How can one implement OO in Lua?

Question

Lua does not have build in support for OO, but it allows you to build it yourself. Could you please share some of the ways one can implement OO?

Please write one example

Answer 1

This is already answered, but anyway, here's my oop implementation: middleclass.

That lib provides the bare minimum for creating classes, instances, inheritance, polymorphism and (primitive) mixins, with an acceptable performance.

Sample:

local class = require 'middleclass'

local Person = class('Person')

function Person:initialize(name)
  self.name = name
end
function Person:speak()
  print('Hi, I am ' .. self.name ..'.')
end

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