The QApplication class manages the application event queue. More...
#include <qapplication.h>
Inherits QObject.
Inherited by QXtApplication.
The QApplication class is central to Qt. It receives events from the underlying window system and sends them to the destination widgets. An application object must be created before any widgets can be created!
Only one single QApplication object should be created. In fact Qt
complains if you create more than one, and this is normally done
in the main() function. Once a QApplication object has been
created, qApp (defined as extern QApplication *qApp)
refers to this object.
Example (a complete Qt application):
#include <qapplication.h> // defines QApplication
#include <qpushbutton.h> // defines QPushButton
int main( int argc, char **argv )
{
QApplication app( argc, argv ); // create application object
QPushButton hello( "Hello, world!" ); // create a push button
app.setMainWidget( &hello ); // define as main widget
connect( &hello, SIGNAL(clicked()), // clicking the button
&app, SLOT(quit()) ); // quits the application
hello.show(); // show button
return app.exec(); // run main event loop
}
Important
Notice that the QApplication object must
be created before any window-system functionality of Qt is used, this
includes widgets, colors, fonts etc.
Note also that for X11, setMainWidget() may change the main widget according to the -geometry option. To preserve this functionality, you must set your defaults before setMainWidget() and any overrides after.
While Qt is not optimized or designed for writing non-GUI programs, it's possible to use some of its classes without creating a QApplication. This can be very useful if you wish to share code between a non-GUI server and a GUI client.
Examples: forever/forever.cpp desktop/desktop.cpp connect/connect.cpp drawdemo/drawdemo.cpp movies/main.cpp picture/picture.cpp xform/xform.cpp menu/menu.cpp progress/progress.cpp cursor/cursor.cpp layout/layout.cpp qmag/qmag.cpp widgets/widgets.cpp
Initializes the window system and onstructs an application object with the command line arguments argc and argv.
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 (includes widgets, pixmaps, bitmaps etc.)
Notice that argc and argv might be changed. Qt removes
command line arguments that it recognizes. argc and argv are
can be accessed later by qApp->argc() and qApp->argv(). The
documentation for argv() contains a detailed description of how to
process command line arguments.
Qt debugging options:
-nograb, tells Qt to never grab the mouse or the keyboard.
-sync (only under X11), switches to synchronous mode for
debugging.
See Debugging Techniques for a more detailed explanation.
The X11 version of Qt supports a few more command line options:
-display display, sets the X display (default is $DISPLAY).
-geometry geometry, sets the client geometry of the
main widget.
-fn or -font font, defines the application font.
-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.
-name name, sets the application name.
-title title, sets the application title (caption).
-style= style, sets the application GUI style. Possible values
are motif and windows
-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 a 8-bit display, if the application is using the
QApplication::ManyColor color specification. If count is
216 then a 6x6x6 color cube is used (ie. 6 levels of red, 6 of green,
and 6 of blue); for 108, a 3x3x3, and 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.
[virtual]Deletes all remaining widgets and cleans up any window system
resources that were allocated by this application. Sets the global
variable qApp to null.
[static]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 to TRUE. A modal widget must be finished before the user can continue with other parts of the program.
The modal widgets are organized in a stack. This function returns the active modal widget on top of the stack.
See also: currentPopupWidget() and topLevelWidgets().
[static]Returns the active popup widget.
A popup widget is a special top level widget that sets the WType_Popup widget flag, e.g. the QPopupMenu widget. When the application opens a popup widget, all events are sent to the popup and 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 on top of the stack.
See also: currentModalWidget() and topLevelWidgets().
[static]Returns a list of all the widgets in the application.
The list is created using new and must be deleted by the caller.
The list is empty if there are no widgets.
Note that some of the widgets may be hidden.
Example:
//
// Updates all widgets.
//
QWidgetList *list = QApplication::allWidgets();
QWidgetListIt it( *list ); // iterate over the widgets
while ( it.current() ) { // for each top level widget...
it.current()->update();
++it;
}
delete list; // delete the list, not the widgets
The QWidgetList class is defined in the qwidcoll.h header file.
Warning: Delete the list away as soon you have finished using it. You can get in serious trouble if you for instance try to access a widget that has been deleted.
See also: topLevelWidgets(), QWidget::isVisible() and QList::isEmpty(),.
Returns the number of command line arguments.
The documentation for argv() contains a detailed description of how to process command line arguments.
See also: argv() and QApplication::QApplication().
Returns the command line argument vector.
argv()[0] is the program name, argv()[1] is the first argument and
argv()[argc()-1] is the last argument.
A QApplication object is constructed by passing argc and argv from
the main() function. Some of the arguments may be recognized as Qt
options removed from the argument vector. For example, the X11
version of Qt knows about -display, -font and a few more options.
Example:
// showargs.cpp - displays program arguments in a list box
#include <qapplication.h>
#include <qlistbox.h>
int main( int argc, char **argv )
{
QApplication a( argc, argv );
QListBox b;
a.setMainWidget( &b );
for ( int i=0; i<a.argc(); i++ ) // a.argc() == argc
b.insertItem( a.argv()[i] ); // a.argv()[i] == argv[i]
b.show();
return a.exec();
}
If you run showargs -display unix:0 -font 9x15bold hello world under X11, the list box contains the three strings "showargs", "hello" and "world".
See also: argc() and QApplication::QApplication().
[static]Sounds the bell, using the default volume and sound.
[static]Returns a pointer to the application global clipboard.
[static]Returns TRUE if the application objects are being destroyed.
See also: startingUp().
[static]Returns the color specification.
See also: QApplication::setColorSpec().
Examples: showimg/showimg.cpp
[static]Returns the desktop widget (also called the root window).
The desktop widget is useful for obtaining the size of the screen. It can also be used to draw on the desktop.
QWidget *d = QApplication::desktop();
int w=d->width(); // returns screen width
int h=d->height(); // returns screen height
d->setBackgroundColor( red ); // makes desktop red
Examples: qmag/qmag.cpp
[static]Returns the maximum duration for a double click.
See also: setDoubleClickInterval().
This function enters the main event loop (recursively). Do not call it unless you are an expert.
See also: exit_loop().
Enters the main event loop and waits until exit() is called or the main widget is destroyed. Returns the value that was specified 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 widget have 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 by using processEvents() and processOneEvent().
See also: quit(), exit(), processEvents(), setMainWidget() and QTimer.
Examples: forever/forever.cpp connect/connect.cpp mainlyQt/editor.cpp drawdemo/drawdemo.cpp movies/main.cpp picture/picture.cpp xform/xform.cpp menu/menu.cpp progress/progress.cpp cursor/cursor.cpp layout/layout.cpp qmag/qmag.cpp widgets/widgets.cpp
[static]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 retcode.
By convention, retcode 0 means success, any non-zero value indicates an error.
Note that unlike the C library exit function, this function does returns to the caller - it is event processing that stops.
Examples: picture/picture.cpp
This function leaves from a recursive call to the main event loop. Do not call it unless you are an expert.
See also: enter_loop().
[static]Flushes the X event queue in the X11 implementation. Does nothing on other platforms.
See also: syncX().
Returns the application widget that has the keyboard input focus, or null if no application widget has the focus.
See also: QWidget::setFocus() and QWidget::hasFocus().
[static]Returns the default application font. There is always an application font, i.e. the returned pointer is guaranteed to be non-null.
See also: setFont(), fontMetrics() and QWidget::font().
[static]Returns display (screen) font metrics for the application font.
See also: font(), setFont(), QWidget::fontMetrics() and QPainter::fontMetrics().
[static]Returns TRUE if global mouse tracking is enabled, otherwise FALSE.
See also: setGlobalMouseTracking().
[signal]This signal is emitted when the user has closed a top level widget and there are no more visible top level widgets left.
The signal is very useful when your application has many top level widgets but no main widget. You can then connect it to the quit() slot.
See also: mainWidget(), topLevelWidgets() and QWidget::isTopLevel().
Returns the main application widget, or 0 if there is not a defined main widget.
See also: setMainWidget().
[virtual]Sends event to receiver: receiver->event( event )
Returns the value that is returned from the receiver's event handler.
Reimplementing this virtual function is one of five ways to process an event:
See also: QObject::event() and installEventFilter().
[static]Returns the active application override cursor.
This function returns 0 if no application cursor has been defined (i.e. the internal cursor stack is empty).
See also: setOverrideCursor() and restoreOverrideCursor().
[static]Returns a pointer to the default application palette. There is always an application palette, i.e. the returned pointer is guaranteed to be non-null.
See also: setPalette() and QWidget::palette().
[static]Stores the event in a queue and returns immediatly.
The event must be allocated on the heap, as it is deleted when the event 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.
See also: sendEvent().
Processes pending events, for 3 seconds or until there are no more events to process, then return.
You can call this function occasionally when your program is busy doing a long operation (e.g. copying a file).
See also: processEvents(), exec() and QTimer.
Processes pending events, for maxtime milliseconds or until there are no more events to process, then return.
You can call this function occasionally when you program is busy doing a long operation (e.g. copying a file).
See also: processOneEvent(), exec() and QTimer.
Waits for an event to occur, processes it, then returns.
This function is useful for adapting Qt to situations where the event processing must be grafted into existing program loops. Beware that using this function in new applications may be an indication of design problems.
See also: processEvents(), exec() and QTimer.
[slot]Tells the application to exit with return code 0 (success). Equivalent to calling QApplication::exit( 0 ).
This function is a slot, i.e. you may connect any signal to activate quit().
Example:
QPushButton *quitButton = new QPushButton( "Quit" );
connect( quitButton, SIGNAL(clicked()), qApp, SLOT(quit()) );
See also: exit().
[static]Restores the effect of setOverrideCursor().
If setOverrideCursor() has been called twice, calling restoreOverrideCursor() will activate the first cursor set. Calling this function a second time restores the original widgets cursors.
Application cursors are stored on an internal stack. setOverrideCursor() pushes the cursor onto the stack, and restoreOverrideCursor() pops the active cursor off the stack. Every setOverrideCursor() must have an corresponding restoreOverrideCursor(), otherwise the stack will get out of sync. overrideCursor() returns 0 if the cursor stack is empty.
See also: setOverrideCursor() and overrideCursor().
Examples: showimg/showimg.cpp
[static]Sends an event directly to a receiver, using the notify() function. Returns the value that was returned from the event handler.
See also: postEvent() and notify().
[static]Immediately dispatches all events which have been previously enqueued with QApplication::postEvent() and which are for the object receiver and have the event_type.
Some event compression may occur. Note that events from the window system are not dispatched by this function.
[static]Sets the color specification for the application to spec.
The color specification controls how your application allocates colors. You must set the color specification before you create the QApplication object.
The choices are:
QApplication::NormalColor.
This is the default color allocation strategy.
Use this choice if your application uses buttons, menus,
texts and pixmaps with few colors.
With this choice, the application allocates system global colors.
This work fine for most applications under X11, but Windows dithers to
the 20 standard colors unless the display has true color support (more
than 256 colors).
QApplication::CustomColor.
Use this choice if your application needs a small number of
custom colors. This choice only makes a difference on Windows
- the application gets more colors when it is active, but the
background windows look less good.
Under X11 this is the same as NormalColor. Under Windows, Qt creates a Windows palette if the display
supports 256 colors.
QApplication::ManyColor.
Use this choice if your application is very color hungry
(e.g. it wants thousands of colors).
Under Windows, this is equal to CustomColor.
Under X11 the effect is:
Example:
int main( int argc, char **argv )
{
QApplication::setColorSpec( QApplication::ManyColor );
QApplication a( argc, argv );
...
}
QColor provides more functionality for controlling color allocation and freeing up certains colors. See QColor::enterAllocContext() for more information.
To see what mode you end up with, you can call QColor::numBitPlanes() once the QApplication object exists. A value greater than 8 (typically 16, 24 or 32) means true color.
The color cube used by Qt are all those colors with red, green, and blue components of either 0x00, 0x33, 0x66, 0x99, 0xCC, or 0xFF.
See also: colorSpec(), QColor::numBitPlanes() and QColor::enterAllocContext().
Examples: widgets/widgets.cpp
[static]Sets the time limit that distinguishes a double click from two consecutive mouse clicks to ms milliseconds. This value is ignored under Windows (the control panel value is used.)
The default value is 400 milliseconds.
See also: doubleClickInterval().
[static]Changes the default application font to font.
The default font depends on the X server in use.
If updateAllWidgets is TRUE, then the font of all existing widgets is set to font.
Widgets created after this call get font as their font.
See also: font(), fontMetrics() and QWidget::setFont().
Examples: desktop/desktop.cpp movies/main.cpp
[static]Enables global mouse tracking if enable is TRUE or disables it if enable is FALSE.
Enabling global mouse tracking makes it possible for widget event filters or application event filters to get all mouse move events, even when no button is depressed. This is useful for special GUI elements, e.g. tool tips.
Global mouse tracking does not affect widgets and their mouseMoveEvent(). For a widget to get mouse move events when no button is depressed, it must do QWidget::setMouseTracking(TRUE).
This function has an internal counter. Each setGlobalMouseTracking(TRUE) must have a corresponding setGlobalMouseTracking(FALSE).
See also: hasGlobalMouseTracking() and QWidget::hasMouseTracking().
Sets the main widget of the application.
The special thing about the main widget is that destroying the main widget (i.e. the program calls QWidget::close() or the user double-clicks the window close box) will leave the main event loop and exit the application.
For X11, this function also resizes and moves the main widget according to the -geometry command-line option, so you should set the default geometry before calling setMainWidget().
See also: mainWidget(), exec() and quit().
Examples: forever/forever.cpp connect/connect.cpp mainlyQt/editor.cpp drawdemo/drawdemo.cpp picture/picture.cpp xform/xform.cpp menu/menu.cpp progress/progress.cpp cursor/cursor.cpp layout/layout.cpp qmag/qmag.cpp widgets/widgets.cpp
[static]Sets the application override cursor to cursor.
Application override cursor are intended for showing the user that the application is in a special state, for example during an operation that might take some time.
This cursor will be displayed in all application widgets until restoreOverrideCursor() or another setOverrideCursor() is called.
Application cursors are stored on an internal stack. setOverrideCursor() pushes the cursor onto the stack, and restoreOverrideCursor() pops the active cursor off the stack. Every setOverrideCursor() must have an corresponding restoreOverrideCursor(), otherwise the stack will get out of sync. overrideCursor() returns 0 if the cursor stack is empty.
If replace is TRUE, the new cursor will replace the last override cursor.
Example:
QApplication::setOverrideCursor( waitCursor );
calculateHugeMandelbrot(); // lunch time...
QApplication::restoreOverrideCursor();
See also: overrideCursor(), restoreOverrideCursor() and QWidget::setCursor().
Examples: showimg/showimg.cpp
[static]Changes the default application palette to palette.
If updateAllWidgets is TRUE, then the palette of all existing widgets is set to palette.
Widgets created after this call get palette as their palette.
See also: QWidget::setPalette() and palette().
Examples: xform/xform.cpp
[static]Sets the application GUI style to style.
The style parameter can be WindowsStyle or MotifStyle.
See also: style() and QWidget::setStyle().
[static]Sets the color used to mark selections in windows style for all widgets in the application. Will repaint all widgets if the color is changed.
The default color is darkBlue.
See also: winStyleHighlightColor().
Examples: widgets/widgets.cpp
[static]Returns TRUE if an application object has not been created yet.
See also: closingDown().
[static]Returns the GUI style of the application.
See also: setStyle().
[static]Synchronizes with the X server in the X11 implementation. Does nothing on other platforms.
See also: flushX().
[static]Returns a list of the top level widgets in the application.
The list is created using new and must be deleted by the caller.
The list is empty if there are no top level widgets.
Note that some of the top level widgets may be hidden.
Example:
//
// Shows all hidden top level widgets.
//
QWidgetList *list = QApplication::topLevelWidgets();
QWidgetListIt it( *list ); // iterate over the widgets
while ( it.current() ) { // for each top level widget...
if ( !it.current()->isVisible() )
it.current()->show();
++it;
}
delete list; // delete the list, not the widgets
The QWidgetList class is defined in the qwidcoll.h header file.
Warning: Delete the list away as soon you have finished using it. You can get in serious trouble if you for instance try to access a widget that has been deleted.
See also: allWidgets(), QWidget::isTopLevel(), QWidget::isVisible() and QList::isEmpty().
[static]Returns a pointer to the widget at global screen position (x,y), or a null pointer if there is no Qt widget there.
If child is FALSE and there is a child widget at position (x,y), the top-level widget containing it is returned. If child is TRUE the child widget at position (x,y) is returned.
See also: QCursor::pos(), QWidget::grabMouse() and QWidget::grabKeyboard().
[static]This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
[static]Returns the color used to mark selections in windows style.
See also: setWinStyleHighlightColor().
Adds a global routine that will be called from the QApplication destructor. This function is normally used to add cleanup routines.
CleanUpFunctions is defined as typedef void
(*CleanUpFunction)(); , i.e. a pointer to a function that
takes no arguments and returns nothing.
Example of use:
static int *global_ptr = 0;
void cleanup_ptr()
{
delete [] global_ptr;
}
void init_ptr()
{
global_ptr = new int[100]; // allocate data
qAddPostRoutine( cleanup_ptr ); // delete later
}
Returns the Qt version number for the library, typically "1.30" or "1.31".
Obtains information about the system.
The system's word size in bits (typically 32) is returned in *wordSize. The *bigEndian is set to TRUE if this is a big-endian machine, or to FALSE if this is a little-endian machine.
This function calls fatal() with a message if the computer is truely weird (i.e. different endianness for 16 bit and 32 bit integers).
Prints a debug message, or calls the message handler (if it has been installed).
This function takes a format string and a stack arguments, similar to the C printf() function.
Example:
debug( "my window handle = %x", myWidget->id() );
Under X11, the text is printed to stderr. Under Windows, the text is sent to the debugger.
Warning: The internal buffer is limited to 512 bytes (including the 0-terminator.
See also: warning(), fatal(), qInstallMsgHandler() and Debugging
Prints a warning message, or calls the message handler (if it has been installed).
This function takes a format string and a stack arguments, similar to the C printf() function.
Example:
void f( int c )
{
if ( c > 200 )
warning( "f: bad argument, c == %d", c );
}
Under X11, the text is printed to stderr. Under Windows, the text is sent to the debugger.
Warning: The internal buffer is limited to 512 bytes (including the 0-terminator.
See also: debug(), fatal(), qInstallMsgHandler() and Debugging
Examples: progress/progress.cpp
Prints a fatal error message and exits, or calls the message handler (if it has been installed).
This function takes a format string and a stack arguments, similar to the C printf() function.
Example:
int divide( int a, int b )
{
if ( b == 0 ) // program error
fatal( "divide: cannot divide by zero" );
return a/b;
}
Under X11, the text is printed to stderr. Under Windows, the text is sent to the debugger.
Warning: The internal buffer is limited to 512 bytes (including the 0-terminator.
See also: debug(), warning(), qInstallMsgHandler() and Debugging
Prints a warning message containing the source code file name and line number if test is FALSE.
This is really a macro defined in qglobal.h.
ASSERT is useful for testing required conditions in your program.
Example:
//
// File: div.cpp
//
#include <qglobal.h>
int divide( int a, int b )
{
ASSERT( b != 0 ); // this is line 9
return a/b;
}
If b is zero, the ASSERT statement will output the following message
using the warning() function:
ASSERT: "b == 0" in div.cpp (9)
See also: warning() and Debugging
If p is null, a fatal messages says that the program ran out of memory and exits. If p is not null, nothing happens.
This is really a macro defined in qglobal.h.
Warning: CHECK_PTR only works for the development release of the Qt library. In the release library, CHECK_PTR will be substituted with nothing.
Example:
int *a;
CHECK_PTR( a = new int[80] ); // never do this!
// do this instead:
a = new int[80];
CHECK_PTR( a ); // this is fine
See also: fatal() and Debugging
Installs a Qt message handler. Returns a pointer to the message handler previously defined.
The message handler is a function that prints out debug messages, warnings and fatal error messages. The Qt library (debug version) contains hundreds of warning messages that are printed when internal errors (usually invalid function arguments) occur. If you implement your own message handler, you get total control of these messages.
The default message handler prints the message to the standard output under X11 or to the debugger under Windows. If it is a fatal message, the application aborts immediately.
Only one message handler can be defined, since this is usually done on an application-wide basis to control debug output.
To restore the message handler, call qInstallMsgHandler(0).
Example:
#include <qapplication.h>
#include <stdio.h>
#include <stdlib.h>
void myMessageOutput( QtMsgType type, const char *msg )
{
switch ( type ) {
case QtDebugMsg:
fprintf( stderr, "Debug: %s\n", msg );
break;
case QtWarningMsg:
fprintf( stderr, "Warning: %s\n", msg );
break;
case QtFatalMsg:
fprintf( stderr, "Fatal: %s\n", msg );
abort(); // dump core on purpose
}
}
int main( int argc, char **argv )
{
qInstallMsgHandler( myMessageOutput );
QApplication a( argc, argv );
...
return a.exec();
}
See also: debug(), warning(), fatal() and Debugging
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