Is nil required if reference is lost?

eezing · May 2014

Been working with Lua for a little while now and didn't really ponder the concept of garbage collection until now.

If we have the following:

local table = {a, b, c}

So the variable 'table' is just a reference to {a, b, c} and not actually a table.

So next, If we do this:

table = {4, 5, 6}

'table' is now referenced to {4, 5, 6} with reference to {a, b, c} lost. {a, b, c} will be garbage collected as any object not referenced will be collected, correct?

So, the last line in the following loop is not needed, correct?

local table

for i = 1, 100 do

table = makeSomeTable() -- Some function that returns a unique table

---[[ Use table in some manner here ]]---
---[[ Use table in some manner here ]]---
---[[ Use table in some manner here ]]---

table = nil <<----- pointless correct???

end

Or is there a benefit to nil for garbage collection?

HubertRonald · May 2014

Hi @eezing
I'm using your expression in my projects because
"Before Older versions of Lua created empty tables with some pre-allocated slots to avoid this overhead when initializing small. However, this approach wastes memory".

As an example, the next loop runs in 2.0 seconds:

for i = 1, 1000000 do
  local a = {}
  a[1] = 1; a[2] = 2; a[3] = 3
end

If we create the tables with the right size, we reduce the run time to 0.7 seconds:

for i = 1, 1000000 do
  local a = {true, true, true}
  a[1] = 1; a[2] = 2; a[3] = 3
end

If you know which will hosting the data table as well as its size, you can define something

local a = {}
for i = 1, 3 do --a,b,c, or more elements
   a[#a+1] = true -- it's more fast than table.insert(a,#a+1,true)
end

and after use this table for your operations

If you want know how garbage collection works check this thread
http://giderosmobile.com/forum/discussion/1423/class-inheritance-and-memory/p1

I hope I've helped you

P.S. samples was took of http://www.lua.org/gems/sample.pdf

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