Lua 5.1 workaround for __gc metadata for tables
I am currently facing the problem that you cannot use the method __gc
for tables in Lua 5.1 as they are implemented in Lua 5.2. However, I want to release the allocated resources as soon as the lua table is built. Is there a workaround that gives me the metatet functionality __gc
in Lua 5.2 for Lua 5.1?
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In lua 5.1, the only lua values that work with a metamethod __gc
are userdata
. Naturally, any hack or workaround has to be enabled in some way userdata
. There is usually no way to just create newuserdata from the lua side, but there is one "hidden" undocumented function for that newproxy
.
newproxy
accepts an optional bool or userdata parameter. If you go to true
, then you will get the user data with the new meta tag added. If you pass in a different one userdata
, then the new userdata will be assigned the same meta-metat as the one that was passed.
So now you can just hack a function that will make __gc
tables work:
function setmt__gc(t, mt)
local prox = newproxy(true)
getmetatable(prox).__gc = function() mt.__gc(t) end
t[prox] = true
return setmetatable(t, mt)
end
And a quick test to confirm the behavior:
iscollected = false
function gctest(self)
iscollected = true
print("cleaning up:", self)
end
test = setmt__gc({}, {__gc = gctest})
collectgarbage()
assert(not iscollected)
test = nil
collectgarbage()
assert(iscollected)
Note that lua 5.2+ and later don't have it anymore newproxy
, as it is __gc
officially supported in tables.
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