How to create a class, subclass and properties in Lua?

Question

I'm having a hard time grokking classes in Lua. Fruitless googling led me to ideas about meta-tables, and implied that third-party libraries are necessary to simulate/write classes.

Here's a sample (just because I've noticed I get better answers when I provide sample code):

public class ElectronicDevice
{
    protected bool _isOn;
    public bool IsOn { get { return _isOn; } set { _isOn = value; } }
    public  void Reboot(){_isOn = false; ResetHardware();_isOn = true; }
}

public class Router : ElectronicDevice
{
}

public class Modem :ElectronicDevice
{
    public void WarDialNeighborhood(string areaCode)
    {
        ElectronicDevice cisco = new Router();
        cisco.Reboot();
        Reboot();
        if (_isOn)
            StartDialing(areaCode);
    }
}

Here is my first attempt to translate the above using the technique suggested by Javier.

I took the advice of RBerteig. However, invocations on derived classes still yield: "attempt to call method 'methodName' (a nil value)"

--Everything is a table
ElectronicDevice = {};

--Magic happens
mt = {__index=ElectronicDevice};

--This must be a constructor
function ElectronicDeviceFactory ()
    -- Seems that the metatable holds the fields
    return setmetatable ({isOn=true}, mt)
end

-- Simulate properties with get/set functions
function ElectronicDevice:getIsOn()  return self.isOn end
function ElectronicDevice:setIsOn(value)  self.isOn = value end
function ElectronicDevice:Reboot() self.isOn = false;
    self:ResetHardware(); self.isOn = true; end
function ElectronicDevice:ResetHardware()  print('resetting hardware...') end

Router = {};
mt_for_router = {__index=Router}

--Router inherits from ElectronicDevice
Router = setmetatable({},{__index=ElectronicDevice});

--Constructor for subclass, not sure if metatable is supposed to be different
function RouterFactory ()
    return setmetatable ({},mt_for_router)
end

Modem ={};
mt_for_modem = {__index=Modem}

--Modem inherits from ElectronicDevice
Modem = setmetatable({},{__index=ElectronicDevice});

--Constructor for subclass, not sure if metatable is supposed to be different
function ModemFactory ()
    return setmetatable ({},mt_for_modem)
end

function Modem:WarDialNeighborhood(areaCode)
        cisco = RouterFactory();
        --polymorphism
        cisco.Reboot(); --Call reboot on a router
        self.Reboot(); --Call reboot on a modem
        if (self.isOn) then self:StartDialing(areaCode) end;
end

function Modem:StartDialing(areaCode)
    print('now dialing all numbers in ' .. areaCode);
end

testDevice = ElectronicDeviceFactory();
print("The device is on? " .. (testDevice:getIsOn() and "yes" or "no") );
testDevice:Reboot(); --Ok

testRouter = RouterFactory();
testRouter:ResetHardware(); -- nil value

testModem = ModemFactory();
testModem:StartDialing('123'); -- nil value

Answer 1

Another simple approach for subclass

local super    = require("your base class")
local newclass = setmetatable( {}, {__index = super } )
local newclass_mt = { __index = newclass }

function newclass.new(...) -- constructor
    local self = super.new(...)
    return setmetatable( self, newclass_mt )
end

You still can use the functions from superclass even if overwritten

function newclass:dostuff(...)
    super.dostuff(self,...)
   -- more code here --
end

don't forget to use ONE dot when pass the self to the superclass function

Answer 2

It's really easy to do class-like OOP in Lua; just put all the 'methods' in the __index field of a metatable:

local myClassMethods = {}
local my_mt = {__index=myClassMethods}

function myClassMethods:func1 (x, y)
    -- Do anything
    self.x = x + y
    self.y = y - x
end

............

function myClass ()
    return setmetatable ({x=0,y=0}, my_mt)

Personally, I've never needed inheritance, so the above is enough for me. If it's not enough, you can set a metatable for the methods table:

local mySubClassMethods = setmetatable ({}, {__index=myClassMethods})
local my_mt = {__index=mySubClassMethods}

function mySubClassMethods:func2 (....)
    -- Whatever
end

function mySubClass ()
    return setmetatable ({....}, my_mt)

update: There's an error in your updated code:

Router = {};
mt_for_router = {__index=Router}
--Router inherits from ElectronicDevice
Router = setmetatable({},{__index=ElectronicDevice});

Note that you initialize Router, and build mt_for_router from this; but then you reassign Router to a new table, while mt_for_router still points to the original Router.

Replace the Router={} with the Router = setmetatable({},{__index=ElectronicDevice}) (before the mt_for_router initialization).

Answer 3

There are a number of ways you can do it but this is how I do (updated with a shot at inheritance):

function newRGB(r, g, b)
  local rgb={
      red = r;
      green = g;
      blue = b;
      setRed = function(self, r)
          self.red = r;
      end;
      setGreen = function(self, g)
          self.green= g;
      end;
      setBlue = function(self, b)
          self.blue= b;
      end;
      show = function(self)
          print("red=",self.red," blue=",self.blue," green=",self.green);
      end;
  }
  return rgb;
end

purple = newRGB(128, 0, 128);
purple:show();
purple:setRed(180);
purple:show();

---// Does this count as inheritance?
function newNamedRGB(name, r, g, b)
    local nrgb = newRGB(r, g, b);
    nrgb.__index = nrgb; ---// who is self?
    nrgb.setName = function(self, n)
        self.name = n;
    end;
    nrgb.show = function(self)
        print(name,": red=",self.red," blue=",self.blue," green=",self.green);
    end;
    return nrgb;
end

orange = newNamedRGB("orange", 180, 180, 0);
orange:show();
orange:setGreen(128);
orange:show();

I don't implement private, protected, etc. although it is possible.

Answer 4

If you don't want to reinvent the wheel, there is a nice Lua library implementing several object models. It's called LOOP.

Answer 5

Here's an example literal transcription of your code, with a helpful Class library that could be moved to another file.

This is by no means a canonical implementation of Class; feel free to define your object model however you like.

Class = {}

function Class:new(super)
    local class, metatable, properties = {}, {}, {}
    class.metatable = metatable
    class.properties = properties

    function metatable:__index(key)
        local prop = properties[key]
        if prop then
            return prop.get(self)
        elseif class[key] ~= nil then
            return class[key]
        elseif super then
            return super.metatable.__index(self, key)
        else
            return nil
        end
    end

    function metatable:__newindex(key, value)
        local prop = properties[key]
        if prop then
            return prop.set(self, value)
        elseif super then
            return super.metatable.__newindex(self, key, value)
        else
            rawset(self, key, value)
        end
    end

    function class:new(...)
        local obj = setmetatable({}, self.metatable)
        if obj.__new then
            obj:__new(...)
        end
        return obj
    end

    return class
end

ElectronicDevice = Class:new()

function ElectronicDevice:__new()
    self.isOn = false
end

ElectronicDevice.properties.isOn = {}
function ElectronicDevice.properties.isOn:get()
    return self._isOn
end
function ElectronicDevice.properties.isOn:set(value)
    self._isOn = value
end

function ElectronicDevice:Reboot()
    self._isOn = false
    self:ResetHardware()
    self._isOn = true
end

Router = Class:new(ElectronicDevice)

Modem = Class:new(ElectronicDevice)

function Modem:WarDialNeighborhood(areaCode)
    local cisco = Router:new()
    cisco:Reboot()
    self:Reboot()
    if self._isOn then
        self:StartDialing(areaCode)
    end
end

If you were to stick to get/set methods for properties, you wouldn't need __index and __newindex functions, and could just have an __index table. In that case, the easiest way to simulate inheritance is something like this:

BaseClass = {}
BaseClass.index = {}
BaseClass.metatable = {__index = BaseClass.index}

DerivedClass = {}
DerivedClass.index = setmetatable({}, {__index = BaseClass.index})
DerivedClass.metatable = {__index = DerivedClass.index}

In other words, the derived class's __index table "inherits" the base class's __index table. This works because Lua, when delegating to an __index table, effectively repeats the lookup on it, so the __index table's metamethods are invoked.

Also, be wary about calling obj.Method(...) vs obj:Method(...). obj:Method(...) is syntactic sugar for obj.Method(obj, ...), and mixing up the two calls can produce unusual errors.

Answer 6

Your updated code is wordy, but should work. Except, you have a typo that is breaking one of the metatables:

--Modem inherits from ElectronicDevice
Modem = setmetatable({},{__index,ElectronicDevice});

should read

--Modem inherits from ElectronicDevice
Modem = setmetatable({},{__index=ElectronicDevice});

The existing fragment made the Modem metatable be an array where the first element was almost certainly nil (the usual value of _G.__index unless you are using strict.lua or something similar) and the second element is ElectronicDevice.

The Lua Wiki description will make sense after you've grokked metatables a bit more. One thing that helps is to build a little infrastructure to make the usual patterns easier to get right.

I'd also recommend reading the chapter on OOP in PiL. You will want to re-read the chapters on tables and metatables too. Also, I've linked to the online copy of the 1st edition, but owning a copy of the 2nd is highly recommended. There is also a couple of articles in the Lua Gems book that relate. It, too, is recommended.

Answer 7

The way I liked to do it was by implementing a clone() function.
Note that this is for Lua 5.0. I think 5.1 has more built-in object oriented constructions.

clone = function(object, ...) 
    local ret = {}

    -- clone base class
    if type(object)=="table" then 
            for k,v in pairs(object) do 
                    if type(v) == "table" then
                            v = clone(v)
                    end
                    -- don't clone functions, just inherit them
                    if type(v) ~= "function" then
                            -- mix in other objects.
                            ret[k] = v
                    end
            end
    end
    -- set metatable to object
    setmetatable(ret, { __index = object })

    -- mix in tables
    for _,class in ipairs(arg) do
            for k,v in pairs(class) do 
                    if type(v) == "table" then
                            v = clone(v)
                    end
                    -- mix in v.
                    ret[k] = v
            end
    end

    return ret
end

You then define a class as a table:

Thing = {
   a = 1,
   b = 2,
   foo = function(self, x) 
     print("total = ", self.a + self.b + x)
   end
}

To instantiate it or to derive from it, you use clone() and you can override things by passing them in another table (or tables) as mix-ins

myThing = clone(Thing, { a = 5, b = 10 })

To call, you use the syntax :

myThing:foo(100);

That will print:

total = 115

To derive a sub-class, you basically define another prototype object:

BigThing = clone(Thing, { 
     -- and override stuff.  
     foo = function(self, x)
         print("hello");
     end
}

This method is REALLY simple, possibly too simple, but it worked well for my project.