I have read a number of post describing how to use a hidden sheet to
store persistent data, however my data is not simple (just a few
strings or numbers) so I was wondering if there was any way to store a
class instance in a hidden sheet.

The problem with my data is that I have an instance of one class,
which contains collections of classes A, B, C, and D. A in turn
contains a collections of D, D a collection of B. C also contains
collections of other classes. Most of those collections will grow as
the user works with the sheet. This means that I can not store the
member of the classes easily in a sheet, since I would have to move
them around when they grow (I would have needed a N-dimentional sheet
instead of just 2D).

--
Erik Wikström