class gui::widget_base
This class is the parent of the "base" class for every widget. The "base" class for a widget is really an interface class; it specifies the functionality that must be implemented by the widget in order to be used. The "actual" widget classes are wrappers for a boost::shared_ptr to an implementation of the widget's interface. The idea is that the user is isolated from the complexity of the underlying implementation; and simultaneously the implementor is isolated from the mistakes of the user.
Virtual destructor.
This is a hook used by the implementor to allow cooperation between widgets. The implementation_base class should be defined by the implementor to provide an API for accessing internal widget attributes. This function is of no use to the end-user, since he cannot know the definition of implementation_base.