Function/variable scope (pass by value or reference?)

Question

I\'m completely confused by Lua\'s variable scoping and function argument passing (value or reference).

See the code below:

local a          


        

Answer 1

Lua's function, table, userdata and thread (coroutine) types are passed by reference. The other types are passed by value. Or as some people like to put it; all types are passed by value, but function, table, userdata and thread are reference types.

string is also a kind of reference type, but is immutable, interned and copy-on-write - it behaves like a value type, but with better performance.

Here's what's happening:

local a = 9
local t = {4,6}

function moda(a)
  a = 10 -- sets 'a', which is a local introduced in the parameter list
end

function modt(t)
  t[1] = 7 -- modifies the table referred to by the local 't' introduced in the parameter list
  t[2] = 8
end

Perhaps this will put things into perspective as to why things are the way they are:

local a = 9
local t = {4,6}

function moda()
  a = 10 -- modifies the upvalue 'a'
end

function modt()
  t[1] = 7 -- modifies the table referred to by the upvalue 't'
  t[2] = 8
end

-- 'moda' and 'modt' are closures already containing 'a' and 't',
-- so we don't have to pass any parameters to modify those variables
moda()
modt()
print(a)  -- now print 10
print(t[1]..t[2])  -- still print 78