Luaにおけるメタテーブルとメタメソッドの使用例

f1 = {a = 1, b = 2} -- represents a fraction a/b.
f2 = {a = 2, b = 3}

-- This is the wrong one.
-- s = f1 + f2

metafraction = {}
function metafraction.__add(f1, f2)
  sum = {}
  sum.b = f1.b * f2.b
  sum.a = f1.a * f2.b + f2.a * f1.b
  return sum
end

setmetatable(f1, metafraction)
setmetatable(f2, metafraction)

s = f1 + f2 -- call the __add(f1, f2) method on the metatable of f1

-- f1, f2 do not have a key that can access their meta-tables, unlike prototype
-- so you have to use getmetatable(f1) to get the meta table. A metatable is a normal table that
-- Lua can access its key in the usual way, such as __add.

-- The following code is wrong, though, because s has no meta table:.
-- t = s + s
-- The following pattern of class form solves this problem.

-- The __index of a meta-table can override the point operator lookup of.
defaultFavs = {animal = 'gru', food = 'donuts'}
myFavs = {food = 'pizza'}
setmetatable(myFavs, {__index = defaultFavs})
eatenBy = myFavs.animal -- it works! This is thanks to the metatable support

Dog = {} -- 1.

function Dog:new() -- 2.
  newObj = {sound = 'woof'} -- 3.
  self.__index = self -- 4.
  return setmetatable(newObj, self) -- 5.
end

function Dog:makeSound() -- 6.
  print('I say ' ... self.sound)
end

mrDog = Dog:new() -- 7.
mrDog:makeSound() -- 'I say woof' -- 8.

-- 1. Dog looks like a class; it's actually a table entirely.
-- 2. The function tablename:fn(...) is the same as the function tablename.fn(self, ...) is the same
-- The colon (:) just adds self as the first argument.
-- #7 and #8 below show how the self variable gets its value.
-- 3. newObj is an instance of class Dog.
-- 4. self is the initialized instance of the class. Normally self = Dog, but inheritance relationships can change this.
-- If you set both the meta table and __index of newObj to self.
-- newObj will then get the function of self.
-- 5. Remember: setmetatable returns its first argument.
-- 6. The colon (:) is working in #2, but here we expect
-- self is an instance, not a class
-- 7. Similar to Dog.new(Dog), so self = Dog in new().
-- 8. Same as mrDog.makeSound(mrDog); self = mrDog.

LoudDog = Dog:new() -- 1.

function LoudDog:makeSound()
  s = self.sound . ' ' -- 2.
  print(s ... s ... s)
end

seymour = LoudDog:new() -- 3.
seymour:makeSound() -- 'woof woof woof' -- 4.

-- 1. LoudDog gets a list of methods and variables for Dog.
-- 2. self has a key of 'sound' from new() via new(), see #3.
-- 3. Same as LoudDog.new(LoudDog), and is converted to
-- Dog.new(LoudDog), since LoudDog does not have a 'new' key.
-- but you can see __index = Dog in its meta table.
-- The result: seymour's meta-table is LoudDog, and
-- LoudDog.__index = LoudDog. so there is seymour.key
-- = seymour.key, LoudDog.key, Dog.key, to see
-- for the given key which table comes first.
-- 4. The key 'makeSound' can be found at LoudDog; this is the same as
-- LoudDog.makeSound(seymour) is the same.

-- If desired, subclasses can also have new(), similar to the base class'.
function LoudDog:new()
  newObj = {}
  -- Initialize the newObj
  self.__index = self
  return setmetatable(newObj, self)
end