QCoreApplication Class

Member Function Documentation

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

Constructs a Qt core application. Core applications are applications without a graphical user interface. Such 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.

[virtual noexcept] QCoreApplication::~QCoreApplication()

Destroys the QCoreApplication object.

[private signal] void QCoreApplication::aboutToQuit()

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 user 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().

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

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 one or two entries. The first is the installation directory for plugins, which is INSTALL/plugins, where INSTALL is the directory where Qt was installed. The second is the application's own directory (not the current directory), but only after the QCoreApplication object is instantiated.

The library paths are reset to the default when an instance of QCoreApplication is destructed.

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

[static] QString QCoreApplication::applicationDirPath()

Returns the directory that contains the application executable.

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

On macOS and iOS this will point to the directory actually containing the executable, which may be inside an application bundle (if the application is bundled).

See also applicationFilePath().

[static] 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".

See also applicationDirPath().

[static] qint64 QCoreApplication::applicationPid()

Returns the current process ID for the application.

[static] 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.

On Windows, the list is built from the argc and argv parameters only if modified argv/argc parameters are passed to the constructor. In that case, encoding problems might occur.

Otherwise, 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.

See also applicationFilePath() and QCommandLineParser.

[since 6.5] Qt::PermissionStatus QCoreApplication::checkPermission(const QPermission &permission)

Checks the status of the given permission

If the result is Qt::PermissionStatus::Undetermined then permission should be requested via requestPermission() to determine the user's intent.

This function was introduced in Qt 6.5.

See also requestPermission() and Application Permissions.

[static] bool QCoreApplication::closingDown()

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

See also startingUp().

[override virtual protected] bool QCoreApplication::event(QEvent *e)

Reimplements: QObject::event(QEvent *e).

[static] QAbstractEventDispatcher *QCoreApplication::eventDispatcher()

Returns a pointer to the event dispatcher object for the main thread. If no event dispatcher exists for the thread, this function returns nullptr.

See also setEventDispatcher().

[static] int QCoreApplication::exec()

Enters the main event loop and waits until exit() is called. Returns the value that was passed 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 (by 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 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 exec() call.

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

[static slot] void 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.

It's good practice to always connect signals to this slot using a QueuedConnection. If a signal connected (non-queued) to this slot is emitted before control enters the main event loop (such as before "int main" calls exec()), the slot has no effect and the application never exits. Using a queued connection ensures that the slot will not be invoked until after control enters the main event loop.

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

Note also that this function is not thread-safe. It should be called only from the main thread (the thread that the QCoreApplication object is processing events on). To ask the application to exit from another thread, either use QCoreApplication::quit() or instead call this function from the main thread with QMetaMethod::invokeMethod().

See also quit() and exec().

void QCoreApplication::installNativeEventFilter(QAbstractNativeEventFilter *filterObj)

Installs an event filter filterObj for all native events received by the application in the main thread.

The event filter filterObj receives events via its nativeEventFilter() function, which is called for all native events received in the main thread.

The QAbstractNativeEventFilter::nativeEventFilter() function should return true if the event should be filtered, i.e. stopped. It should return false to allow normal Qt processing to continue: the native event can then be translated into a QEvent and handled by the standard Qt event filtering, e.g. QObject::installEventFilter().

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

For maximum portability, you should always try to use QEvent and QObject::installEventFilter() whenever possible.

See also QObject::installEventFilter().

[static] bool QCoreApplication::installTranslator(QTranslator *translationFile)

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 widgets, 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.

The function returns true on success and false on failure.

See also removeTranslator(), translate(), QTranslator::load(), and Prepare for Dynamic Language Changes.

[static noexcept] QCoreApplication *QCoreApplication::instance()

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

If no instance has been allocated, nullptr is returned.

[static] bool QCoreApplication::isSetuidAllowed()

Returns true if the application is allowed to run setuid on UNIX platforms.

See also QCoreApplication::setSetuidAllowed().

[static] QStringList QCoreApplication::libraryPaths()

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

The return value of this function may change when a QCoreApplication is created. It is not recommended to call it before creating a QCoreApplication. The directory of the application executable (not the working directory) is part of the list if it is known. In order to make it known a QCoreApplication has to be constructed as it will use argv[0] to find it.

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. Note that if the qt.conf file is in the directory of the application executable, it may not be found until a QCoreApplication is created. If it is not found when calling this function, the default library paths will be used.

The 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 colon separated entries of the QT_PLUGIN_PATH environment variable are always added. The plugin installation directory (and its existence) may change when the directory of the application executable becomes known.

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

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

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 most common, 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.

Future direction: This function will not be called for objects that live outside the main thread in Qt 7. Applications that need that functionality should find other solutions for their event inspection needs in the meantime. The change may be extended to the main thread, causing this function to be deprecated.

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

[static] void QCoreApplication::postEvent(QObject *receiver, QEvent *event, int priority = Qt::NormalEventPriority)

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.

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

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

Processes some pending events for the calling thread according to the specified flags.

Use of this function is discouraged. Instead, prefer to move long operations out of the GUI thread into an auxiliary one and to completely avoid nested event loop processing. If event processing is really necessary, consider using QEventLoop instead.

In the event that 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, and returns after all available events have been processed. Available events are events queued before the function call. This means that events that are posted while the function runs will be queued until a later round of event processing.

Note: This function is thread-safe.

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

[static] void QCoreApplication::processEvents(QEventLoop::ProcessEventsFlags flags, int ms)

This is an overloaded function.

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

This is equivalent to calling:

QCoreApplication::processEvents(flags, QDeadlineTimer(ms));

[static, since 6.7] void QCoreApplication::processEvents(QEventLoop::ProcessEventsFlags flags, QDeadlineTimer deadline)

This is an overloaded function.

Processes pending events for the calling thread untile deadline has expired, or until there are no more events to process, whichever happens first.

Use of this function is discouraged. Instead, prefer to move long operations out of the GUI thread into an auxiliary one and to completely avoid nested event loop processing. If event processing is really necessary, consider using QEventLoop instead.

Calling this function processes events only for the calling thread.

Note: This function is thread-safe.

This function was introduced in Qt 6.7.

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

[static slot] void QCoreApplication::quit()

Asks the application to quit.

The request may be ignored if the application prevents the quit, for example if one of its windows can't be closed. The application can affect this by handling the QEvent::Quit event on the application level, or QEvent::Close events for the individual windows.

If the quit is not interrupted the application will exit with return code 0 (success).

To exit the application without a chance of being interrupted, call exit() directly. Note that method is not thread-safe.

It's good practice to always connect signals to this slot using a QueuedConnection. If a signal connected (non-queued) to this slot is emitted before control enters the main event loop (such as before "int main" calls exec()), the slot has no effect and the application never exits. Using a queued connection ensures that the slot will not be invoked until after control enters the main event loop.

Example:

QPushButton *quitButton = new QPushButton("Quit");
connect(quitButton, &QPushButton::clicked, &app, &QCoreApplication::quit, Qt::QueuedConnection);

Thread-safety note: this function may be called from any thread to thread-safely cause the currently-running main application loop to exit. However, thread-safety is not guaranteed if the QCoreApplication object is being destroyed at the same time.

Note: This function is thread-safe.

See also exit() and aboutToQuit().

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

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

The library paths are reset to the default when an instance of QCoreApplication is destructed.

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

void QCoreApplication::removeNativeEventFilter(QAbstractNativeEventFilter *filterObject)

Removes an event filterObject 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 nativeEventFilter() function).

See also installNativeEventFilter().

[static] void QCoreApplication::removePostedEvents(QObject *receiver, int eventType = 0)

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 nullptr, the events of eventType are removed for all objects. If eventType is 0, all the events are removed for receiver. You should never call this function with eventType of 0.

Note: This function is thread-safe.

[static] bool QCoreApplication::removeTranslator(QTranslator *translationFile)

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.)

The function returns true on success and false on failure.

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

[since 6.5] template <typename Functor> void QCoreApplication::requestPermission(const QPermission &permission, Functor &&functor)

Requests the given permission.

When the request is ready, functor will be called as functor(const QPermission &permission), with permission describing the result of the request.

The functor can be a free-standing or static member function:

qApp->requestPermission(QCameraPermission{}, &permissionUpdated);

or a lambda:

qApp->requestPermission(QCameraPermission{}, [](const QPermission &permission) {
});

If the user explicitly grants the application the requested permission, or the permission is known to not require user authorization on the given platform, the status will be Qt::PermissionStatus::Granted.

If the user explicitly denies the application the requested permission, or the permission is known to not be accessible or applicable to applications on the given platform, the status will be Qt::PermissionStatus::Denied.

The result of a request will never be Qt::PermissionStatus::Undetermined.

This function was introduced in Qt 6.5.

See also checkPermission() and Application Permissions.

[since 6.5] template <typename Functor> void QCoreApplication::requestPermission(const QPermission &permission, const QObject *context, Functor functor)

Requests the given permission, in the context of context.

When the request is ready, functor will be called as functor(const QPermission &permission), with permission describing the result of the request.

The functor can be a free-standing or static member function:

qApp->requestPermission(QCameraPermission{}, context, &permissionUpdated);

a lambda:

qApp->requestPermission(QCameraPermission{}, context, [](const QPermission &permission) {
});

or a slot in the context object:

qApp->requestPermission(QCameraPermission{}, this, &CamerWidget::permissionUpdated);

The functor will be called in the thread of the context object. If context is destroyed before the request completes, the functor will not be called.

If the user explicitly grants the application the requested permission, or the permission is known to not require user authorization on the given platform, the status will be Qt::PermissionStatus::Granted.

If the user explicitly denies the application the requested permission, or the permission is known to not be accessible or applicable to applications on the given platform, the status will be Qt::PermissionStatus::Denied.

The result of a request will never be Qt::PermissionStatus::Undetermined.

This is an overloaded function.

This function was introduced in Qt 6.5.

See also checkPermission() and Application Permissions.

[static] 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().

[static] void QCoreApplication::sendPostedEvents(QObject *receiver = nullptr, int event_type = 0)

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

See also postEvent().

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

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

See also testAttribute().

[static] void QCoreApplication::setEventDispatcher(QAbstractEventDispatcher *eventDispatcher)

Sets the event dispatcher for the main thread to eventDispatcher. This is only possible as long as there is no event dispatcher installed yet. That is, before QCoreApplication has been instantiated. This method takes ownership of the object.

See also eventDispatcher().

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

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

The library paths are reset to the default when an instance of QCoreApplication is destructed.

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

[static] void QCoreApplication::setSetuidAllowed(bool allow)

Allows the application to run setuid on UNIX platforms if allow is true.

If allow is false (the default) and Qt detects the application is running with an effective user id different than the real user id, the application will be aborted when a QCoreApplication instance is created.

Qt is not an appropriate solution for setuid programs due to its large attack surface. However some applications may be required to run in this manner for historical reasons. This flag will prevent Qt from aborting the application when this is detected, and must be set before a QCoreApplication instance is created.

See also isSetuidAllowed().

[static] bool QCoreApplication::startingUp()

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

See also closingDown().

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

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

See also setAttribute().

[static] QString QCoreApplication::translate(const char *context, const char *sourceText, const char *disambiguation = nullptr, int n = -1)

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() provides 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 nullptr.

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

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

Note: This function is thread-safe.

See also QObject::tr(), installTranslator(), removeTranslator(), and Internationalization and Translations.