Quick Example

This provides basic insight on what the lib is able to do.

local class = require 'middleclass'

Person = class('Person') --this is the same as class('Person', Object) or Object:subclass('Person')
function Person:initialize(name)
  self.name = name
end
function Person:speak()
  print('Hi, I am ' .. self.name ..'.')
end

AgedPerson = class('AgedPerson', Person) -- or Person:subclass('AgedPerson')
AgedPerson.static.ADULT_AGE = 18 --this is a class variable
function AgedPerson:initialize(name, age)
  Person.initialize(self, name) -- this calls the parent's constructor (Person.initialize) on self
  self.age = age
end
function AgedPerson:speak()
  Person.speak(self) -- prints "Hi, I am xx."
  if(self.age < AgedPerson.ADULT_AGE) then --accessing a class variable from an instance method
    print('I am underaged.')
  else
    print('I am an adult.')
  end
end

local p1 = AgedPerson:new('Billy the Kid', 13) -- this is equivalent to AgedPerson('Billy the Kid', 13) - the :new part is implicit
local p2 = AgedPerson:new('Luke Skywalker', 21)
p1:speak()
p2:speak()

Output:

Hi, I'm Billy the Kid.
I am underaged.
Hi, I'm Luke Skywalker.
I am an adult.

Notes:

The standard way to do inheritance is class(‘A’, B). In addition to that, you can also do B:subclass(‘A’). Both expressions will return A (a subclass of B). A.super will return B.

When you call A:new(), only A:initialize() gets called – the constructor of B is not called by default. There is no super, instead you have to use an explicit self:

function A:initialize()
  ...
  B.initialize(self, ...)
  ...
end

The same applies for non-constructor methods – there is no super keyword-like construction, so you must use explicit self calls.