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<title>TQt Tutorial - Chapter 7: One Thing Leads to Another</title>
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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>TQt Tutorial - Chapter 7: One Thing Leads to Another</h1>
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<p> <center><img src="t7.png" alt="Screenshot of tutorial seven"></center>
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<p> This example shows how to create custom widgets with signals and
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slots, and how to connect them together in more complex ways. For the
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first time, the source is split among several files which we've placed
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in the <tt>t7</tt> subdirectory.
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<p> <ul>
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<li> <a href="t7-lcdrange-h.html">t7/lcdrange.h</a> contains the LCDRange class definition.
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<li> <a href="t7-lcdrange-cpp.html">t7/lcdrange.cpp</a> contains the LCDRange implementation.
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<li> <a href="t7-main-cpp.html">t7/main.cpp</a> contains MyWidget and main.
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</ul>
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<p> <h2> Line-by-line Walkthrough
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</h2>
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<a name="1"></a><p> <h3> <a href="t7-lcdrange-h.html">t7/lcdrange.h</a>
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</h3>
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<a name="1-1"></a><p> This file is mainly lifted from <a href="tutorial1-06.html#main">main.cpp</a> in
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Chapter 6; only the changes are noted here.
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<p>
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<p> <pre> #ifndef LCDRANGE_H
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#define LCDRANGE_H
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</pre>
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<p> This is the classic C construction to avoid errors if a header file
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happens to be included more than once. If you don't use it already,
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it is a very good habit to develop. The #ifndef should enclose <em>all</em> of the
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header file.
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<p> <pre> #include <<a href="qvbox-h.html">ntqvbox.h</a>>
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</pre>
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<p> <a href="qvbox-h.html">ntqvbox.h</a> is included. LCDRange inherits <a href="ntqvbox.html">TQVBox</a>, and the header file
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of a parent class must always be included. We cheated a bit in the
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previous chapters, and we let <a href="qwidget-h.html">ntqwidget.h</a> be included indirectly via
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other header files such as <a href="qpushbutton-h.html">ntqpushbutton.h</a>.
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<p> <pre> class TQSlider;
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</pre>
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<p> This is another classic trick, but one that's much less used often. Because
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we don't need <a href="ntqslider.html">TQSlider</a> in the <em>interface</em> of the class, only in the
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implementation, we use a forward declaration of the class in the
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header file and include the header file for TQSlider in the .cpp
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file.
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<p> This makes the compilation of big projects much faster, because when a
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header file has changed, fewer files need to be recompiled. It can
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often speed up big compilations by a factor of two or more.
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<p> <pre> class LCDRange : public <a href="ntqvbox.html">TQVBox</a>
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{
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<a href="metaobjects.html#TQ_OBJECT">TQ_OBJECT</a>
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public:
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LCDRange( <a href="ntqwidget.html">TQWidget</a> *parent=0, const char *name=0 );
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</pre>
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<p> Note the TQ_OBJECT. This macro must be included in <em>all</em> classes that
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contain signals and/or slots. If you are curious, it defines the
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functions that are implemented in the
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<a href="metaobjects.html">meta object file</a>.
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<p> <pre> int value() const;
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public slots:
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void setValue( int );
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signals:
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void valueChanged( int );
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</pre>
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<p> These three members make up an interface between this widget and other
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components in a program. Until now, LCDRange didn't really have an
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interface at all.
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<p> value() is a public function for accessing the value of the LCDRange.
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setValue() is our first custom slot and valueChanged() is our first
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custom signal.
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<p> Slots must be implemented in the normal way (remember that a slot is also
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a C++ member function). Signals are automatically implemented in the
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<a href="signalsandslots.html">meta object</a> file. Signals follow the
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access rules of protected C++ functions (i.e., they can be emitted only
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by the class they are defined in or by classes inheriting from it).
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<p> The signal valueChanged() is used when the LCDRange's value has
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changed - just as you guessed from the name. This is not the last
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signal you'll see called <i>something</i>Changed().
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<p> <h3> <a href="t7-lcdrange-cpp.html">t7/lcdrange.cpp</a>
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</h3>
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<a name="1-2"></a><p>
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<p> This file is mainly lifted from <a href="tutorial1-06.html#main">t6/main.cpp</a>, and
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only the changes are noted here.
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<p> <pre> <a name="x2333"></a> <a href="ntqobject.html#connect">connect</a>( slider, TQ_SIGNAL(<a href="ntqslider.html#valueChanged">valueChanged</a>(int)),
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<a name="x2330"></a> lcd, TQ_SLOT(<a href="ntqlcdnumber.html#display">display</a>(int)) );
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<a href="ntqobject.html#connect">connect</a>( slider, TQ_SIGNAL(<a href="ntqslider.html#valueChanged">valueChanged</a>(int)),
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TQ_SIGNAL(valueChanged(int)) );
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</pre>
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<p> This code is from the LCDRange constructor.
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<p> The first connect is the same that you have seen in the previous chapter.
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The second is new; it connects slider's valueChanged() signal to this
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object's valueChanged <em>signal</em>. Connect() with 3 arguments always
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connects to signals or slots in <tt>this</tt> object.
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<p> Yes, that's right. Signals can be connected to other signals. When
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the first is emitted, the second signal is also emitted.
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<p> Let's look at what happens when the user operates the slider. The
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slider sees that its value has changed and emits the valueChanged()
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signal. That signal is connected both to the display() slot of the
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<a href="ntqlcdnumber.html">TQLCDNumber</a> and to the valueChanged() signal of the LCDRange.
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<p> Thus, when the signal is emitted, LCDRange emits its own
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valueChanged() signal. In addition, <a href="ntqlcdnumber.html#display">TQLCDNumber::display</a>() is called
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and shows the new number.
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<p> Note that you're not guaranteed any particular order of execution -
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LCDRange::valueChanged() may be emitted before or after
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TQLCDNumber::display()and is entirely arbitrary.
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<p> <pre> int LCDRange::value() const
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{
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<a name="x2332"></a> return slider-><a href="ntqslider.html#value">value</a>();
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}
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</pre>
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<p> The implementation of value() is straightforward; it simply returns
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the slider's value.
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<p> <pre> void LCDRange::setValue( int value )
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{
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<a name="x2331"></a> slider-><a href="ntqslider.html#setValue">setValue</a>( value );
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}
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</pre>
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<p> The implementation of setValue() is equally straightforward. Note
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that because the slider and LCD number are connected, setting the
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slider's value automatically updates the LCD number as well. In
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addition, the slider will automatically adjust the value if it is
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outside its legal range.
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<p> <h3> <a href="t7-main-cpp.html">t7/main.cpp</a>
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</h3>
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<a name="1-3"></a><p>
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<p> <pre> LCDRange *previous = 0;
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for( int r = 0 ; r < 4 ; r++ ) {
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for( int c = 0 ; c < 4 ; c++ ) {
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LCDRange* lr = new LCDRange( grid );
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if ( previous )
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<a href="ntqobject.html#connect">connect</a>( lr, TQ_SIGNAL(valueChanged(int)),
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previous, TQ_SLOT(setValue(int)) );
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previous = lr;
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}
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}
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</pre>
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<p> All of main.cpp is copied from the previous chapter except in
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the constructor for MyWidget. When we create the 16 LCDRange object, we
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now connect them using the <a href="signalsandslots.html">signal/slot</a> mechanism. Each has its valueChanged() signal
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connected to the setValue() slot in the previous one. Because LCDRange
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emits the signal valueChanged() when its value changes (surprise!), we
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are here creating a "chain" of signals and slots.
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<p> <a name="compiling"></a>
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<h2> Compiling
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</h2>
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<a name="2"></a><p> Creating a makefile for a multi-file application is no different from
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creating one for a single-file application. If you've saved all the
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files in this example in their own directory, all you have to do is:
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<pre>
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qmake -project
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qmake
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</pre>
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<p> The first command tells <a href="qmake-manual.html">qmake</a> to
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create a <tt>.pro</tt> (project) file. The second command tells it to create
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a (platform-specific) makefile based on the project file. You should
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now be able to type <tt>make</tt> (or <tt>nmake</tt> if you're using Visual
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Studio) to build your application.
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<p> <h2> Behavior
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</h2>
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<a name="3"></a><p> On startup, the program's appearance is identical to the previous one.
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Try operating the slider to the bottom right...
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<p> <h2> Exercises
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</h2>
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<a name="4"></a><p> Use the bottom right slider to set all LCDs to 50. Then set the top
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half to 40 by clicking once to the left of the slider handle. Now,
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use the one to the left of the last one operated to set the first
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seven LCDs back to 50.
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<p> Click to the left of the handle on the bottom right slider. What
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happens? Why is this the correct behavior?
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<p> You're now ready for <a href="tutorial1-08.html">Chapter 8.</a>
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<p> [<a href="tutorial1-06.html">Previous tutorial</a>]
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[<a href="tutorial1-08.html">Next tutorial</a>]
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[<a href="tutorial.html">Main tutorial page</a>]
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<p>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>TQt 3.3.8</div>
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