A very simple OOP implementation in Lua

Simple_Lua_OOP.lua

--------  Simple Lua OOP

local BaseClass = {}
BaseClass.class = BaseClass

function BaseClass:init(...)
end

function BaseClass:new(...)
    local instance = setmetatable({}, {__index = self})
    instance:init(...)
    return instance
end

function BaseClass:instanceOf(class)
    if self.class == class then
        return true
    end
    if self.super then
        return self.super:instanceOf(class)
    end
    return false
end

function BaseClass:createSubClass()
    local SubClass = {}
    SubClass.class = SubClass
    SubClass.super = self
    return setmetatable(SubClass, {__index = self})
end

--------  Usage Example

local Animal = BaseClass:createSubClass()

local numAnimals = 0

function Animal:init(name)
    -- self = the instance to be initialized
    self.name = name
    numAnimals = numAnimals + 1
    self.id = numAnimals
end

local Cat = Animal:createSubClass()

function Cat:init(name, breed)
    -- self = the instance to be initialized
    -- self.super = the parent class of the class of the instance
    self.super.init(self, name)
    self.breed = breed
end

local cat = Cat:new("cat name", "cat breed")