiver, int event_type ) `[static]`

Immediately dispatches all events which have been previously queued with QCoreApplication::postEvent() and which are for the object receiver and have the event type event_type.

Note that events from the window system are not dispatched by this function, but by processEvents().

If receiver is null, the events of event_type are sent for all objects. If event_type is 0, all the events are sent for receiver.

See also flush() and postEvent().

### void QCoreApplication::sendPostedEvents () `[static]`

This is an overloaded member function, provided for convenience.

Dispatches all posted events, i.e. empties the event queue.

### void QCoreApplication::setAttribute ( Qt::ApplicationAttribute attribute, bool on = true ) `[static]`

Sets the attribute attribute if on is true; otherwise clears the attribute.

One of the attributes that can be set with this method is Qt::AA_ImmediateWidgetCreation. It tells Qt to create toplevel windows immediately. Normally, resources for widgets are allocated on demand to improve efficiency and minimize resource usage. Therefore, if it is important to minimize resource consumption, do not set this attribute.

See also testAttribute().

### EventFilter QCoreApplication::setEventFilter ( EventFilter filter )

Sets the event filter filter. Returns a pointer to the filter function previously defined.

The event filter is a function that is called for every message received in all threads. This does not include messages to objects that are not handled by Qt.

The function can return true to stop the event to be processed by Qt, or false to continue with the standard event processing.

Only one filter can be defined, but the filter can use the return value to call the previously set event filter. By default, no filter is set (i.e., the function returns 0).

See also installEventFilter().

### void QCoreApplication::setLibraryPaths ( const QStringList & paths ) `[static]`

Sets the list of directories to search when loading libraries to paths. All existing paths will be deleted and the path list will consist of the paths given in paths.

See also libraryPaths(), addLibraryPath(), removeLibraryPath(), and QLibrary.

### bool QCoreApplication::startingUp () `[static]`

Returns true if an application object has not been created yet; otherwise returns false.

See also closingDown().

### bool QCoreApplication::testAttribute ( Qt::ApplicationAttribute attribute ) `[static]`

Returns true if attribute attribute is set; otherwise returns false.

See also setAttribute().

### QString QCoreApplication::translate ( const char * context, const char * sourceText, const char * comment, Encoding encoding, int n ) `[static]`

Returns the translation text for sourceText, by querying the installed translation files. The translation files are searched from the most recently installed file back to the first installed file.

QObject::tr() and QObject::trUtf8() provide this functionality more conveniently.

context is typically a class name (e.g., "MyDialog") and sourceText is either English text or a short identifying text.

comment is a disambiguating comment, for when the same sourceText is used in different roles within the same context. By default, it is null. encoding indicates the 8-bit encoding of character stings See the QTranslator documentation for more information about contexts and comments.

n is used in conjunction with `%n` to support plural forms. See QObject::tr() for details.

If none of the translation files contain a translation for sourceText in context, this function returns a QString equivalent of sourceText. The encoding of sourceText is specified by encoding; it defaults to CodecForTr.

This function is not virtual. You can use alternative translation techniques by subclassing QTranslator.

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 most likely result in crashes or other undesirable behavior.

Note: This function is reentrant.

See also QObject::tr(), installTranslator(), and QTextCodec::codecForTr().

### QString QCoreApplication::translate ( const char * context, const char * sourceText, const char * comment = 0, Encoding encoding = CodecForTr ) `[static]`

This is an overloaded member function, provided for convenience.

### bool QCoreApplication::winEventFilter ( MSG * msg, long * result ) `[virtual]`

The message procedure calls this function for every message received. Reimplement this function if you want to process window messages msg that are not processed by Qt. If you don't want the event to be processed by Qt, then return true and set result to the value that the window procedure should return. Otherwise return false.

It is only directly addressed messages that are filtered. To handle system wide messages, such as messages from a registered hot key, you need to install an event filter on the event dispatcher, which is returned from QAbstractEventDispatcher::instance().

## Related Non-Members

### void qAddPostRoutine ( QtCleanUpFunction ptr )

Adds a global routine that will be called from the QApplication destructor. This function is normally used to add cleanup routines for program-wide functionality.

The function specified by ptr should take no arguments and should return nothing. For example:

     static int *global_ptr = 0;

     static void cleanup_ptr()
     {
         delete [] global_ptr;
         global_ptr = 0;
     }

     void init_ptr()
     {
         global_ptr = new int[100];      // allocate data
         qAddPostRoutine(cleanup_ptr);   // delete later
     }

Note that for an application- or module-wide cleanup, qAddPostRoutine() is often not suitable. For example, if the program is split into dynamically loaded modules, the relevant module may be unloaded long before the QApplication destructor is called.

For modules and libraries, using a reference-counted initialization manager or Qt's parent-child deletion mechanism may be better. Here is an example of a private class that uses the parent-child mechanism to call a cleanup function at the right time:

     class MyPrivateInitStuff : public QObject
     {
     public:
         static MyPrivateInitStuff *initStuff(QObject *parent)
         {
             if (!p)
                 p = new MyPrivateInitStuff(parent);
             return p;
         }

         ~MyPrivateInitStuff()
         {
             // cleanup goes here
         }

     private:
         MyPrivateInitStuff(QObject *parent)
             : QObject(parent)
         {
             // initialization goes here
         }

         MyPrivateInitStuff *p;
     };

By selecting the right parent object, this can often be made to clean up the module's data at the right moment.

## Macro Documentation

### Q_DECLARE_TR_FUNCTIONS ( context )

The Q_DECLARE_TR_FUNCTIONS() macro declares and implements two translation functions, `tr()` and `trUtf8()`, with these signatures:

     static inline QString tr(const char *sourceText,
                              const char *comment = 0);
     static inline QString trUtf8(const char *sourceText,
                                  const char *comment = 0);

This macro is useful if you want to use QObject::tr() or QObject::trUtf8() in classes that don't inherit from QObject.

Q_DECLARE_TR_FUNCTIONS() must appear at the very top of the class definition (before the first `public:` or `protected:`). For example:

     class MyMfcView : public CView
     {
         Q_DECLARE_TR_FUNCTIONS(MyMfcView)

     public:
         MyMfcView();
         ...
     };

The context parameter is normally the class name, but it can be any string.

See also Q_OBJECT, QObject::tr(), and QObject::trUtf8().