Re: Prevent userdata from being marked for garbage collection?

[Carsten Fuchs <carsten.fuchs@...>]

Hi Roberto,

Am 2012-07-04 22:42, schrieb Roberto Ierusalimschy:
> I guess the following structure does what you want.

Thank you very much!
However it requires ephemerons and thus Lua 5.2 to work, doesn't it?

I read http://www.inf.puc-rio.br/~roberto/docs/ry08-06.pdf when trying to understand 
what it does, which was very instructive, but I still don't understand how calling 
mksentinel(obj) again in the sentinel's __gc method can save the obj. Isn't it too late 
also for obj when the sentinel's __gc is run?

> (Note that you cannot use the __gc metamethod of
> 'myobject', as it will be called once the object becomes gargage; you
> must use 'collect' to finalize the object.)

So myobject's __gc metamethod is called every time the (sentinel, obj) pair is dropped 
from the anchor table?
I understand that we thus have to do any finalization work in collect() instead, but I 
still wonder how mksentinel(obj) can save the obj when it seems that internally to Lua 
it was marked for collection at the time when the sentinel key in anchor became collectible.


Well, sorry that I didn't mention earlier that I'm still using Lua 5.1: we're not quite 
ready to upgrade to 5.2 yet (but I'll be among the first to purchase the PiL3 the day it 
comes out :-) ).

I assume there is no easy alternative of this code for Lua 5.1?

My current, simple but somewhat clumsy, work-around plan is to keep normal, strong 
references to my C++ objects by default, and once in a while (e.g once per frame, or 
every n-th frame, or whatever) iterate the strong references table, checking which of 
the objects still have references in C++ (smart pointer reference count > 1), and for 
those that don't, simply drop the entry from the strong references table.
(And possibly re-insert it as required, e.g. whenever the object occurs in the stack of 
a CFunction again, the type checking mechanism would restore the strong reference...)

Best regards,
Carsten



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