Metatables and OOP

Code

-- Vector class via metatable
local Vector = {}
Vector.__index = Vector  -- lookups fall through to Vector

function Vector.new(x, y)
  local self = setmetatable({}, Vector)
  self.x = x or 0
  self.y = y or 0
  return self
end

-- Operator overloading
function Vector.__add(a, b)
  return Vector.new(a.x + b.x, a.y + b.y)
end

function Vector.__tostring(self)
  return string.format("(%d, %d)", self.x, self.y)
end

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

-- Usage
local v1 = Vector.new(3, 4)
local v2 = Vector.new(1, 2)
local v3 = v1 + v2          -- (4, 6)
print(v3)                   -- (4, 6) (uses __tostring)
print(v3:length())          -- 7.21
print(getmetatable(v1))     -- Vector table

Explanation

Metatables hook into Lua's operations (add, index, call, etc.). __index makes inheritance work — when a key isn't found in the table, Lua looks in __index. setmetatable returns the table so you can chain. __add, __sub, __eq overload operators. __tostring controls print output. This is how all Lua OOP libraries work.

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