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+/****************************************************************************
+**
+** Explanation of the Qt object model
+**
+** Copyright (C) 2000-2008 Trolltech ASA.  All rights reserved.
+**
+** This file is part of the Qt GUI Toolkit.
+**
+** This file may be used under the terms of the GNU General
+** Public License versions 2.0 or 3.0 as published by the Free
+** Software Foundation and appearing in the files LICENSE.GPL2
+** and LICENSE.GPL3 included in the packaging of this file.
+** Alternatively you may (at your option) use any later version
+** of the GNU General Public License if such license has been
+** publicly approved by Trolltech ASA (or its successors, if any)
+** and the KDE Free Qt Foundation.
+**
+** Please review the following information to ensure GNU General
+** Public Licensing retquirements will be met:
+** http://trolltech.com/products/qt/licenses/licensing/opensource/.
+** If you are unsure which license is appropriate for your use, please
+** review the following information:
+** http://trolltech.com/products/qt/licenses/licensing/licensingoverview
+** or contact the sales department at sales@trolltech.com.
+**
+** This file may be used under the terms of the Q Public License as
+** defined by Trolltech ASA and appearing in the file LICENSE.QPL
+** included in the packaging of this file.  Licensees holding valid Qt
+** Commercial licenses may use this file in accordance with the Qt
+** Commercial License Agreement provided with the Software.
+**
+** This file is provided "AS IS" with NO WARRANTY OF ANY KIND,
+** INCLUDING THE WARRANTIES OF DESIGN, MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE. Trolltech reserves all rights not granted
+** herein.
+**
+**********************************************************************/
+
+/*!
+\page object.html
+
+\title Qt Object Model
+
+The standard C++ Object Model provides very efficient runtime support
+for the object paradigm. But the C++ Object Model's static nature is
+inflexibile in certain problem domains. Graphical User Interface
+programming is a domain that retquires both runtime efficiency and a
+high level of flexibility. Qt provides this, by combining the speed of
+C++ with the flexibility of the Qt Object Model.
+
+Qt adds these features to C++:
+
+\list
+\i a very powerful mechanism for seamless object
+  communication called \link signalsandslots.html signals and
+  slots \endlink;
+\i queryable and designable \link properties.html object
+  properties \endlink;
+\i powerful \link eventsandfilters.html events and event filters \endlink,
+\i contextual \link i18n.html string translation for internationalization \endlink;
+\i sophisticated interval driven \link timers.html timers \endlink
+  that make it possible to elegantly integrate many tasks in an
+  event-driven GUI;
+\i hierarchical and queryable \link objecttrees.html object
+  trees \endlink that organize object ownership in a natural way;
+\i guarded pointers, \l QGuardedPtr, that are automatically
+  set to 0 when the referenced object is destroyed, unlike normal C++
+  pointers which become "dangling pointers" when their objects are destroyed.
+\endlist
+
+Many of these Qt features are implemented with standard C++
+techniques, based on inheritance from \l QObject. Others, like the
+object communication mechanism and the dynamic property system,
+retquire the \link metaobjects.html Meta Object System \endlink provided
+by Qt's own \link moc.html Meta Object Compiler (moc) \endlink. 
+
+The Meta Object System is a C++ extension that makes the language
+better suited to true component GUI programming. Although templates can
+be used to extend C++, the Meta Object System provides benefits using
+standard C++ that cannot be achieved with templates; see \link
+templates.html Why doesn't Qt use templates for signals and
+slots? \endlink.
+
+*/
+
+
+
+
+/*!
+\page timers.html
+
+\title Timers
+
+\l QObject, the base class of all Qt objects, provides the basic timer
+support in Qt. With \l QObject::startTimer(), you start a timer with
+an \e interval in milliseconds as argument. The function returns a
+unique integer timer id. The timer will now "fire" every \e interval
+milliseconds, until you explicitly call \l QObject::killTimer() with
+the timer id.
+
+For this mechanism to work, the application must run in an event
+loop. You start an event loop with \l QApplication::exec(). When a
+timer fires, the application sends a QTimerEvent, and the flow of
+control leaves the event loop until the timer event is processed. This
+implies that a timer cannot fire while your application is busy doing
+something else. In other words: the accuracy of timers depends on the
+granularity of your application.
+
+There is practically no upper limit for the interval value (more than
+one year is possible). The accuracy depends on the underlying operating
+system. Windows 95/98 has 55 millisecond (18.2 times per second)
+accuracy; other systems that we have tested (UNIX X11 and Windows NT)
+can handle 1 millisecond intervals.
+
+The main API for the timer functionality is \l QTimer. That class
+provides regular timers that emit a signal when the timer fires, and
+inherits \l QObject so that it fits well into the ownership structure
+of most GUI programs. The normal way of using it is like this:
+\code
+    QTimer * counter = new QTimer( this );
+    connect( counter, SIGNAL(timeout()),
+	     this, SLOT(updateCaption()) );
+    counter->start( 1000 );
+\endcode
+
+The counter timer is made into a child of this widget, so that when
+this widget is deleted, the timer is deleted too. Next, its timeout
+signal is connected to the slot that will do the work, and finally
+it's started.
+
+QTimer also provides a simple one-shot timer API. \l QButton uses this
+to show the button being pressed down and then (0.1 seconds later) be
+released when the keyboard is used to "press" a button, for example:
+
+\code
+    QTimer::singleShot( 100, this, SLOT(animateTimeout()) );
+\endcode
+
+0.1 seconds after this line of code is executed, the same button's
+animateTimeout() slot is called.
+
+Here is an outline of a slightly longer example that combines object
+communication via signals and slots with a QTimer object. It
+demonstrates how to use timers to perform intensive calculations in a
+single-threaded application without blocking the user interface.
+
+  \code
+    // The Mandelbrot class uses a QTimer to calculate the mandelbrot
+    // set one scanline at a time without blocking the CPU. It
+    // inherits QObject to use signals and slots. Calling start()
+    // starts the calculation. The done() signal is emitted when it
+    // has finished. Note that this example is not complete, just an
+    // outline.
+
+    class Mandelbrot : public QObject
+    {
+	Q_OBJECT // retquired for signals/slots
+    public:
+	Mandelbrot( QObject *parent=0, const char *name );
+	...
+    public slots:
+	void start();
+    signals:
+	void done();
+    private slots:
+	void calculate();
+	private:
+	QTimer timer;
+	...
+    };
+
+    //
+    // Constructs and initializes a Mandelbrot object.
+    //
+
+    Mandelbrot::Mandelbrot( QObject *parent=0, const char *name )
+    : QObject( parent, name )
+    {
+	connect( &timer, SIGNAL(timeout()), SLOT(calculate()) );
+	...
+    }
+
+    //
+    // Starts the calculation task. The internal calculate() slot
+    // will be activated every 10 milliseconds.
+    //
+
+    void Mandelbrot::start()
+    {
+	if ( !timer.isActive() ) // not already running
+	    timer.start( 10 );   // timeout every 10 ms
+    }
+
+    //
+    // Calculates one scanline at a time.
+    // Emits the done() signal when finished.
+    //
+
+    void Mandelbrot::calculate()
+    {
+	...                // perform the calculation for a scanline
+	if ( finished ) {  // no more scanlines
+	    timer.stop();
+	    emit done();
+	}
+    }
+  \endcode
+
+
+*/
+
+
+/*!
+\page properties.html
+
+\title Properties
+
+Qt provides a sophisticated property system similar to those supplied
+by some compiler vendors. However, as a compiler- and
+platform-independent library, Qt cannot rely on non-standard compiler
+features like \c __property or \c [property]. Our solution works with
+\e any standard C++ compiler on every platform we support. It's based
+on the meta-object system that also provides object communication
+through \link signalsandslots.html signals and slots\endlink.
+
+The \c Q_PROPERTY macro in a class declaration declares a
+property. Properties can only be declared in classes that inherit \l
+QObject. A second macro, \c Q_OVERRIDE, can be used to override some
+aspects of an inherited property in a subclass. (See \link #override
+Q_OVERRIDE\endlink.)
+
+To the outer world, a property appears to be similar to a data member.
+But properties have several features that distinguish them from
+ordinary data members:
+
+\list
+\i A read function. This always exists.
+
+\i A write function. This is optional: read-only properties like \l
+QWidget::isDesktop() do not have one.
+
+\i An attribute "stored" that indicates persistence. Most properties
+are stored, but a few virtual properties are not. For example, \l
+QWidget::minimumWidth() isn't stored, since it's just a view of
+\l QWidget::minimumSize(), and has no data of its own.
+
+\i A reset function to set a property back to its context specific
+default value. This is very rare, but for example, \l QWidget::font()
+needs this, since no call to \l QWidget::setFont() can mean 'reset to
+the context specific font'.
+
+\i An attribute "designable" that indicates whether it makes sense to
+make the property available in a GUI builder (e.g. \link
+designer-manual.book Qt Designer\endlink). For most properties this 
+makes sense, but not for all, e.g. \l QButton::isDown(). The user can
+press buttons, and the application programmer can make the program
+press its own buttons, but a GUI design tool can't press buttons.
+
+\endlist
+
+The read, write, and reset functions must be public member functions
+from the class in which the property is defined.
+
+Properties can be read and written through generic functions in
+QObject without knowing anything about the class in use. These two
+function calls are equivalent:
+
+\code
+    // QButton *b and QObject *o point to the same button
+    b->setDown( TRUE );
+    o->setProperty( "down", TRUE );
+\endcode
+
+Equivalent, that is, except that the first is faster, and provides
+much better diagnostics at compile time. When practical, the first is
+better. However, since you can get a list of all available properties
+for any QObject through its \l QMetaObject, \l QObject::setProperty()
+can give you control over classes that weren't available at compile
+time.
+
+As well as QObject::setProperty(), there is a corresponding \l
+QObject::property() function. \l QMetaObject::propertyNames() returns
+the names of all available properties. \l QMetaObject::property()
+returns the property data for a named property: a \l QMetaProperty
+object.
+
+Here's a simple example that shows the most important property
+functions in use:
+
+\code
+    class MyClass : public QObject
+    {
+        Q_OBJECT
+    public:
+        MyClass( QObject * parent=0, const char * name=0 );
+        ~MyClass();
+
+        enum Priority { High, Low, VeryHigh, VeryLow };
+        void setPriority( Priority );
+        Priority priority() const;
+    };
+\endcode
+
+The class has a property "priority" that is not yet known to the meta
+object system. In order to make the property known, you must
+declare it with the \c Q_PROPERTY macro. The syntax is as follows:
+
+\code
+Q_PROPERTY( type name READ getFunction [WRITE setFunction]
+            [RESET resetFunction] [DESIGNABLE bool] 
+            [SCRIPTABLE bool] [STORED bool] )
+\endcode
+
+For the declaration to be valid, the get function must be const and
+to return either the type itself, a pointer to it, or a reference to
+it. The optional write function must return void and must take exactly
+one argument, either the type itself, a pointer or a const reference
+to it. The meta object compiler enforces this.
+
+The type of a property can be any \l QVariant supported type or an
+enumeration type declared in the class itself. Since \c MyClass uses
+the enumeration type \c Priority for the property, this type must be
+registered with the property system as well.
+
+There are two exceptions to the above: The type of a property can also
+be either \link QValueList QValueList\<QVariant\>\endlink or \link
+QMap QMap\<QString,QVariant\>\endlink. In
+these cases the type must be specified as \c QValueList or as \c QMap
+(i.e. without their template parameters).
+
+It is possible to set a value by name, like this:
+\code
+    obj->setProperty( "priority", "VeryHigh" );
+\endcode
+In the case of \c QValueList and \c QMap properties the value passes
+is a QVariant whose value is the entire list or map.
+
+Enumeration types are registered with the \c Q_ENUMS macro. Here's the
+final class declaration including the property related declarations:
+
+\code
+    class MyClass : public QObject
+    {
+        Q_OBJECT
+        Q_PROPERTY( Priority priority READ priority WRITE setPriority )
+        Q_ENUMS( Priority )
+    public:
+        MyClass( QObject * parent=0, const char * name=0 );
+        ~MyClass();
+
+        enum Priority { High, Low, VeryHigh, VeryLow };
+        void setPriority( Priority );
+        Priority priority() const;
+    };
+\endcode
+
+Another similar macro is \c Q_SETS. Like \c Q_ENUMS, it registers an
+enumeration type but marks it in addition as a "set", i.e. the
+enumeration values can be OR-ed together. An I/O class might have
+enumeration values "Read" and "Write" and accept "Read|Write": such an
+enum is best handled with \c Q_SETS, rather than \c Q_ENUMS.
+
+The remaining keywords in the \c Q_PROPERTY section are \c RESET, \c
+DESIGNABLE, \c SCRIPTABLE and \c STORED.
+
+\c RESET names a function that will set the property to its default
+state (which may have changed since initialization). The function
+must return void and take no arguments.
+
+\c DESIGNABLE declares whether this property is suitable for
+modification by a GUI design tool. The default is \c TRUE for
+writable properties; otherwise \c FALSE. Instead of \c TRUE or \c
+FALSE, you can specify a boolean member function.
+
+\c SCRIPTABLE declares whether this property is suited for access by a
+scripting engine. The default is \c TRUE. Instead of \c TRUE or \c FALSE,
+you can specify a boolean member function.
+
+\c STORED declares whether the property's value must be remembered
+when storing an object's state. Stored makes only sense for writable
+properties. The default value is \c TRUE. Technically superfluous
+properties (like QPoint pos if QRect geometry is already a property)
+define this to be \c FALSE.
+
+
+Connected to the property system is an additional macro, "Q_CLASSINFO",
+that can be used to attach additional name/value-pairs to a class'
+meta object, for example:
+
+\code
+    Q_CLASSINFO( "Version", "3.0.0" )
+\endcode
+
+Like other meta data, class information is accessible at runtime
+through the meta object, see \l QMetaObject::classInfo() for details.
+
+\target override
+\section1 Q_OVERRIDE
+
+When you inherit a QObject subclass you may wish to override some
+aspects of some of the class's properties.
+
+For example, in QWidget we have the autoMask property defined like
+this:
+\code
+    Q_PROPERTY( bool autoMask READ autoMask WRITE setAutoMask DESIGNABLE false SCRIPTABLE false )
+\endcode
+
+But we need to make the auto mask property designable in some QWidget
+subclasses. Similarly some classes will need this property to be
+scriptable (e.g. for QSA). This is achieved by overriding these
+features of the property in a subclass. In QCheckBox, for example, we
+achieve this using the following code:
+\code
+    Q_OVERRIDE( bool autoMask DESIGNABLE true SCRIPTABLE true )
+\endcode
+
+Another example is QToolButton. By default QToolButton has a read-only
+"toggleButton" property, because that's what it inherits from QButton:
+\code
+    Q_PROPERTY( bool toggleButton READ isToggleButton )
+\endcode
+
+But we want to make our tool buttons able to be toggled, so we write a
+WRITE function in QToolButton, and use the following property override
+to make it acessible:
+\code
+    Q_OVERRIDE( bool toggleButton WRITE setToggleButton )
+\endcode
+The result is read-write (and scriptable and designable, since we now
+have a WRITE function) boolean property "toggleButton" for tool
+buttons.
+
+*/
+
+/*!
+\page eventsandfilters.html
+
+\title Events and Event Filters
+
+In Qt, an event is an object that inherits \l QEvent. Events are
+delivered to objects that inherit \l QObject through calling \l
+QObject::event(). Event delivery means that an event has occurred, the
+QEvent indicates precisely what, and the QObject needs to respond. Most
+events are specific to \l QWidget and its subclasses, but there are
+important events that aren't related to graphics, for example, socket
+activation, which is the event used by \l QSocketNotifier for its
+work.
+
+Some events come from the window system, e.g. \l QMouseEvent, some
+from other sources, e.g. \l QTimerEvent, and some come from the
+application program. Qt is symmetric, as usual, so you can send
+events in exactly the same ways as Qt's own event loop does.
+
+Most events types have special classes, most commonly \l QResizeEvent,
+\l QPaintEvent, \l QMouseEvent, \l QKeyEvent and \l QCloseEvent.
+There are many others, perhaps forty or so, but most are rather odd.
+
+Each class subclasses QEvent and adds event-specific functions; see,
+for example, \l QResizeEvent. In the case of QResizeEvent, \l
+QResizeEvent::size() and \l QResizeEvent::oldSize() are added.
+
+Some classes support more than one event type. \l QMouseEvent
+supports mouse moves, presses, shift-presses, drags, clicks,
+right-presses, etc.
+
+Since programs need to react in varied and complex ways, Qt's
+event delivery mechanisms are flexible. The documentation for
+\l QApplication::notify() concisely tells the whole story, here we
+will explain enough for 99% of applications.
+
+The normal way for an event to be delivered is by calling a virtual
+function. For example, \l QPaintEvent is delivered by calling \l
+QWidget::paintEvent(). This virtual function is responsible for
+reacting appropriately, normally by repainting the widget. If you
+do not perform all the necessary work in your implementation of the
+virtual function, you may need to call the base class's
+implementation; for example:
+\code
+    MyTable::contentsMouseMoveEvent( QMouseEvent *me )
+    {
+	// my implementation
+
+	QTable::contentsMouseMoveEvent( me ); // hand it on
+    }
+\endcode
+If you want to replace the base class's function then you must
+implement everything yourself; but if you only want to extend the base
+class's functionality, then you implement what you want and then call
+the base class.
+
+Occasionally there isn't such an event-specific function, or the
+event-specific function isn't sufficient. The most common example is
+tab key presses. Normally, those are interpreted by QWidget to move
+the keyboard focus, but a few widgets need the tab key for themselves.
+
+These objects can reimplement \l QObject::event(), the general event
+handler, and either do their event handling before or after the usual
+handling, or replace it completely. A very unusual widget that both
+interprets tab and has an application-specific custom event might
+contain:
+
+\code
+  bool MyClass:event( QEvent *evt ) {
+      if ( evt->type() == QEvent::KeyPress ) {
+	  QKeyEvent *ke = (QKeyEvent *)evt;
+	  if ( ke->key() == Key_Tab ) {
+	      // special tab handling here
+	      ke->accept();
+	      return TRUE;
+	  }
+      } else if ( evt->type() >= QEvent::User ) {
+	  QCustomEvent *ce = (QCustomEvent*) evt;
+	  // custom event handling here
+	  return TRUE;
+      }
+      return QWidget::event( evt );
+  }
+\endcode
+
+More commonly, an object needs to look at another's events. Qt
+supports this using \l QObject::installEventFilter() (and the
+corresponding remove). For example, dialogs commonly want to filter
+key presses for some widgets, e.g. to modify Return-key handling.
+
+An event filter gets to process events before the target object does.
+The filter's \l QObject::eventFilter() implementation is called, and
+can accept or reject the filter, and allow or deny further processing
+of the event. If all the event filters allow further processing of an
+event, the event is sent to the target object itself. If one of them
+stops processing, the target and any later event filters don't get to
+see the event at all.
+
+It's also possible to filter \e all events for the entire application,
+by installing an event filter on \l QApplication. This is what \l
+QToolTip does in order to see \e all the mouse and keyboard activity.
+This is very powerful, but it also slows down event delivery of every
+single event in the entire application, so it's best avoided.
+
+The global event filters are called before the object-specific
+filters.
+
+Finally, many applications want to create and send their own events.
+
+Creating an event of a built-in type is very simple: create an object
+of the relevant type, and then call \l QApplication::sendEvent() or \l
+QApplication::postEvent().
+
+sendEvent() processes the event immediately - when sendEvent()
+returns, (the event filters and) the object have already processed the
+event. For many event classes there is a function called isAccepted()
+that tells you whether the event was accepted or rejected by the last
+handler that was called.
+
+postEvent() posts the event on a queue for later dispatch. The next
+time Qt's main event loop runs, it dispatches all posted events, with
+some optimization. For example, if there are several resize events,
+they are are compacted into one. The same applies to paint events: \l
+QWidget::update() calls postEvent(), which minimizes flickering and
+increases speed by avoiding multiple repaints.
+
+postEvent() is also often used during object initialization, since the
+posted event will typically be dispatched very soon after the
+initialization of the object is complete.
+
+To create events of a custom type, you need to define an event number,
+which must be greater than \c QEvent::User, and probably you also need
+to subclass \l QCustomEvent in order to pass characteristics about
+your custom event. See the documentation to \l QCustomEvent for
+details.
+
+*/
+
+
+/*!
+\page objecttrees.html
+
+\title Object Trees and Object Ownership
+
+\link QObject QObjects\endlink organize themselves in object trees.
+When you create a QObject with another object as parent, it's added to
+the parent's \link QObject::children() children() \endlink list, and
+is deleted when the parent is. It turns out that this approach fits
+the needs of GUI objects very well. For example, a \l QAccel (keyboard
+accelerator) is a child of the relevant window, so when the user closes
+that window, the accelerator is deleted too.
+
+The static function \l QObject::objectTrees() provides access to all
+the root objects that currently exist.
+
+\l QWidget, the base class of everything that appears on the screen,
+extends the parent-child relationship. A child normally also becomes a
+child widget, i.e. it is displayed in its parent's coordinate system
+and is graphically clipped by its parent's boundaries. For example,
+when the an application deletes a message box after it has been
+closed, the message box's buttons and label are also deleted, just as
+we'd want, because the buttons and label are children of the message
+box.
+
+You can also delete child objects yourself, and they will remove
+themselves from their parents. For example, when the user removes a
+toolbar it may lead to the application deleting one of its \l QToolBar
+objects, in which case the tool bar's \l QMainWindow parent would
+detect the change and reconfigure its screen space accordingly.
+
+The debugging functions \l QObject::dumpObjectTree() and \l
+QObject::dumpObjectInfo() are often useful when an application looks or
+acts strangely.
+
+*/
+
+
+/*!
+\page templates.html
+
+\title Why doesn't Qt use templates for signals and slots?
+
+A simple answer is that when Qt was designed, it was not possible to
+fully exploit the template mechanism in multi-platform applications due
+to the inadequacies of various compilers. Even today, many widely used
+C++ compilers have problems with advanced templates. For example, you
+cannot safely rely on partial template instantiation, which is essential
+for some non-trivial problem domains. Thus Qt's usage of templates has
+to be rather conservative. Keep in mind that Qt is a multi-platform
+toolkit, and progress on the Linux/g++ platform does not necessarily
+improve the situation elsewhere.
+
+Eventually those compilers with weak template implementations will
+improve. But even if all our users had access to a fully standards
+compliant modern C++ compiler with excellent template support, we would
+not abandon the string-based approach used by our meta object compiler.
+Here are five reasons why:
+
+<h3>1. Syntax matters</h3>
+
+Syntax isn't just sugar: the syntax we use to express our algorithms can
+significantly affect the readability and maintainability of our code.
+The syntax used for Qt's signals and slots has proved very successful in
+practice. The syntax is intuitive, simple to use and easy to read.
+People learning Qt find the syntax helps them understand and utilize the
+signals and slots concept -- despite its highly abstract and generic
+nature. Furthermore, declaring signals in class definitions ensures that
+the signals are protected in the sense of protected C++ member
+functions. This helps programmers get their design right from the very
+beginning, without even having to think about design patterns.
+
+<h3>2. Precompilers are good</h3>
+
+Qt's <tt>moc</tt> (Meta Object Compiler) provides a clean way to go
+beyond the compiled language's facilities. It does so by generating
+additional C++ code which can be compiled by any standard C++ compiler.
+The <tt>moc</tt> reads C++ source files. If it finds one or more class
+declarations that contain the "Q_OBJECT" macro, it produces another C++
+source file which contains the meta object code for those classes. The
+C++ source file generated by the <tt>moc</tt> must be compiled and
+linked with the implementation of the class (or it can be
+<tt>#included</tt> into the class's source file). Typically <tt>moc</tt>
+is not called manually, but automatically by the build system, so it
+retquires no additional effort by the programmer.
+
+There are other precompilers, for example, <tt>rpc</tt> and
+<tt>idl</tt>, that enable programs or objects to communicate over
+process or machine boundaries. The alternatives to precompilers are
+hacked compilers, proprietary languages or graphical programming tools
+with dialogs or wizards that generate obscure code. Rather than locking
+our customers into a proprietary C++ compiler or into a particular
+Integrated Development Environment, we enable them to use whatever tools
+they prefer. Instead of forcing programmers to add generated code into
+source repositories, we encourage them to add our tools to their build
+system: cleaner, safer and more in the spirit of UNIX.
+
+
+<h3>3. Flexibility is king</h3>
+
+C++ is a standarized, powerful and elaborate general-purpose language.
+It's the only language that is exploited on such a wide range of
+software projects, spanning every kind of application from entire
+operating systems, database servers and high end graphics
+applications to common desktop applications. One of the keys to C++'s
+success is its scalable language design that focuses on maximum
+performance and minimal memory consumption whilst still maintaining
+ANSI-C compatibility.
+
+For all these advantages, there are some downsides. For C++, the static
+object model is a clear disadvantage over the dynamic messaging approach
+of Objective C when it comes to component-based graphical user interface
+programming. What's good for a high end database server or an operating
+system isn't necessarily the right design choice for a GUI frontend.
+With <tt>moc</tt>, we have turned this disadvantage into an advantage,
+and added the flexibility retquired to meet the challenge of safe and
+efficient graphical user interface programming.
+
+Our approach goes far beyond anything you can do with templates. For
+example, we can have object properties. And we can have overloaded
+signals and slots, which feels natural when programming in a language
+where overloads are a key concept. Our signals add zero bytes to the
+size of a class instance, which means we can add new signals without
+breaking binary compatibility. Because we do not rely on excessive
+inlining as done with templates, we can keep the code size smaller.
+Adding new connections just expands to a simple function call rather
+than a complex template function. 
+
+Another benefit is that we can explore an object's signals and slots at
+runtime. We can establish connections using type-safe call-by-name,
+without having to know the exact types of the objects we are connecting.
+This is impossible with a template based solution. This kind of runtime
+introspection opens up new possibilities, for example GUIs that are
+generated and connected from Qt Designer's XML <tt>ui</tt> files. 
+
+<h3>4. Calling performance is not everything</h3>
+
+Qt's signals and slots implementation is not as fast as a template-based
+solution. While emitting a signal is approximately the cost of four
+ordinary function calls with common template implementations, Qt
+retquires effort comparable to about ten function calls. This is not
+surprising since the Qt mechanism includes a generic marshaller,
+introspection and ultimately scriptability. It does not rely on
+excessive inlining and code expansion and it provides unmatched runtime
+safety. Qt's iterators are safe while those of faster template-based
+systems are not. Even during the process of emitting a signal to several
+receivers, those receivers can be deleted safely without your program
+crashing. Without this safety, your application would eventually crash
+with a difficult to debug free'd memory read or write error.
+
+Nonetheless, couldn't a template-based solution improve the performance
+of an application using signals and slots? While it is true that Qt adds
+a small overhead to the cost of calling a slot through a signal, the
+cost of the call is only a small proportion of the entire cost of a
+slot. Benchmarking against Qt's signals and slots system is typically
+done with empty slots. As soon as you do anything useful in your slots,
+for example a few simple string operations, the calling overhead becomes
+negligible. Qt's system is so optimized that anything that retquires
+operator new or delete (for example, string operations or
+inserting/removing something from a template container) is significantly
+more expensive than emitting a signal.
+
+Aside: If you have a signals and slots connection in a tight inner loop
+of a performance critical task and you identify this connection as the
+bottleneck, think about using the standard listener-interface pattern
+rather than signals and slots. In cases where this occurs, you probably
+only retquire a 1:1 connection anyway. For example, if you have an object
+that downloads data from the network, it's a perfectly sensible design
+to use a signal to indicate that the requested data arrived. But if you
+need to send out every single byte one by one to a consumer, use a
+listener interface rather than signals and slots.
+
+<h3>5. No limits</h3>
+
+Because we had the <tt>moc</tt> for signals and slots, we could add
+other useful things to it that could not not be done with templates.
+Among these are scoped translations via a generated <tt>tr()</tt>
+function, and an advanced property system with introspection and
+extended runtime type information. The property system alone is a great
+advantage: a powerful and generic user interface design tool like Qt
+Designer would be a lot harder to write - if not impossible - without a
+powerful and introspective property system.
+
+C++ with the <tt>moc</tt> preprocessor essentially gives us the
+flexibility of Objective-C or of a Java Runtime Environment, while
+maintaining C++'s unique performance and scalability advantages. It is
+what makes Qt the flexible and comfortable tool we have today.
+
+*/