QApplication Class Reference
[QtGui module]

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.

Qt debugging options (not available if Qt was compiled without the QT_DEBUG flag defined):

• -nograb, tells Qt that it must never grab the mouse or the keyboard.
• -dograb (only under X11), running under a debugger can cause an implicit -nograb, use -dograb to override.
• -sync (only under X11), switches to synchronous mode for debugging.

See Debugging Techniques for a more detailed explanation.

All Qt programs automatically support the following command line options:

• -style= style, sets the application GUI style. Possible values are motif, windows, and platinum. If you compiled Qt with additional styles or have additional styles as plugins these will be available to the -style command line option.
• -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.
• -session= session, restores the application from an earlier session.
• -session session, 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
• -graphicssystem, sets the backend to be used for on-screen widgets and QPixmaps. Available options are raster and opengl.

The X11 version of Qt supports some traditional X11 command line options:

• -display display, sets the X display (default is $DISPLAY).
• -geometry geometry, sets the client geometry of the first window that is shown.
• -fn or -font font, defines the application font. The font should be specified using an X logical font description.
• -bg or -background color, sets the default background color and an application palette (light and dark shades are calculated).
• -fg or -foreground color, sets the default foreground color.
• -btn or -button color, sets the default button color.
• -name name, sets the application name.
• -title title, sets the application title.
• -visual TrueColor, forces the application to use a TrueColor visual on an 8-bit display.
• -ncols count, limits the number of colors allocated in the color cube on an 8-bit display, if the application is using the QApplication::ManyColor color specification. If count is 216 then a 6x6x6 color cube is used (i.e. 6 levels of red, 6 of green, and 6 of blue); for other values, a cube approximately proportional to a 2x3x1 cube is used.
• -cmap, causes the application to install a private color map on an 8-bit display.
• -im, sets the input method server (equivalent to setting the XMODIFIERS environment variable)
• -inputstyle, defines how the input is inserted into the given widget, e.g., onTheSpot makes the input appear directly in the widget, while overTheSpot makes the input appear in a box floating over the widget and is not inserted until the editing is done.

See also arguments().

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

Constructs an application object with argc command line arguments in argv. If GUIenabled is true, a GUI application is constructed, otherwise a non-GUI (console) application is created.

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.

Set GUIenabled to false for programs without a graphical user interface that should be able to run without a window system.

On X11, the window system is initialized if GUIenabled is true. If GUIenabled is false, the application does not connect to the X server. On Windows and Mac OS, currently the window system is always initialized, regardless of the value of GUIenabled. This may change in future versions of Qt.

The following example shows how to create an application that uses a graphical interface when available.

 int main(int argc, char **argv)
 {
 #ifdef Q_WS_X11
     bool useGUI = getenv("DISPLAY") != 0;
 #else
     bool useGUI = true;
 #endif
     QApplication app(argc, argv, useGUI);

     if (useGUI) {
        // start GUI version
        ...
     } else {
        // start non-GUI version
        ...
     }
     return app.exec();
 }

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

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.

With Qt for Embedded Linux, passing QApplication::GuiServer for type makes this application the server (equivalent to running with the -qws option).

QApplication::QApplication ( Display * display, Qt::HANDLE visual = 0, Qt::HANDLE colormap = 0 )

Creates an application, given an already open display display. If visual and colormap are non-zero, the application will use those values as the default Visual and Colormap contexts.

Warning: Qt only supports TrueColor visuals at depths higher than 8 bits-per-pixel.

This function is only available on X11.

QApplication::QApplication ( Display * display, int & argc, char ** argv, Qt::HANDLE visual = 0, Qt::HANDLE colormap = 0 )

Creates an application, given an already open display and using argc command line arguments in argv. If visual and colormap are non-zero, the application will use those values as the default Visual and Colormap contexts.

Warning: Qt only supports TrueColor visuals at depths higher than 8 bits-per-pixel.

This function is only available on X11.

QApplication::QApplication ( QApplication::QS60MainApplicationFactory factory, int & argc, char ** argv )

Creates an application using the application factory given in factory, and using argc command line arguments in argv. factory can be leaving, but the error will be converted to a standard exception.

This function is only available on S60.

This function was introduced in Qt 4.6.

 void updateAllWidgets()
 {
     foreach (QWidget *widget, QApplication::allWidgets())
         widget->update();
 }

     exitAct = new QAction(tr("E&xit"), this);
     exitAct->setShortcuts(QKeySequence::Quit);
     exitAct->setStatusTip(tr("Exit the application"));
     connect(exitAct, SIGNAL(triggered()), qApp, SLOT(closeAllWindows()));

 int main(int argc, char *argv[])
 {
     QApplication::setColorSpec(QApplication::ManyColor);
     QApplication app(argc, argv);
     ...
     return app.exec();
 }

 int main(int argc, char *argv[])
 {
     QApplication::setDesktopSettingsAware(false);
     QApplication app(argc, argv);
     ...
     return app.exec();
 }

 QApplication::setOverrideCursor(QCursor(Qt::WaitCursor));
 calculateHugeMandelbrot();              // lunch time...
 QApplication::restoreOverrideCursor();

 QApplication::setStyle(new QWindowsStyle);

 void showAllHiddenTopLevelWidgets()
 {
     foreach (QWidget *widget, QApplication::topLevelWidgets()) {
         if (widget->isHidden())
             widget->show();
     }
 }