다단계 상속

function Animal:extend()
  local cls = {}
  cls.__index = cls
  setmetatable(cls, {__index = self})
  cls.super = self
  function cls:new(...)
    local inst = setmetatable({}, cls)
    if inst.init then inst:init(...) end
    return inst
  end
  return cls
end

local Dog = Animal:extend()
function Dog:init(name)
  Dog.super.init(self, name, "Woof")
  self.tricks = {}
end
function Dog:learnTrick(trick)
  self.tricks[#self.tricks+1] = trick
end
local d = Dog:new("Rex")
d:learnTrick("sit")
d:speak()  -- Rex says Woof

local GuideDog = Dog:extend()
function GuideDog:init(name, owner)
  GuideDog.super.init(self, name)
  self.owner = owner
end
function GuideDog:guide()
  print(self.name .. " guides " .. self.owner)
end

function Dog:init(name)
  Dog.super.init(self, name, "Woof")  -- explicit super call
end

function Dog:speak()
  print(self.name .. " barks!")
  Dog.super.speak(self)  -- also call parent speak
end

local function isA(instance, cls)
  local mt = getmetatable(instance)
  while mt do
    if mt == cls then return true end
    mt = getmetatable(mt)
  end
  return false
end
print(isA(d, Dog))     -- true
print(isA(d, Animal))  -- true