QApplication Class

The QApplication class manages the GUI application's control flow and main settings. More...

• List of all members, including inherited members
• Obsolete members

Properties

Public Functions

Reimplemented Public Functions

Public Slots

Signals

Static Public Members

Reimplemented Protected Functions

Macros

Detailed Description

QApplication specializes QGuiApplication with some functionality needed for QWidget-based applications. It handles widget specific initialization, finalization.

For any GUI application using Qt, there is precisely one QApplication object, no matter whether the application has 0, 1, 2 or more windows at any given time. For non-QWidget based Qt applications, use QGuiApplication instead, as it does not depend on the QtWidgets library.

Some GUI applications provide a special batch mode ie. provide command line arguments for executing tasks without manual intervention. In such non-GUI mode, it is often sufficient to instantiate a plain QCoreApplication to avoid unnecessarily initializing resources needed for a graphical user interface. The following example shows how to dynamically create an appropriate type of application instance:

QCoreApplication* createApplication(int &argc, char *argv[])
{
    for (int i = 1; i < argc; ++i) {
        if (!qstrcmp(argv[i], "-no-gui"))
            return new QCoreApplication(argc, argv);
    }
    return new QApplication(argc, argv);
}

int main(int argc, char* argv[])
{
    QScopedPointer<QCoreApplication> app(createApplication(argc, argv));

    if (qobject_cast<QApplication *>(app.data())) {
       // start GUI version...
    } else {
       // start non-GUI version...
    }

    return app->exec();
}

The QApplication object is accessible through the instance() function that returns a pointer equivalent to the global qApp pointer.

QApplication's main areas of responsibility are:

• It initializes the application with the user's desktop settings such as palette(), font() and doubleClickInterval(). It keeps track of these properties in case the user changes the desktop globally, for example through some kind of control panel.
• It performs event handling, meaning that it receives events from the underlying window system and dispatches them to the relevant widgets. By using sendEvent() and postEvent() you can send your own events to widgets.
• It parses common command line arguments and sets its internal state accordingly. See the constructor documentation below for more details.
• It defines the application's look and feel, which is encapsulated in a QStyle object. This can be changed at runtime with setStyle().
• It provides localization of strings that are visible to the user via translate().
• It provides some magical objects like the desktop() and the clipboard().
• It knows about the application's windows. You can ask which widget is at a certain position using widgetAt(), get a list of topLevelWidgets() and closeAllWindows(), etc.
• It manages the application's mouse cursor handling, see setOverrideCursor()

Since the QApplication object does so much initialization, it must be created before any other objects related to the user interface are created. QApplication also deals with common command line arguments. Hence, it is usually a good idea to create it before any interpretation or modification of argv is done in the application itself.

See also QCoreApplication, QAbstractEventDispatcher, QEventLoop, and QSettings.

if ((startPos - currentPos).manhattanLength() >=
        QApplication::startDragDistance())
    startTheDrag();

Member Function Documentation

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

Initializes the window system and constructs an application object with argc command line arguments in argv.

Warning: The data referred to by argc and argv must stay valid for the entire lifetime of the QApplication object. In addition, argc must be greater than zero and argv must contain at least one valid character string.

The global qApp pointer refers to this application object. Only one application object should be created.

This application object must be constructed before any paint devices (including widgets, pixmaps, bitmaps etc.).

Note: argc and argv might be changed as Qt removes command line arguments that it recognizes.

All Qt programs automatically support the following command line options:

• -style= style, sets the application GUI style. Possible values depend on your system configuration. If you compiled Qt with additional styles or have additional styles as plugins these will be available to the -style command line option. You can also set the style for all Qt applications by setting the QT_STYLE_OVERRIDE environment variable.
• -style style, is the same as listed above.
• -stylesheet= stylesheet, sets the application styleSheet. The value must be a path to a file that contains the Style Sheet.

  Note: Relative URLs in the Style Sheet file are relative to the Style Sheet file's path.

• -stylesheet stylesheet, is the same as listed above.
• -widgetcount, prints debug message at the end about number of widgets left undestroyed and maximum number of widgets existed at the same time
• -reverse, sets the application's layout direction to Qt::RightToLeft
• -qmljsdebugger=, activates the QML/JS debugger with a specified port. The value must be of format port:1234[,block], where block is optional and will make the application wait until a debugger connects to it.

See also QCoreApplication::arguments().

[static slot] void QApplication::aboutQt()

Displays a simple message box about Qt. The message includes the version number of Qt being used by the application.

This is useful for inclusion in the Help menu of an application, as shown in the Menus example.

This function is a convenience slot for QMessageBox::aboutQt().

[static slot] void QApplication::closeAllWindows()

Closes all top-level windows.

This function is particularly useful for applications with many top-level windows. It could, for example, be connected to a Exit entry in the File menu:

    const QIcon exitIcon = QIcon::fromTheme("application-exit");
    QAction *exitAct = fileMenu->addAction(exitIcon, tr("E&xit"), qApp, &QApplication::closeAllWindows);
    exitAct->setShortcuts(QKeySequence::Quit);
    exitAct->setStatusTip(tr("Exit the application"));
    fileMenu->addAction(exitAct);

The windows are closed in random order, until one window does not accept the close event. The application quits when the last window was successfully closed; this can be turned off by setting quitOnLastWindowClosed to false.

See also quitOnLastWindowClosed, lastWindowClosed(), QWidget::close(), QWidget::closeEvent(), lastWindowClosed(), QCoreApplication::quit(), topLevelWidgets(), and QWidget::isWindow().

[signal] void QApplication::focusChanged(QWidget *old, QWidget *now)

This signal is emitted when the widget that has keyboard focus changed from old to now, i.e., because the user pressed the tab-key, clicked into a widget or changed the active window. Both old and now can be nullptr.

The signal is emitted after both widget have been notified about the change through QFocusEvent.

This function was introduced in Qt 4.1.

See also QWidget::setFocus(), QWidget::clearFocus(), and Qt::FocusReason.

[virtual] QApplication::~QApplication()

Cleans up any window system resources that were allocated by this application. Sets the global variable qApp to nullptr.

[static] QWidget *QApplication::activeModalWidget()

Returns the active modal widget.

A modal widget is a special top-level widget which is a subclass of QDialog that specifies the modal parameter of the constructor as true. A modal widget must be closed before the user can continue with other parts of the program.

Modal widgets are organized in a stack. This function returns the active modal widget at the top of the stack.

See also activePopupWidget() and topLevelWidgets().

[static] QWidget *QApplication::activePopupWidget()

Returns the active popup widget.

A popup widget is a special top-level widget that sets the Qt::WType_Popup widget flag, e.g. the QMenu widget. When the application opens a popup widget, all events are sent to the popup. Normal widgets and modal widgets cannot be accessed before the popup widget is closed.

Only other popup widgets may be opened when a popup widget is shown. The popup widgets are organized in a stack. This function returns the active popup widget at the top of the stack.

See also activeModalWidget() and topLevelWidgets().

[static] QWidget *QApplication::activeWindow()

Returns the application top-level window that has the keyboard input focus, or nullptr if no application window has the focus. There might be an activeWindow() even if there is no focusWidget(), for example if no widget in that window accepts key events.

See also setActiveWindow(), QWidget::setFocus(), QWidget::hasFocus(), and focusWidget().

[static] void QApplication::alert(QWidget *widget, int msec = 0)

Causes an alert to be shown for widget if the window is not the active window. The alert is shown for msec miliseconds. If msec is zero (the default), then the alert is shown indefinitely until the window becomes active again.

Currently this function does nothing on Qt for Embedded Linux.

On macOS, this works more at the application level and will cause the application icon to bounce in the dock.

On Windows, this causes the window's taskbar entry to flash for a time. If msec is zero, the flashing will stop and the taskbar entry will turn a different color (currently orange).

On X11, this will cause the window to be marked as "demands attention", the window must not be hidden (i.e. not have hide() called on it, but be visible in some sort of way) in order for this to work.

This function was introduced in Qt 4.3.

[static] QWidgetList QApplication::allWidgets()

Returns a list of all the widgets in the application.

The list is empty (QList::isEmpty()) if there are no widgets.

Note: Some of the widgets may be hidden.

Example:

void updateAllWidgets()
{
    const QWidgetList allWidgets = QApplication::allWidgets();
    for (QWidget *widget : allWidgets)
        widget->update();
}

See also topLevelWidgets() and QWidget::isVisible().

[static] void QApplication::beep()

Sounds the bell, using the default volume and sound. The function is not available in Qt for Embedded Linux.

[static] QDesktopWidget *QApplication::desktop()

Returns the desktop widget (also called the root window).

The desktop may be composed of multiple screens, so it would be incorrect, for example, to attempt to center some widget in the desktop's geometry. QDesktopWidget has various functions for obtaining useful geometries upon the desktop, such as QDesktopWidget::screenGeometry() and QDesktopWidget::availableGeometry().

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

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

[static] int QApplication::exec()

Enters the main event loop and waits until exit() is called, then 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.

Generally, no user interaction can take place before calling exec(). As a special case, modal widgets like QMessageBox can be used before calling exec(), because modal widgets call exec() to start a local event loop.

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. This is because, on some platforms the QApplication::exec() call may not return. For example, on the Windows platform, 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 QApplication::exec() call.

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

[static] QWidget *QApplication::focusWidget()

Returns the application widget that has the keyboard input focus, or nullptr if no widget in this application has the focus.

See also QWidget::setFocus(), QWidget::hasFocus(), activeWindow(), and focusChanged().

[static] QFont QApplication::font()

Returns the default application font.

See also setFont(), fontMetrics(), and QWidget::font().

[static] QFont QApplication::font(const QWidget *widget)

This is an overloaded function.

Returns the default font for the widget.

See also fontMetrics() and QWidget::setFont().

[static] QFont QApplication::font(const char *className)

This is an overloaded function.

Returns the font for widgets of the given className.

See also setFont() and QWidget::font().

[static] QFontMetrics QApplication::fontMetrics()

Returns display (screen) font metrics for the application font.

See also font(), setFont(), QWidget::fontMetrics(), and QPainter::fontMetrics().

[static] bool QApplication::isEffectEnabled(Qt::UIEffect effect)

Returns true if effect is enabled; otherwise returns false.

By default, Qt will try to use the desktop settings. To prevent this, call setDesktopSettingsAware(false).

Note: All effects are disabled on screens running at less than 16-bit color depth.

See also setEffectEnabled() and Qt::UIEffect.

[static] Qt::NavigationMode QApplication::navigationMode()

Returns what kind of focus navigation Qt is using.

This feature is available in Qt for Embedded Linux only.

This function was introduced in Qt 4.6.

See also setNavigationMode().

[override virtual] bool QApplication::notify(QObject *receiver, QEvent *e)

Reimplements: QGuiApplication::notify(QObject *object, QEvent *event).

[static] QPalette QApplication::palette(const QWidget *widget)

If a widget is passed, the default palette for the widget's class is returned. This may or may not be the application palette. In most cases there is no special palette for certain types of widgets, but one notable exception is the popup menu under Windows, if the user has defined a special background color for menus in the display settings.

See also setPalette() and QWidget::palette().

[static] QPalette QApplication::palette(const char *className)

This is an overloaded function.

Returns the palette for widgets of the given className.

See also setPalette() and QWidget::palette().

[static] void QApplication::setActiveWindow(QWidget *active)

Sets the active window to the active widget in response to a system event. The function is called from the platform specific event handlers.

Warning: This function does not set the keyboard focus to the active widget. Call QWidget::activateWindow() instead.

It sets the activeWindow() and focusWidget() attributes and sends proper WindowActivate/WindowDeactivate and FocusIn/FocusOut events to all appropriate widgets. The window will then be painted in active state (e.g. cursors in line edits will blink), and it will have tool tips enabled.

See also activeWindow() and QWidget::activateWindow().

[static] void QApplication::setEffectEnabled(Qt::UIEffect effect, bool enable = true)

Enables the UI effect effect if enable is true, otherwise the effect will not be used.

Note: All effects are disabled on screens running at less than 16-bit color depth.

See also isEffectEnabled(), Qt::UIEffect, and setDesktopSettingsAware().

[static] void QApplication::setFont(const QFont &font, const char *className = nullptr)

Changes the default application font to font. If className is passed, the change applies only to classes that inherit className (as reported by QObject::inherits()).

On application start-up, the default font depends on the window system. It can vary depending on both the window system version and the locale. This function lets you override the default font; but overriding may be a bad idea because, for example, some locales need extra large fonts to support their special characters.

Warning: Do not use this function in conjunction with Qt Style Sheets. The font of an application can be customized using the "font" style sheet property. To set a bold font for all QPushButtons, set the application styleSheet() as "QPushButton { font: bold }"

See also font(), fontMetrics(), and QWidget::setFont().

[static] void QApplication::setNavigationMode(Qt::NavigationMode mode)

Sets the kind of focus navigation Qt should use to mode.

This feature is available in Qt for Embedded Linux only.

This function was introduced in Qt 4.6.

See also navigationMode().

[static] void QApplication::setPalette(const QPalette &palette, const char *className = nullptr)

Changes the application palette to palette.

If className is passed, the change applies only to widgets that inherit className (as reported by QObject::inherits()). If className is left 0, the change affects all widgets, thus overriding any previously set class specific palettes.

The palette may be changed according to the current GUI style in QStyle::polish().

Warning: Do not use this function in conjunction with Qt Style Sheets. When using style sheets, the palette of a widget can be customized using the "color", "background-color", "selection-color", "selection-background-color" and "alternate-background-color".

Note: Some styles do not use the palette for all drawing, for instance, if they make use of native theme engines. This is the case for the Windows Vista and macOS styles.

See also QWidget::setPalette(), palette(), and QStyle::polish().

[static] void QApplication::setStyle(QStyle *style)

Sets the application's GUI style to style. Ownership of the style object is transferred to QApplication, so QApplication will delete the style object on application exit or when a new style is set and the old style is still the parent of the application object.

Example usage:

QApplication::setStyle(QStyleFactory::create("Fusion"));

When switching application styles, the color palette is set back to the initial colors or the system defaults. This is necessary since certain styles have to adapt the color palette to be fully style-guide compliant.

Setting the style before a palette has been set, i.e., before creating QApplication, will cause the application to use QStyle::standardPalette() for the palette.

Warning: Qt style sheets are currently not supported for custom QStyle subclasses. We plan to address this in some future release.

See also style(), QStyle, setPalette(), and desktopSettingsAware().

[static] QStyle *QApplication::setStyle(const QString &style)

This is an overloaded function.

Requests a QStyle object for style from the QStyleFactory.

The string must be one of the QStyleFactory::keys(), typically one of "windows", "windowsvista", "fusion", or "macintosh". Style names are case insensitive.

Returns nullptr if an unknown style is passed, otherwise the QStyle object returned is set as the application's GUI style.

Warning: To ensure that the application's style is set correctly, it is best to call this function before the QApplication constructor, if possible.

[static] QStyle *QApplication::style()

Returns the application's style object.

See also setStyle() and QStyle.

[static] QWidget *QApplication::topLevelAt(const QPoint &point)

Returns the top-level widget at the given point; returns nullptr if there is no such widget.

[static] QWidget *QApplication::topLevelAt(int x, int y)

This is an overloaded function.

Returns the top-level widget at the point (x, y); returns 0 if there is no such widget.

[static] QWidgetList QApplication::topLevelWidgets()

Returns a list of the top-level widgets (windows) in the application.

Note: Some of the top-level widgets may be hidden, for example a tooltip if no tooltip is currently shown.

Example:

void showAllHiddenTopLevelWidgets()
{
    const QWidgetList topLevelWidgets = QApplication::topLevelWidgets();
    for (QWidget *widget : topLevelWidgets) {
        if (widget->isHidden())
            widget->show();
    }
}

See also allWidgets(), QWidget::isWindow(), and QWidget::isHidden().

[static] QWidget *QApplication::widgetAt(const QPoint &point)

Returns the widget at global screen position point, or nullptr if there is no Qt widget there.

This function can be slow.

See also QCursor::pos(), QWidget::grabMouse(), and QWidget::grabKeyboard().

[static] QWidget *QApplication::widgetAt(int x, int y)

This is an overloaded function.

Returns the widget at global screen position (x, y), or nullptr if there is no Qt widget there.