RE: Strings and GC - was RE: Using full userdata as table index?

[RLake@...]

Kevin Baca preguntó:

> Forgive me, but why when a string
is used as a key for a weak-keyed
> table is it not GC'd?

Ah, now that *is* a good question. Wim
Couwenberg and I had a fascinating correspondence about it, a while back.

The essence of a weak-keyed table is
that the key will never come back once it is no longer reachable, so once
the only reference to the key is the table itself, the key-value pair can
be removed (possibly losing the last reference to the value as well.) So,
as Wim pointed out, if you can't (or even don't) iterate a table, all of
its keys which are mutable objects might as well be weak. In that sense,
non-iterability is the underlying feature rather than weak-keyness; the
key in a non-iterable table is weak precisely when it is a mutable object.

However, a string is not a mutable object
(in Lua). So a string can disappear and then be recreated *exactly as though
it had never gone away*. It is therefore not safe to remove a (string)key-value
pair from a weak-keyed table even if there is no other existing reference
to the string key, because you might later recreate the string. If Lua
had mutable strings, then they would be GC'd from weak-keyed tables.

Rici