----------------------------------------------------
-- 3.1 Metatables and metamethods.
----------------------------------------------------
-- A table can have a metatable that gives the table
-- operator-overloadish behavior. Later we'll see
-- how metatables support js-prototypey behavior.
f1 = {a = 1, b = 2} -- Represents the fraction a/b.
f2 = {a = 2, b = 3}
-- This would fail:
-- 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 __add(f1, f2) on f1's metatable
-- f1, f2 have no key for their metatable, unlike
-- prototypes in js, so you must retrieve it as in
-- getmetatable(f1). The metatable is a normal table
-- with keys that Lua knows about, like __add.
-- But the next line fails since s has no metatable:
-- t = s + s
-- Class-like patterns given below would fix this.
-- An __index on a metatable overloads dot lookups:
defaultFavs = {animal = 'gru', food = 'donuts'}
myFavs = {food = 'pizza'}
setmetatable(myFavs, {__index = defaultFavs})
eatenBy = myFavs.animal -- works! thanks, metatable
-- Direct table lookups that fail will retry using
-- the metatable's __index value, and this recurses.
-- An __index value can also be a function(tbl, key)
-- for more customized lookups.
-- Values of __index,add, .. are called metamethods.
-- Full list. Here a is a table with the metamethod.
-- __add(a, b) for a + b
-- __sub(a, b) for a - b
-- __mul(a, b) for a * b
-- __div(a, b) for a / b
-- __mod(a, b) for a % b
-- __pow(a, b) for a ^ b
-- __unm(a) for -a
-- __concat(a, b) for a .. b
-- __len(a) for #a
-- __eq(a, b) for a == b
-- __lt(a, b) for a < b
-- __le(a, b) for a <= b
-- __index(a, b) <fn or a table> for a.b
-- __newindex(a, b, c) for a.b = c
-- __call(a, ...) for a(...)
----------------------------------------------------
-- 3.2 Class-like tables and inheritance.
----------------------------------------------------
-- Classes aren't built in; there are different ways
-- to make them using tables and metatables.
-- Explanation for this example is below it.
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 acts like a class; it's really a table.
-- 2. function tablename:fn(...) is the same as
-- function tablename.fn(self, ...)
-- The : just adds a first arg called self.
-- Read 7 & 8 below for how self gets its value.
-- 3. newObj will be an instance of class Dog.
-- 4. self = the class being instantiated. Often
-- self = Dog, but inheritance can change it.
-- newObj gets self's functions when we set both
-- newObj's metatable and self's __index to self.
-- 5. Reminder: setmetatable returns its first arg.
-- 6. The : works as in 2, but this time we expect
-- self to be an instance instead of a class.
-- 7. Same as Dog.new(Dog), so self = Dog in new().
-- 8. Same as mrDog.makeSound(mrDog); self = mrDog.
----------------------------------------------------
-- Inheritance example:
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 Dog's methods and variables.
-- 2. self has a 'sound' key from new(), see 3.
-- 3. Same as LoudDog.new(LoudDog), and converted to
-- Dog.new(LoudDog) as LoudDog has no 'new' key,
-- but does have __index = Dog on its metatable.
-- Result: seymour's metatable is LoudDog, and
-- LoudDog.__index = LoudDog. So seymour.key will
-- = seymour.key, LoudDog.key, Dog.key, whichever
-- table is the first with the given key.
-- 4. The 'makeSound' key is found in LoudDog; this
-- is the same as LoudDog.makeSound(seymour).
-- If needed, a subclass's new() is like the base's:
function LoudDog:new()
newObj = {}
-- set up newObj
self.__index = self
return setmetatable(newObj, self)
end
----------------------------------------------------
-- 4. Modules.
----------------------------------------------------
--[[ I'm commenting out this section so the rest of
-- this script remains runnable.
-- Suppose the file mod.lua looks like this:
local M = {}
local function sayMyName()
print('Hrunkner')
end
function M.sayHello()
print('Why hello there')
sayMyName()
end
return M
-- Another file can use mod.lua's functionality:
local mod = require('mod') -- Run the file mod.lua.
-- require is the standard way to include modules.
-- require acts like: (if not cached; see below)
local mod = (function ()
<contents of mod.lua>
end)()
-- It's like mod.lua is a function body, so that
-- locals inside mod.lua are invisible outside it.
-- This works because mod here = M in mod.lua:
mod.sayHello() -- Says hello to Hrunkner.
-- This is wrong; sayMyName only exists in mod.lua:
mod.sayMyName() -- error
-- require's return values are cached so a file is
-- run at most once, even when require'd many times.
-- Suppose mod2.lua contains "print('Hi!')".
local a = require('mod2') -- Prints Hi!
local b = require('mod2') -- Doesn't print; a=b.
-- dofile is like require without caching:
dofile('mod2.lua') --> Hi!
dofile('mod2.lua') --> Hi! (runs it again)
-- loadfile loads a lua file but doesn't run it yet.
f = loadfile('mod2.lua') -- Call f() to run it.
-- loadstring is loadfile for strings.
g = loadstring('print(343)') -- Returns a function.
g() -- Prints out 343; nothing printed before now.
--]]
Difference between . and : in Lua
The colon is for implementing methods that pass self as the first parameter. So x:bar(3,4)should be the same as x.bar(x,3,4).
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