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<title>Qt Tutorial - Chapter 4: Let There Be Widgets</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>Qt Tutorial - Chapter 4: Let There Be Widgets</h1>
<p> <center><img src="t4.png" alt="Screenshot of tutorial four"></center>
<p> This example shows how to create your own widget, describes how to control the
minimum and maximum sizes of a widget, and introduces widget names.
<p> <pre>/****************************************************************
**
** Qt tutorial 4
**
****************************************************************/
#include <<a href="qapplication-h.html">qapplication.h</a>>
#include <<a href="qpushbutton-h.html">qpushbutton.h</a>>
#include <<a href="qfont-h.html">qfont.h</a>>
class MyWidget : public <a href="qwidget.html">QWidget</a>
{
public:
MyWidget( <a href="qwidget.html">QWidget</a> *parent=0, const char *name=0 );
};
<a name="f552"></a>MyWidget::MyWidget( <a href="qwidget.html">QWidget</a> *parent, const char *name )
: <a href="qwidget.html">QWidget</a>( parent, name )
{
<a href="qwidget.html#setMinimumSize">setMinimumSize</a>( 200, 120 );
<a href="qwidget.html#setMaximumSize">setMaximumSize</a>( 200, 120 );
<a href="qpushbutton.html">QPushButton</a> *quit = new <a href="qpushbutton.html">QPushButton</a>( "Quit", this, "quit" );
quit-><a href="qwidget.html#setGeometry">setGeometry</a>( 62, 40, 75, 30 );
quit-><a href="qwidget.html#setFont">setFont</a>( QFont( "Times", 18, QFont::Bold ) );
<a href="qobject.html#connect">connect</a>( quit, SIGNAL(<a href="qbutton.html#clicked">clicked</a>()), qApp, SLOT(<a href="qapplication.html#quit">quit</a>()) );
}
int main( int argc, char **argv )
{
<a href="qapplication.html">QApplication</a> a( argc, argv );
MyWidget w;
w.<a href="qwidget.html#setGeometry">setGeometry</a>( 100, 100, 200, 120 );
a.<a href="qapplication.html#setMainWidget">setMainWidget</a>( &w );
w.<a href="qwidget.html#show">show</a>();
return a.<a href="qapplication.html#exec">exec</a>();
}
</pre>
<p> <h2> Line-by-line Walkthrough
</h2>
<a name="1"></a><p> <pre> class MyWidget : public <a href="qwidget.html">QWidget</a>
{
public:
MyWidget( <a href="qwidget.html">QWidget</a> *parent=0, const char *name=0 );
};
</pre>
<p> Here we create a new class. Because this class inherits from <a href="qwidget.html">QWidget</a>,
the new class is a widget and may be a top level window or a child
widget (like the push button in Chapter 3).
<p> This class has only one member, a constructor (in addition to the
members it inherits from QWidget). The constructor is a standard Qt
widget constructor; you should always include a similar constructor
when you create widgets.
<p> The first argument is its parent widget. To create a top-level window
you specify a null pointer as the parent. As you can see, this widget
defaults to be a top-level window.
<p> The second argument is the widget's name. This is <em>not</em> the text
that appears in the window's title bar or in the button. It is a name
associated with a widget to make it possible to <a href="qobject.html#queryList">look up</a> this widget later, and there is
also a <a href="qobject.html#dumpObjectTree">handy debugging function</a> that will list a complete widget hierarchy.
<p> <pre> MyWidget::MyWidget( <a href="qwidget.html">QWidget</a> *parent, const char *name )
: <a href="qwidget.html">QWidget</a>( parent, name )
</pre>
<p> The implementation of the constructor starts here. Like most widgets,
it just passes on the <tt>parent</tt> and <tt>name</tt> to the <a href="qwidget.html">QWidget</a>
constructor.
<p> <pre> {
<a href="qwidget.html#setMinimumSize">setMinimumSize</a>( 200, 120 );
<a href="qwidget.html#setMaximumSize">setMaximumSize</a>( 200, 120 );
</pre>
<p> Because this widget doesn't know how to handle resizing, we fix its size
by setting the minimum and maximum to be equal. In the next chapter
we will show how a widget can respond to resize event from the user.
<p> <pre> <a href="qpushbutton.html">QPushButton</a> *quit = new <a href="qpushbutton.html">QPushButton</a>( "Quit", this, "quit" );
<a name="x2308"></a> quit-><a href="qwidget.html#setGeometry">setGeometry</a>( 62, 40, 75, 30 );
<a name="x2307"></a> quit-><a href="qwidget.html#setFont">setFont</a>( QFont( "Times", 18, QFont::Bold ) );
</pre>
<p> Here we create and set up a child widget of this widget (the new widget's
parent is <tt>this</tt>) which has the widget name "quit". The widget
name has nothing to do with the button text; it just happens to be
similar in this case.
<p> Note that <tt>quit</tt> is a local variable in the constructor. MyWidget
does not keep track of it, but Qt does, and will by default delete it
when MyWidget is deleted. This is why MyWidget doesn't need a
destructor. (On the other hand, there is no harm in deleting a child
when you choose to, the child will automatically tell Qt about its
imminent death.)
<p> The setGeometry() call does the same as move() and resize() did in the
previous chapters.
<p> <pre> <a name="x2306"></a><a name="x2304"></a> <a href="qobject.html#connect">connect</a>( quit, SIGNAL(<a href="qbutton.html#clicked">clicked</a>()), qApp, SLOT(<a href="qapplication.html#quit">quit</a>()) );
}
</pre>
<p> Because the MyWidget class doesn't know about the application object, it
has to connect to Qt's pointer to it, <tt>qApp</tt>.
<p> A widget is a software component and should know as little as possible
about its environment in order to be as general and reusable as
possible.
<p> Knowing the name of the application object would break this principle,
so Qt offers an alias, qApp, for the cases in which a component such as
MyWidget needs to talk to the application object.
<p> <pre> int main( int argc, char **argv )
{
<a href="qapplication.html">QApplication</a> a( argc, argv );
MyWidget w;
w.<a href="qwidget.html#setGeometry">setGeometry</a>( 100, 100, 200, 120 );
<a name="x2305"></a> a.<a href="qapplication.html#setMainWidget">setMainWidget</a>( &w );
<a name="x2309"></a> w.<a href="qwidget.html#show">show</a>();
<a name="x2303"></a> return a.<a href="qapplication.html#exec">exec</a>();
}
</pre>
<p> Here we instantiate our new child, set it to be the main widget, and
execute the application.
<p> <h2> Behavior
</h2>
<a name="2"></a><p> This program is very similar in behavior to the previous one. The
difference lies in the way we have implemented it. It does behave
slightly differently, however. Just try to resize it to see.
<p> (See <a href="tutorial1-01.html#compiling">Compiling</a> for how to create a
makefile and build the application.)
<p> <h2> Exercises
</h2>
<a name="3"></a><p> Try to create another MyWidget object in main(). What happens?
<p> Try to add more buttons or put in widgets other than <a href="qpushbutton.html">QPushButton</a>.
<p> You're now ready for <a href="tutorial1-05.html">Chapter 5.</a>
<p> [<a href="tutorial1-03.html">Previous tutorial</a>]
[<a href="tutorial1-05.html">Next tutorial</a>]
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