Qt Tutorial 3 - Family Values

Line by Line Walkthrough

 #include <QWidget>

We add an include of <QWidget> to get the base widget class we'll use.

     QWidget window;

Here we simply create a plain widget object. The QWidget class is the base class of all user interface objects. The widget is the atom of the user interface: It receives mouse, keyboard and other events from the window system, and paints a representation of itself on the screen. A widget is clipped by its parent and by the widgets in front of it.

A widget that isn't embedded in a parent widget, like this particular widget, is called a window. Usually, windows have their own window frame and taskbar entry, provided by the window system. A widget without a parent widget is always an independent window. Its initial position on the screen is controlled by the window system.

     window.resize(200, 120);

We set the window's width to 200 pixels and its height to 120 pixels.

     QPushButton quit("Quit", &window);

A child is born. This QPushButton is created with a parent widget (window). A child widget is always displayed within its parent's area. When displayed, it is clipped by its parent's bounds. By default, it is rooted at the top-left corner of its parent, at position (0, 0).

     quit.setGeometry(10, 40, 180, 40);

The QWidget::setGeometry() function takes four arguments: The first two arguments are the x and y coordinates of the button's top-left corner. The coordinates are relative to the parent widget. The last two arguments are the button's width and height. The result is a button that extends from position (10, 40) to position (190, 80).

     window.show();

When a parent widget is shown, it will call show for all its children (except those that were explicitly hidden using QWidget::hide()).