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(following-up to myself...)

Adam D. Moss wrote:
A simpler way is to store the dummy created-on-demant heavyuserdata
directly in your {}, using a reserved key (e.g., __userdata).

That's possible but pollutes the original table (but that
might not be a problem for objects, where you wouldn't
usually for example want to do a pairs() over the table).

Oh of course, silly me; most ways of actually implementing
objects and classes use a proxy table so pairs() doesn't
work properly anyway.

You then need a way to map back from the userdata to the table

Since this userdata is dummy anyway, you can store the table as the
userdata's metatable.

That's a very nice idea (though then, surely, the __gc field has
to go into the table itself and the same pollution occurs as above).

The table itself could go inside the userdata's metatable somewhere
(creating a loop, but oh well).  The penalty is that this removes
the ability to share this metatable between all such userdata.

Adam D. Moss   -