A simple OOP library for Lua. It has inheritance, metamethods (operators), class variables and weak mixin support.

local class = require 'middleclass'

local Fruit = class('Fruit') -- 'Fruit' is the class' name

function Fruit:initialize(sweetness)
  self.sweetness = sweetness
end

Fruit.static.sweetness_threshold = 5 -- class variable (also admits methods)

function Fruit:isSweet()
  return self.sweetness > Fruit.sweetness_threshold
end

local Lemon = class('Lemon', Fruit) -- subclassing

function Lemon:initialize()
  Fruit.initialize(self, 1) -- invoking the superclass' initializer
end

local lemon = Lemon:new()

print(lemon:isSweet()) -- false

See the github wiki page for examples & documentation.

Just copy the middleclass.lua file wherever you want it (for example on a lib/ folder). Then write this in any Lua file where you want to use it:

local class = require 'middleclass'

This project uses busted for its specs. If you want to run the specs, you will have to install it first. Then just execute the following:

cd /folder/where/the/spec/folder/is
busted

Middleclass also comes with a small performance test suite. Just run the following command:

lua performance/run.lua

Middleclass is distributed under the MIT license.

Middleclass used to expose several global variables on the main scope. It does not do that anymore.

class is now returned by require 'middleclass', and it is not set globally. So you can do this:

local class = require 'middleclass'
local MyClass = class('MyClass') -- works as before

Object is not a global variable any more. But you can get it from class.Object

local class = require 'middleclass'
local Object = class.Object

print(Object) -- prints 'class Object'

The public functions instanceOf, subclassOf and includes have been replaced by Object.isInstanceOf, Object.static.isSubclassOf and Object.static.includes.

Prior to 3.x:

instanceOf(MyClass, obj)
subclassOf(Object, aClass)
includes(aMixin, aClass)

Since 3.x:

obj:isInstanceOf(MyClass)
aClass:isSubclassOf(Object)
aClass:includes(aMixin)

The 3.x code snippet will throw an error if obj is not an object, or if aClass is not a class (since they will not implement isInstanceOf, isSubclassOf or includes). If you are unsure of whether obj and aClass are an object or a class, you can use the methods in Object. They are prepared to work with random types, not just classes and instances:

Object.isInstanceOf(obj, MyClass)
Object.isSubclassOf(aClass, Object)
Object.includes(aClass, aMixin)

Notice that the parameter order is not the same now as it was in 2.x. Also note the change in naming: isInstanceOf instead of instanceOf, and isSubclassOf instead of subclassOf.