Chapter 2: Calling it Quits
Having created a window in chapter one, we will
now go on to make the application quit properly when the user tells it to.
We will also use a font that is more exciting than the default one.
/****************************************************************
**
** Qt tutorial 2
**
****************************************************************/
#include <qapplication.h>
#include <qpushbutton.h>
#include <qfont.h>
int main( int argc, char **argv )
{
QApplication a( argc, argv );
QPushButton quit( "Quit", 0 );
quit.resize( 75, 30 );
quit.setFont( QFont( "Times", 18, QFont::Bold ) );
QObject::connect( &quit, SIGNAL(clicked()), &a, SLOT(quit()) );
a.setMainWidget( &quit );
quit.show();
return a.exec();
}
Line by Line Walk-Through
#include <qfont.h>
Since this program uses QFont, it needs to include qfont.h. Qt's font
abstraction is rather different from the horror provided by X, and
loading and using fonts has been highly optimized.
QPushButton quit( "Quit", 0 );
This time, the button says "Quit" and that's exactly what the program
will do when the user clicks the button. This is not a coincidence.
We still pass 0 as the parent, since the button is a top-level window.
quit.resize( 75, 30 );
We've chosen another size for the button since the text is a bit
shorter than "Hello world!". We could also have used QFontMetrics
to set right size.
quit.setFont( QFont( "Times", 18, QFont::Bold ) );
Here we choose a new font for the button, an 18-point bold font from
the Times family. Note that we create the font on the spot.
It is also possible to change the default font (using QApplication::setFont()) for the whole application.
QObject::connect( &quit, SIGNAL(clicked()), &a, SLOT(quit()) );
connect() is perhaps the most central feature of Qt.
Note that connect() is a static function in QObject. Do not confuse it
with the connect() function in the socket library.
This line establishes a one-way connection between two Qt objects (objects
that inherit QObject, directly or indirectly). Every Qt object can have
both signals
(to send messages) and slots
(to receive messages). All
widgets are Qt objects. They inherit QWidget which in turn inherits
QObject.
Here, the clicked() signal of quit is connected to the quit() slot of a, so that when the button is clicked, the
application quits.
The Signals and Slots documentation
describes this topic in detail.
Behavior
When you run this program, you will see an even smaller window than in
chapter one, filled with an even smaller button.
Exercises
Try to resize the window. Press the button. Oops! That connect()
would seem to make some difference :)
Are there any other signals in QPushButton you can connect to quit?
Hint: The QPushButton inherits most of its behavior from QButton.
You may now go on to chapter three.
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