How Lua compares with Object-Oriented languages like Python

Sep 18, 2015

The essence of Lua:

A table in Lua is an object in more than one sense. Like objects, tables have a state. Like objects, tables have an identity (a selfness) that is independent of their values; specifically, two objects (tables) with the same value are different objects, whereas an object can have different values at different times, but it is always the same object.

More than convenience?

function Account.withdraw (self, v)
    	self.balance = self.balance - v
a.withdraw(a, 100.00)
function Account:withdraw (v)
    	self.balance = self.balance - v

Tables are class objects with metadata:

a,b = {},{}

b.test = "passes objects"
a = setmetatable(a, {__index = b})
print(a.test)          -->  "passes objects"

b[2] = "forwards"
print(a[2])            -->  "forwards"

a["3"] = "NOT backwards"
print(b["3"])          -->  nil

After that[ assignment with setmetatable], a looks up in b for any operation that it does not have. To see b as the class of object a is not much more than a change in terminology.

A Benefit

For example, an object invoking this class:

function Account:new (o)
    o = o or {}   			-- create object if user does not provide one
    setmetatable(o, self)  	-- i.e., setmetatable(object, add_a_class)
    self.__index = self
    return o

also inherits other _methods_ from that class, i.e.,

b = Account:new()
print(b.balance)    --> 0

A Cost

Multiple inheritance can increase access-overhead, but, tricks like below short-circuit access points and keep costs low:

setmetatable(c, {__index = function (t, k)
    local v = search(k, arg)
    t[k] = v       -- save for next access
    return v


After direct access, no-self-style implementations are fastest:

function newAccount (initialBalance)

    local self = {balance = initialBalance}

    local withdraw = function (v)
                        self.balance = self.balance - v

    local deposit = function (v)
                        self.balance = self.balance + v

    local getBalance = function () return self.balance end

    return {
    	withdraw = withdraw,
    	deposit = deposit,
    	getBalance = getBalance
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