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    GObject
    ╰── GInitiallyUnowned
        ╰── GtkObject
            ╰── GtkWidget
                ╰── GtkContainer
                    ╰── GtkBin
                        ├── GtkWindow
                        ├── GtkAlignment
                        ├── GtkFrame
                        ├── GtkButton
                        ├── GtkItem
                        ├── GtkComboBox
                        ├── GtkEventBox
                        ├── GtkExpander
                        ├── GtkHandleBox
                        ├── GtkToolItem
                        ├── GtkScrolledWindow
                        ╰── GtkViewport
The GtkBin widget is a container with just one child. It is not very useful itself, but it is useful for deriving subclasses, since it provides common code needed for handling a single child widget.
Many GTK+ widgets are subclasses of GtkBin, including GtkWindow, GtkButton, GtkFrame, GtkHandleBox or GtkScrolledWindow.