Example from the Lua docs on creating class like behaviour using metatables

account.lua

-- Basic lua class 
-- http://www.lua.org/pil/16.html
-- http://www.lua.org/pil/16.1.html
Account = { balance=0 }

function Account:new (o)
  o = o or {
    balance = 0
  }
  setmetatable(o, self)
  self.__index = self
  return o
end

function Account:withdraw (v) 
  self.balance = self.balance - v
end

function Account:deposit (v)
  self.balance = self.balance + v
end

x = Account:new()
x:deposit(200.00)
x:withdraw(100.00)

y = Account:new({ balance=50.00 })
y:withdraw(20.49)

print(x.balance) -- 100.0
print(y.balance) -- 29.51