Object-Oriented Lua: Classes and Inheritance

The Fundamentals course showed how setmetatable and __index turn a plain table into something like an instance. Here we build a full class hierarchy with inheritance.

A base class

Lua

local Animal = {}
Animal.__index = Animal

function Animal.new(name)
  return setmetatable({ name = name }, Animal)
end

function Animal:speak()
  return self.name .. " makes a sound."
end

local Animal = {}
Animal.__index = Animal

function Animal.new(name)
  return setmetatable({ name = name }, Animal)
end

function Animal:speak()
  return self.name .. " makes a sound."
end

Inheriting from it

Lua

local Dog = setmetatable({}, { __index = Animal }) -- Dog falls back to Animal
Dog.__index = Dog

function Dog.new(name)
  local self = Animal.new(name)   -- reuse the base constructor
  return setmetatable(self, Dog)   -- but tag the instance as a Dog
end

function Dog:speak() -- overrides Animal:speak
  return self.name .. " says woof!"
end

local generic = Animal.new("Creature")
local rex = Dog.new("Rex")

print(generic:speak()) -- Creature makes a sound.
print(rex:speak())      -- Rex says woof!

local Dog = setmetatable({}, { __index = Animal }) -- Dog falls back to Animal
Dog.__index = Dog

function Dog.new(name)
  local self = Animal.new(name)   -- reuse the base constructor
  return setmetatable(self, Dog)   -- but tag the instance as a Dog
end

function Dog:speak() -- overrides Animal:speak
  return self.name .. " says woof!"
end

local generic = Animal.new("Creature")
local rex = Dog.new("Rex")

print(generic:speak()) -- Creature makes a sound.
print(rex:speak())      -- Rex says woof!

Two __index links are at work: Dog's metatable points __index at Animal, so a Dog instance whose class table (Dog) doesn't have a method falls through to Animal. Because Dog:speak is defined, it wins over Animal:speak, this is how overriding works.

Calling the parent's method

Lua

function Dog:speak()
  return Animal.speak(self) .. " (a very good boy)"
end

function Dog:speak()
  return Animal.speak(self) .. " (a very good boy)"
end

Calling Animal.speak(self) directly (dot syntax, explicit self) is how Lua does "super.method()" calls, there's no dedicated super keyword.

TIP

This two-metatable pattern (class table linked to a parent class table, instances linked to their class table) is the idiomatic way OOP frameworks implement inheritance under the hood, now you know exactly what they're doing for you.

Object-Oriented Lua: Classes and Inheritance

A base class

Inheriting from it

Calling the parent's method

📝 OOP & Inheritance Quiz