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- Subject: weak references
- From: "John Belmonte" <jvb@...>
- Date: Sat, 11 Nov 2000 01:49:49 +0900
Taking Roberto's hint I've worked out weak references, at least on paper.
p.s. I'm wondering about the Technical Notes... is this kind of content
** Weak references for Lua **
ref = weakref( val )
-- ref is an object, use function call syntax to dereference it
val = ref()
Function weakref is implemented in C. It creates a userdata
object. Only one value is needed as state (the number returned by
lua_ref), so the userdata pointer itself can be used instead of
allocating memory. The "function" tagmethod is set on the object,
and when called it returns the result of lua_getref. The "gc"
tagmethod is also set on the object, so that lua_unref can be
called to free the reference resource when the weak reference
object is collected.
Where to from here
Using the interface above, it is possible to implement weak
tables (from Lua). A weak table works like any other table,
except that for garbage collection it doesn't count as a
reference of the values it holds.
Actually the description above is technically for a "weak value
hash". There are also variants known as "weak key hash"
(strong reference to value is released when key is collected)
and "doubly weak hash" (both value and key are weak). These
could be implemented also, but perhaps would require
Again with help from C, it's possible to generate callbacks
when the value pointed to by a weak reference is collected.
This could be implemented by keeping a list of such references
and then, using the end-of-garbage-collection hook, scan for
references that have nil values.