QCoreApplication Class

The QCoreApplication class provides an event loop for Qt applications without UI. More...

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Properties

Public Functions

Public Slots

Signals

Static Public Members

Reimplemented Protected Functions

Related Non-Members

Macros

Detailed Description

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 QGuiApplication. For applications that use the Qt Widgets module, 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, QGuiApplication 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 with sendEvent() or posted to an event queue with postEvent(). Pending events can be removed with removePostedEvents() or dispatched with sendPostedEvents().

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: QCoreApplication removes option -qmljsdebugger="...". It parses the argument of qmljsdebugger, and then removes this option plus its argument.

For more advanced command line option handling, create a QCommandLineParser.

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, QGuiApplication or QCoreApplication to reset the locale that is used for number formatting to "C"-locale.

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

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.

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.

[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.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also quit().

[static slot] void QCoreApplication::quit()

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

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

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, SIGNAL(clicked()), &app, SLOT(quit()), Qt::QueuedConnection);

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

[virtual] QCoreApplication::~QCoreApplication()

Destroys the QCoreApplication object.

[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 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.

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).

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 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".

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().

[static] qint64 QCoreApplication::applicationPid()

Returns the current process ID for the application.

This function was introduced in Qt 4.4.

[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.

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 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.

This function was introduced in Qt 4.1.

See also applicationFilePath() and QCommandLineParser.

[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] 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.

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.

Note: The filter function set here receives native messages, i.e. MSG or XCB event structs.

Note: Native event filters will be disabled in the application when the Qt::AA_PluginApplication attribute is set.

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

This function was introduced in Qt 5.0.

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 Dynamic Translation.

[static] 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.

This function was introduced in Qt 5.3.

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.

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.

[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 6. 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.

Warning: If you override this function, you must ensure all threads that process events stop doing so before your application object begins destruction. This includes threads started by other libraries that you may be using, but does not apply to Qt's own threads.

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.

This function was introduced in Qt 4.3.

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

[static] void 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 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(), flush(), and sendPostedEvents().

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

This function overloads processEvents().

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

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

Calling this function processes events only for the calling thread.

Note: Unlike the processEvents() overload, this function also processes events that are posted while the function runs.

Note: This function is thread-safe.

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

[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).

This function was introduced in Qt 5.0.

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.

This function was introduced in Qt 4.3.

[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().

[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.

Note: This method must be called from the thread in which its QObject parameter, receiver, lives.

See also flush() and postEvent().

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

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

Note: Some application attributes must be set before creating a QCoreApplication instance. Refer to the Qt::ApplicationAttribute documentation for more information.

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 libraries to paths. All existing paths will be deleted and the path list will consist of the paths given in paths.

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.

Note: It is strongly recommended not to enable this option since it introduces security risks.

This function was introduced in Qt 5.3.

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 translate().

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
}

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;
};

// Called once QCoreApplication exists
static void preRoutineMyDebugTool()
{
    MyDebugTool* tool = new MyDebugTool(QCoreApplication::instance());
    QCoreApplication::instance()->installEventFilter(tool);
}

Q_COREAPP_STARTUP_FUNCTION(preRoutineMyDebugTool)

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

class MyMfcView : public CView
{
    Q_DECLARE_TR_FUNCTIONS(MyMfcView)

public:
    MyMfcView();
    ...
};