Qt Tutorial 1 - Hello World!

Line by Line Walkthrough

 #include <QApplication>

This line includes the QApplication class definition. There has to be exactly one QApplication object in every GUI application that uses Qt. QApplication manages various application-wide resources, such as the default font and cursor.

 #include <QPushButton>

This line includes the QPushButton class definition. For each class that's part of the public Qt API, there exists a header file of the same name that contains its definition.

QPushButton is a 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 QIcon.

 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.

The argc parameter is the number of command-line arguments and argv is the array of command-line arguments. This is a standard C++ feature.

     QApplication app(argc, argv);

The app object is this program's QApplication instance. Here it is created. We pass argc and argv to the QApplication constructor so that it can process certain standard command-line arguments (such as -display under X11). All command-line arguments recognized by Qt are removed from argv, and argc is decremented accordingly. See the QApplication::arguments() documentation for details.

The QApplication object must be created before any GUI-related features of Qt are used.

     QPushButton hello("Hello world!");

Here, after the QApplication, comes the first GUI-related code: A push button is created.

The button is set up to display the text "Hello world!". Because we don't specify a parent window (as second argument to the QPushButton constructor), the button will be a window of its own, with its own window frame and title bar.

The button's size is determined by its default size hint. We could call QWidget::move() to assign a specific screen position to the widget, but instead we let the windowing system choose a position.

     hello.show();

A widget is never visible when you create it. You must call QWidget::show() to make it visible.

     return app.exec();
 }

This is where main() passes control to Qt. QCoreApplication::exec() will return when the application exits. (QCoreApplication is QApplication's base class. It implements QApplication's core, non-GUI functionality and can be used when developing non-GUI applications.)

In QCoreApplication::exec(), Qt receives and processes user and system events and passes these on to the appropriate widgets.

You should now compile and run this program.

 qmake -project
 qmake