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QCoreApplication Class Reference
[QtCore module]

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

Inherits QObject.

Inherited by QApplication.

Types

Methods

Static Methods

Qt Signals


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 Event Loop and Event Handling

The event loop is started with a call to exec_(). Long running operations can call processEvents() to keep the application responsive.

In general, we recommend that you create a QCoreApplication or a QApplication object in your main() function as early as possible. exec_() will not return until the event loop exits; e.g., when quit() is called.

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().

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

Application and Library Paths

An application has an applicationDirPath() and an applicationFilePath(). Library paths (see QLibrary) can be retrieved with libraryPaths() and manipulated by setLibraryPaths(), addLibraryPath(), and removeLibraryPath().

Internationalization and Translations

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().

Accessing Command Line Arguments

The command line arguments which are passed to QCoreApplication's constructor should be accessed using the arguments() function. Note that some arguments supplied by the user may have been processed and removed by QCoreApplication.

In cases where command line arguments need to be obtained using the argv() function, you must convert them from the local string encoding using QString.fromLocal8Bit().

Locale Settings

On Unix/Linux Qt is configured to use the system locale settings by default. This can cause a conflict when using POSIX functions, for instance, when converting between data types such as floats and strings, since the notation may differ between locales. To get around this problem, call the POSIX function setlocale(LC_NUMERIC,"C") right after initializing QApplication or QCoreApplication to reset the locale that is used for number formatting to "C"-locale.


Type Documentation

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().


Method Documentation

QCoreApplication.__init__ (self, list 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 referred to by argc and argv must stay valid for the entire lifetime of the QCoreApplication object. In addition, argc must be greater than zero and argv must contain at least one valid character string.

QCoreApplication.addLibraryPath (QString)

Prepends path to the beginning of the library path list, ensuring that it is searched for libraries first. 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.

In Symbian this function is only useful for adding paths for finding Qt extension plugin stubs, since the OS can only load libraries from the /sys/bin directory.

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

QString QCoreApplication.applicationDirPath ()

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 Linux, this function will try to get the path from the /proc file system. If that fails, it assumes that argv[0] contains the absolute file name of the executable. The function also assumes that the current directory has not been changed by the application.

In Symbian this function will return the application private directory, not the path to executable itself, as those are always in /sys/bin. If the application is in a read only drive, i.e. ROM, then the private path on the system drive will be returned.

See also applicationFilePath().

QString QCoreApplication.applicationFilePath ()

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 Linux, this function will try to get the path from the /proc file system. If that fails, it assumes that argv[0] contains the absolute file name of the executable. The function also assumes that the current directory has not been changed by the application.

See also applicationDirPath().

QString QCoreApplication.applicationName ()

int QCoreApplication.applicationPid ()

Returns the current process ID for the application.

This function was introduced in Qt 4.4.

QString QCoreApplication.applicationVersion ()

int QCoreApplication.argc ()

QStringList QCoreApplication.arguments ()

Returns the list of command-line arguments.

Usually arguments().at(0) is the program name, arguments().at(1) is the first argument, and arguments().last() is the last argument. See the note below about Windows.

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, for example, 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. On Windows, the arguments() are not built from the contents of argv/argc, as the content does not support Unicode. Instead, the arguments() are constructed from the return value of GetCommandLine(). As a result of this, the string given by arguments().at(0) might not be the program name on Windows, depending on how the application was started.

For Symbian applications started with RApaLsSession.StartApp one can specify arguments using CApaCommandLine.SetTailEndL function. Such arguments are only available via this method; they will not be passed to main function. Also note that only 8-bit string data set with CApaCommandLine.SetTailEndL is supported by this function.

This function was introduced in Qt 4.1.

See also applicationFilePath().

list QCoreApplication.argv ()

bool QCoreApplication.closingDown ()

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

See also startingUp().

bool QCoreApplication.event (self, QEvent)

Reimplemented from QObject.event().

int QCoreApplication.exec_ ()

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().

We recommend that you connect clean-up code to the aboutToQuit() signal, instead of putting it in your application's main() function because on some platforms the QCoreApplication.exec() call may not return. For example, on Windows when the user logs off, the system terminates the process after Qt closes all top-level windows. Hence, there is no guarantee that the application will have time to exit its event loop and execute code at the end of the main() function after the QCoreApplication.exec() call.

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

QCoreApplication.exit (int returnCode = 0)

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_().

QCoreApplication.flush ()

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 ()

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().

QCoreApplication.installTranslator (QTranslator)

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 reverse order in which they were installed, so the most recently installed translation file is searched first and the first translation file installed is searched last. The search stops as soon as a translation containing a matching string is found.

Installing or removing a QTranslator, or changing an installed QTranslator generates a LanguageChange event for the QCoreApplication instance. A QApplication instance will propagate the event to all toplevel windows, where a reimplementation of changeEvent can re-translate the user interface by passing user-visible strings via the tr() function to the respective property setters. User-interface classes generated by Qt Designer provide a retranslateUi() function that can be called.

See also removeTranslator(), translate(), QTranslator.load(), and Dynamic Translation.

QCoreApplication QCoreApplication.instance ()

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

If no instance has been allocated, null is returned.

QStringList QCoreApplication.libraryPaths ()

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 (const QString &path, app.libraryPaths())
     do_something(path);

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

bool QCoreApplication.notify (self, QObject, QEvent)

Sends event to receiver: receiver->event(event). Returns the value that is returned from the receiver's event handler. Note that this function is called for all events sent to any object in any thread.

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. Note that application event filters are only called for objects that live in the main thread.
  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, including Tab and Shift+Tab key press events, as long as they do not change the focus widget.

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

QString QCoreApplication.organizationDomain ()

QString QCoreApplication.organizationName ()

QCoreApplication.postEvent (QObject receiver, QEvent event)

The event argument has it's ownership transferred to Qt.

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 access 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().

QCoreApplication.postEvent (QObject receiver, QEvent event, int priority)

The event argument has it's ownership transferred to Qt.

This function overloads postEvent().

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 access 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.

QCoreApplication.processEvents (QEventLoop.ProcessEventsFlags flags = QEventLoop.AllEvents)

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().

QCoreApplication.processEvents (QEventLoop.ProcessEventsFlags flags, int maxtime)

This function overloads processEvents().

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().

QCoreApplication.quit ()

This method is also a Qt slot with the C++ signature void quit().

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().

QCoreApplication.removeLibraryPath (QString)

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().

QCoreApplication.removePostedEvents (QObject receiver)

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.

QCoreApplication.removePostedEvents (QObject receiver, int eventType)

This function overloads removePostedEvents().

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.

QCoreApplication.removeTranslator (QTranslator)

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)

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().

QCoreApplication.sendPostedEvents (QObject receiver, int event_type)

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.

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.

Note: This method must be called from the same thread as its QObject parameter, receiver.

See also flush() and postEvent().

QCoreApplication.sendPostedEvents ()

This function overloads sendPostedEvents().

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

QCoreApplication.setApplicationName (QString application)

QCoreApplication.setApplicationVersion (QString version)

QCoreApplication.setAttribute (Qt.ApplicationAttribute attribute, bool on = True)

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().

QCoreApplication.setLibraryPaths (QStringList)

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.

In Symbian this function is only useful for setting paths for finding Qt extension plugin stubs, since the OS can only load libraries from the /sys/bin directory.

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

QCoreApplication.setOrganizationDomain (QString orgDomain)

QCoreApplication.setOrganizationName (QString orgName)

bool QCoreApplication.startingUp ()

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

See also closingDown().

bool QCoreApplication.testAttribute (Qt.ApplicationAttribute attribute)

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

See also setAttribute().

QString QCoreApplication.translate (str context, object sourceText, str disambiguation = None, Encoding encoding = QCoreApplication.CodecForTr)

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.

disambiguation is an identifying string, for when the same sourceText is used in different roles within the same context. By default, it is null.

See the QTranslator and QObject.tr() documentation for more information about contexts, disambiguations and comments.

encoding indicates the 8-bit encoding of character strings.

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.

This function was introduced in Qt 4.5.

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

QString QCoreApplication.translate (str context, object sourceText, str disambiguation, Encoding encoding, int n)

This function overloads translate().


Qt Signal Documentation

void aboutToQuit ()

This is the default overload of this 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().


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