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If your widget only contains child widgets, you probably do not need to implement any event handlers.If you want to detect a mouse click in a child widget call the child's under Mouse() function inside the widget's mouse Press Event().Many of the examples provided with Qt use this approach, and it is also covered in the Qt Tutorials.Since QWidget is a subclass of QPaint Device, subclasses can be used to display custom content that is composed using a series of painting operations with an instance of the QPainter class.In Qt, QMain Window and the various subclasses of QDialog are the most common window types.Every widget's constructor accepts one or two standard arguments: QWidget has many member functions, but some of them have little direct functionality; for example, QWidget has a font property, but never uses this itself.There are many subclasses which provide real functionality, such as QLabel, QPush Button, QList Widget, and QTab Widget.A widget without a parent widget is always an independent window (top-level widget).
Composite widgets can also be created by subclassing a standard widget, such as QWidget or QFrame, and adding the necessary layout and child widgets in the constructor of the subclass.
Every widget is rectangular, and they are sorted in a Z-order.
A widget is clipped by its parent and by the widgets in front of it.
A widget that is not embedded in a parent widget is called a window.
Usually, windows have a frame and a title bar, although it is also possible to create windows without such decoration using suitable window flags).