Another basic OOP in lua with inheritance and virtual methods.

oop.lua

-- creates a class table which has a "new" method to create object instances.
class = function (parentclass, classdef)
  local cls = {
    __classdef__ = classdef or {},
    __parent__ = parentclass or {},

    -- instance constructor
    new = function(cls, ...)
      local instance = {
        super = cls.__parent__.__classdef__,
        __class__ = cls
      }

      -- if instance attr is not defined, the accessor will be referred to
      -- class definition attribute
      setmetatable(instance, {__index = cls.__classdef__})

      -- if class definition attribute is not defined, the accessor should
      -- be referred to parent's class definition attribute (inheritance)
      setmetatable(
        cls.__classdef__,
        {__index = cls.__parent__.__classdef__ }
      )

      -- call init method, if any
      if instance.init then
        instance:init(...)
      end

      return instance
    end,
  }

  return cls
end

-- a short form of class() call
define = function(classdef) return class(nil, classdef) end

-- a "syntactic sugar" for defining class inheritance
extend = function(parent)
  return function(classdef) return class(parent, classdef) end
end