Re: Metamethods as weak reference?

On 1 June 2011 14:31, Chris Redden <kaelic@gmail.com> wrote:

May have discovered the issue, it's two-fold.

1- I needed a metatable on the metatable to make it have weak-refs on __index/__newindex

2- Weak refs are left until GCed (makes sense), so I'd need to invoke a full GC whenever I wanted to make sure the weak ref isn't accessible

Example:
local EntityList = {}

function main()

EntityList[0] = { foo = "bar" }

local EntityVar_mt_mt = { __mode = "kv" }
local EntityVar_mt = { __index = EntityList[0], __newindex = EntityList[0] }

setmetatable( EntityVar_mt, EntityVar_mt_mt )
local EntityVar = {}
setmetatable( EntityVar, EntityVar_mt )

print(EntityVar.foo)

EntityList[0] = nil
collectgarbage("collect")

print(EntityVar.foo)
end

main()

On 1 June 2011 14:06, Chris Redden <kaelic@gmail.com> wrote:

When you say "If your metatable has weak values, then its entries will be cleared
when the values are no longer accessible." are you saying this includes metamethods?

local EntityList = {}

function main()

EntityList[0] = { foo = "bar" }

local EntityVar_mt = { __mode = "kv", __index = EntityList[0], __newindex = EntityList[0] }
local EntityVar = {}

setmetatable( EntityVar, EntityVar_mt )

print(EntityVar.foo)

EntityList[0] = nil

print(EntityVar.foo)
end

main()

In this example, I'd expect the following output:
bar
nil

but I get:
bar
bar

On 1 June 2011 13:27, Roberto Ierusalimschy <roberto@inf.puc-rio.br> wrote:

> It seems all of the metamethods retain a strong reference, which is causing
> me problems trying to use proxy objects. Is there a specific reason they're
> a strong reference? It seems to be this line that makes them all strong:
>
> ltm.c: 39
> for (i=0; i<TM_N; i++) {
> G(L)->tmname[i] = luaS_new(L, luaT_eventname[i]);
> luaS_fix(G(L)->tmname[i]); /* never collect these names */
>
> What would be the implications of removing this luaS_fix on the metamethods?

This 'fix' only prevents the collection of the strings '__add', '__index',
etc. (Anyway, strings are never removed by themselves from weak tables.)
If your metatable has weak values, then its entries will be cleared
when the values are no longer accessible.

-- Roberto