A SizeGroup provides a mechanism for grouping a number of widgets together so they all request the same amount of space. This is typically useful when you want a column of widgets to have the same size, but you can't use a Gtk.Table widget. Its use is effectively demonstrated by the Human Interface Guidelines of the Gnome project.

In detail, the size requested for each widget in a SizeGroup is the maximum of the sizes that would have been requested for each widget in the SizeGroup without a SizeGroup. The SizeGroup.Mode of the size group determines whether this applies to the horizontal size, the vertical size, or both sizes.

Note that SizeGroups only affect the amount of space requested, not the size that the widgets finally receive. If you want the widgets in a SizeGroup to actually be the same size, you need to pack them in such a way that they get the size they request and not more. For example, if you are packing your widgets into a table, you would not include the Fill flag.

SizeGroup objects are referenced by each widget in the size group, so once you have added all widgets to a SizeGroup, you can drop the initial reference to the SizeGroup by calling GLib.Object.Unref. If the widgets in the SizeGroup are subsequently destroyed, then they will be removed from the SizeGroup and drop their references on the SizeGroup; when all widgets have been removed, the size group will be freed.

Widgets can be part of multiple size groups; GTK will compute the horizontal size of a widget from the horizontal requisition of all widgets that can be reached from the widget by a chain of size groups of type SizeGroupMode.Horizontal or SizeGroupMode.Both. Likewise, the vertical size is computed from the vertical requisition of all widgets that can be reached from the widget by a chain of size groups of type SizeGroupMode.Vertical or SizeGroupMode.Both.