Qt Tutorial - Chapter 1: Hello, World!Jump
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This first program is a simple hello-world example. It contains only
the bare minimum you need to get a Qt application up and running. The
picture above is a snapshot of this program.
/****************************************************************
**
** Qt tutorial 1
**
****************************************************************/
#include <qapplication.h>
#include <qpushbutton.h>
int main( int argc, char **argv )
{
QApplication a( argc, argv );
QPushButton hello( "Hello world!", 0 );
hello.resize( 100, 30 );
a.setMainWidget( &hello );
hello.show();
return a.exec();
}
Line-by-line Walkthrough
#include <qapplication.h>
This line includes the QApplication
class definition. There has to be exactly one QApplication object in every
application that uses Qt. QApplication manages various application-wide
resources, such as the default font and cursor.
#include <qpushbutton.h>
This line includes the QPushButton
class definition. The reference
documentation for each class mentions at the top which file needs to
be included to use that class.
QPushButton is a classical GUI push button that the user can press and
release. It manages its own look and feel, like every other QWidget. A
widget is a user interface object that can process user input and draw
graphics. The programmer can change both the overall look
and feel and many minor properties of it (such as color), as well as
the widget's content. A QPushButton can show either a text or a QPixmap.
int main( int argc, char **argv )
{
The main() function is the entry point to the program. Almost always
when using Qt, main() only needs to perform some kind of initialization
before passing the control to the Qt library, which then tells the program
about the user's actions via events.
argc is the number of command-line arguments and argv
is the array of command-line arguments. This is a C/C++ feature. It is not
specific to Qt; however, Qt needs to process these arguments (see
following).
QApplication a( argc, argv );
a is this program's QApplication.
Here it is created and processes some of the command-line arguments (such
as -display under X Window). Note that all command-line arguments
recognized by Qt are removed from argv (and argc is
decremented accordingly). See the QApplication::argv()
documentation for details.
Note: It is essential that the QApplication object be
created before any window-system parts of Qt are used.
QPushButton hello( "Hello world!", 0 );
Here, after the QApplication, comes the first window-system
code: A push button is created.
The button is set up to display the text "Hello world!" and be a window
of its own (because the constructor specifies 0 for the parent window,
inside which the button should be located).
hello.resize( 100, 30 );
The button is set up to be 100 pixels wide and 30 pixels high (plus the
window system frame). In this case we don't care about the button's
position, and we accept the default value.
a.setMainWidget( &hello );
The push button is chosen as the main widget for the application. If
the user closes a main widget, the application exits.
You don't have to have a main widget, but most programs do have one.
hello.show();
A widget is never visible when you create it. You must call show() to
make it visible.
return a.exec();
This is where main() passes control to Qt, and exec() will return when
the application exits.
In exec(), Qt receives and processes user and system events and passes
these on to the appropriate widgets.
}
You should now try to compile and run this program.
Compiling
To compile a C++ application you need to create a makefile. The easiest
way to create a makefile for Qt is to use the qmake
build tool supplied with Qt. If you've saved main.cpp in its own
directory, all you have to do is: qmake -project
qmake
The first command tells qmake
to create a .pro (project) file. The second command tells it to
create a (platform-specific) makefile based on the project file. You
should now be able to type make (or nmake if you're
using Visual Studio) and then run your first Qt application!
Behavior
When you run it, you will see a small window filled with a single
button, and on it you can read the famous words, Hello World!
Exercises
Try to resize the window. Press the button. If you're running X Window,
try running the program with the -geometry option (for example,
-geometry 100x200+10+20).
You're now ready for Chapter
2.
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