Interface data/multiple inheritance

[David Nečas <yeti physics muni cz>]

Hello,

GObject does not support multiple inheritance.
GInterface cannot have any data members (g_type_class_add_private() is
not possible either).  

Is there any other way to create a class that has is-a relation to two
classes/interfaces/whatever that each carries data?

I have control over all the classes/interfaces involved.  The thing I
want to avoid is replication of identical code handling the data that
logically goes with the interface in each class that implements the
interface.

Yeti