Passing a class instance to a thread via spawn()

[Joseph Rushton Wakeling]

Hello all,

I have a data structure which is a final class.  Once created, the contents of
the class will not be mutated (only its const methods will be called).

Is there any way to pass this to a thread via spawn() or via message-passing?
I've seen instructions to use shared() but if I try and declare the class this
way I get errors such as "non-shared method is not callable using a shared
object" and "non-shared const method is not callable using a shared object".

The ideal would be to mark this object, once created, as immutable -- but trusty
methods like assumeUnique() don't work for classes!

Can anyone advise?

Thanks & best wishes,

     -- Joe