Metatables and Modules

Metatables: customizing table behavior

A metatable is a table attached to another table that defines how it behaves for operations it doesn't natively support, like addition, or missing-key lookups. This is how Lua achieves "object-oriented programming" without a built-in class keyword.

local Vector = {}
Vector.__index = Vector -- missing keys on an instance fall back to Vector

function Vector.new(x, y)
  local self = setmetatable({}, Vector)
  self.x = x
  self.y = y
  return self
end

function Vector:length()
  return math.sqrt(self.x ^ 2 + self.y ^ 2)
end

local v = Vector.new(3, 4)
print(v:length()) -- 5.0

• setmetatable(t, mt) attaches metatable mt to table t.
• __index is a special metamethod: when you look up a key that v doesn't have directly (like length), Lua checks __index next, this is how "instances" share methods defined on a common table.
• function Vector:length() is sugar for function Vector.length(self), the colon automatically adds a hidden self first parameter, and v:length() automatically passes v as that self.

Modules

Lua organizes reusable code into modules: files that return a table of functions. In a real project this would be two separate files:

-- mathutils.lua
local M = {}

function M.double(n)
  return n * 2
end

return M

-- main.lua
local mathutils = require("mathutils")
print(mathutils.double(21)) -- 42

require loads a module by name, runs it once, and caches its returned value, calling require again for the same module returns the cached table instead of re-running the file. This course's sandbox only runs one file at a time, so here's the same idea inlined into a single runnable script:

local function newMathUtils() -- stands in for a separate mathutils.lua file
  local M = {}

  function M.double(n)
    return n * 2
  end

  return M
end

local mathutils = newMathUtils() -- in a real project: require("mathutils")
print(mathutils.double(21)) -- 42