Qt Tutorial - Chapter 1: Hello, World!

    #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

qmake -project
qmake