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In the C++ GUI Programming with Qt 4 book, it mentions in an example in the first chapter that QWidget serves as the application's main window.
And, on the Qt Reference Documentation: http://doc.qt.io/qt-4.8/qwidget.html there is plenty of information about QWidget.
But, what is the baseline? What does QWidget mainly do? When should I think about it?
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A QDialog is based on QWidget , but designed to be shown as a window. It will always appear in a window, and has functions to make it work well with common buttons on dialogs (accept, reject, etc.). QMainWindow is designed around common needs for a main window to have.
All Qt widgets inherit QObject. The convenience function isWidgetType() returns whether an object is actually a widget. It is much faster than qobject_cast<QWidget *>(obj) or obj->inherits("QWidget"). Some QObject functions, e.g. children(), return a QObjectList.
The tree-like organization of QWidgets (and in fact of any QObjects ) is part of the memory management strategy used within the Qt-Framework. Assigning a QObject to a parent means that ownership of the child object is transferred to the parent object. If the parent is deleted, all its children are also deleted.
Qt Main Window Framework A main window provides a framework for building an application's user interface. Qt has QMainWindow and its related classes for main window management. QMainWindow has its own layout to which you can add QToolBars, QDockWidgets, a QMenuBar, and a QStatusBar.
One way to think about it is any object that knows how to display itself on the screen is a QWidget (in particular, some subclass of QWidget).
There are some objects like QPicture that represent an image, but a QPicture by itself doesn't know how to put itself on the screen. You usually need to use it in combination with a QLabel for instance (which is a kind of QWidget).
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It is an abstract of window objects. Every visible/invisible Qt window-related object inherits from QWidget.
Just consider a vehicle, it is the abstract of cars, trucks and other stuffs.
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