Object Classes for Lua

This package provide simple object-oriented capabilities to Lua. Each class is defined with a metatable, which contains methods. Inheritance is achieved by setting metatables over metatables. An efficient type checking is provided.

Typical Example

local class = require 'class'

-- define some dummy A class
local A = class('A')

function A:__init(stuff)
  self.stuff = stuff
end

function A:run()
  print(self.stuff)
end

-- define some dummy B class, inheriting from A
local B = class('B', 'A')

function B:run5()
  for i=1,5 do
    print(self.stuff)
  end
end

-- create some instances of both classes
local a = A('hello world from A')
local b = B('hello world from B')

-- run stuff
a:run()
b:run()
b:run5()

Documentation

First, require the package

local class = require 'class'

Note that class does not clutter the global namespace.

Class metatables are then created with class(name) or equivalently class.new(name).

local A = class('A')
local B = class('B', 'A') -- B inherit from A

You then have to fill-up the returned metatable with methods.

function A:myMethod()
  -- do something
end

There are two special methods: new(), which already exists when the class is created and should not be overrided and __init() which is called by new() at the creation of the class.

function A:__init(args)
  -- do something with args
  -- note that self exists
end

Creation of an instance is then achieved with the new() function or (equivalently) using the Lua __call metamethod:

local a = A('blah blah') -- an instance of A
local aa = A.new('blah blah') -- equivalent of the above

class.new(name[, parentname])

Creates a new class called name, which might optionally inherit from parentname. Returns a table, in which methods should be defined.

Note that the returned table is not the metatable, but a constructor table (with a __call function defined). In that respect, one can use the following shorthand:

local A = class.new('A')
local a = A.new() -- instance.
local aa = A()    -- another instance (shorthand).

There is also a shorthandclass.new(), which is class().

class.factory(name)

Return a new (empty) instance of the class name. No __init method will be called.

class.metatable(name)

Return the metatable (i.e. the table containing all methods) related to class name.

class.type(obj)

Return the type of the object obj (if this is a known class), or the type returned by the standard lua type() function (if it is not known).

class.istype(obj, name)

Check is obj is an instance (or a child) of class name. Returns a boolean.