Beginner Question - Classes/OOP in LUA

gyrosp · November 2012

Hi

.

I'm new to Lua and I have some difficulties in understanding Classes/OOP in Lua.

I'm just trying to create a simple "player"-class. This is what I have so far:

--[[
filename : Player.lua
--]]

Player = Core.class()

function Player:init()
self.health = 100
self.speed = 3
end

function Player:startRunning()
end

function Player:stopRunning()
end

I create the instance and call a method like this:

require ("Class/Player")

local player = Player.new()
player.startRunning()

Is this the right approach? How do I declare local variables like health and access them from outside the class?
I've read several articles about this issue but after that I'm more confused than before

. There are many different oppinions out there.

Another thing. What's the point with the . and : syntax? I have seen some methods/functions like this

function object.print(self)
print("Hello from ", self)
end

What's the difference between function object.print(self) and function object:print(self)?

plamen · November 2012

you don't need "require ("Class/Player")" and "player.startRunning()" should be "player:startRunning()" if you want to access health its done like this: player.health=100 or player.speed=200 :-)
question about . and : here it is: object:method() - calls object method and object.property - access to objects property.

techdojo · November 2012

When you use the single dot ( . ) method to call the function in the table, it does just that, it calls the function with the parameter list as specified.

When you use the double dot ( : ) method, the lua interpreter actually puts "self" as a hidden first parameter so

        object.print(self)

Is the same as

        object:print()

Likewise when defining functions like

    function myClass:printSelfAndA(a)
        print("Self = ",self,a)
    end

Will work because "self" is actually the first "hidden" parameter.

You can call the above function in two ways

     object = myClass.new()
 
     object:printSelfAndA(a)
     object.printSelfAndA(object,a)

In Lua "self" is a reference to the table that contains the function being called - like "this" in C++.

If the above has completely confused you then just remember it like this.

For accessing variables stored in a table use ( . )
For defining and calling functions stored in a table use ( : )

You can then always use "self" within a function to refer to the current "instance" of the class for accessing variables and related functions.

gyrosp · November 2012

Thanks for your answers, it's getting clearer now

.

But I have two more questions left

...

1. Why don't I need

"require ("Class/Player")"

? Is this some sort of 'Gideros-Magic'?
2. So I don't need to declare my class variables in Lua? I just write self.myVar in my class, this is all?

Mells · November 2012

Hi @gyrosp

1. Yes, unless you specifically 'Exclude from execution" your file (right click on your file in the project hierarchy)
2. The following works :

self.myVar = 1
self.myVar = "test"
local x, y, z = 2, 4, 6
self.myVar = {1, "apple", {x, y, z}}

gyrosp · November 2012

This seems to be a great forum

. Thank you all very much for your fast and competent answers

!

plamen · November 2012

@gyrosp Lua gives unbelievable freedom and with freedom comes responsibility

gyrosp · November 2012

@gyrosp Lua gives unbelievable freedom and with freedom comes responsibility

You are so right

techdojo · November 2012

1. You don't need "require" because the Gideros IDE/Engine will run all of the scripts in the project in the order specified (see the execution order tab), so effectively they will have already been "loaded" by the time your code runs.

If you "exclude" a file from execution, then it won't be run and Lua will not know anything about your file ( and will then need "requiring" )

2. When Lua run's "self.foo = 10" it''s actually setting the variable "foo" in the "current" table to equal "10", if that variable doesn't exist then it's added to the table, so effectively by running that statement you are both "declaring" and initialising the variable in a single step.

Because your working on "self" ie the "instance" of the table that's created when you call Class.new() then each "instance" of the table will be initialised to contain the same variables.

You mentioned "class" variables, in this instance I'm talking about "instance" variables - one's where each object "instance" has their own separate copy. I would normally use the term "class" variable to mean a (in C++ terms) static variable, one that is shared across all instances of a class. You can create a static class variable by simply declaring a variable as being "local" at the top of the file, this will also mean it can only be accessed from within the file (which effectively makes it "private" )

see the following...

-- File - MyPlayerClass.lua
 
MyPlayer = Core.class(Sprite)      -- I usually derive all onscreen objects from a "Sprite"
 
-- these variables are accessible by ANY instance of a MyPlayer object, 
-- changing the value in one instance will mean ALL instances see the new value
-- also because they are "local" to this file, no other object can see or access them!
 
local privateSharedVarA = 1      
local privateSharedVarB = 2
 
-- again a "static" function, accessible to any MyPlayer instance, might be more efficient
-- as there is no table lookup to perform, however this wouldn't be able to access 
-- MyPlayer instance variables unless they are passed as parameters...
 
local function privateFunctionA(a, b)
     return a+b
end
 
-- this static function CAN access MyPlayer instance variables as "self" is passed as a 
-- parameter - as it's NOT a table function there is no . or : malarky to worry about
-- but YOU do have to pass "self" (ideally) as the first parameter
 
local function privateFunctionB(self,a)
     print(self.health,a)
end
 
-- This is the "constructor" function, any parameters passed to the "new" function will
-- need to be declared, as Sprite.new() doesn't take a parameter it easy to extend 
-- the class and still retain the Sprite functionality, it gets a little more complex if you
-- try and extend say a Bitmap or other class that takes parameters as I'm not 100% 
-- sure what happens to the parent (super) class or if the parent (super) class 
-- constructor is automatically called.  So to make like easy for yourself, just 
-- extend Sprite if need be.
 
function MyPlayer:init(startingHealth)
    -- declares and initialises the current instance variables
    self.health = startingHealth 
end
 
-- This is a normal function, the "self" is implied as a "hidden" first parameter as it's 
-- declared using : 
 
function MyPlayer:update(xyz)
    local val = privateFunction(self.health, privateSharedVarA)
    self.health = (self.health + xyz) - val
 
    privateFunctionB(self,privateSharedVarB)
 
end

Ideally each "class" should be in a separate file, which if it's added to the Gideros project won't need requiring

So to use this in main you could write.

    -- starting health - note the . here as new() actually returns a table that is
   --  "filled in"  automatically by the :init() function.
   local p1 = MyPlayer.new(100)    
 
    -- these two function do EXACTLY the same thing!
    p1:update(4)         -- note the : here
    p1.update(p1,4)    -- note the . here

Hope this helps!

techdojo · November 2012

Also remember - because of Lua's "untyped / don't declare variables before you use them" open nature, if you ever misspell a variable name it'll just assume your talking about a new one and promptly (silently) create it for you.

This is usually the number 1 source of "bugs" in Lua apps! - remember what @plamen said.

gyrosp · November 2012

Ok, I think I have to get used to not declaring my vars

. I am a friend of explicit declaration to avoid such bugs as techdojo mentioned.

I stumbled about another thing:

I have read a post about class inheritance with gideros (http://www.giderosmobile.com/documentation/classes_in_gideros.html) but have some issues I didn't get.

I have following inheritance:

--[[
 filename : ZombieBase.lua
--]]
 
ZombieBase = Core.class()
 
function ZombieBase:init()
    self.health = 100
    self.speed = 3
end
 
function ZombieBase:startWalking()
end
 
function ZombieBase:stopWalking()
end

--[[
 filename : SlowZombie.lua
--]]
 
SlowZombie = Core.class(ZombieBase)
 
function SlowZombie:init()
end

local slowZombie = SlowZombie.new()
slowZombie:startWalking()

Executing this throws following error

main.lua:14: attempt to call method 'startWalking' (a nil value)
stack traceback:
	main.lua:14: in main chunk

Why does the class Zombie does not inherit the function startWalking() from ZombieBase?

Edit:
What is the tag I need to use to format my code like techdojo did

?

vitalitymobile · November 2012

@gyrosp
If you want to display your code, you can use < pre lang="lua" > Your code </ pre>

(remember to clear blank on tags)

local slowZombie = SlowZombie.new()
slowZombie:startWalking()

gyrosp · November 2012

@vitalitymobile: Thanks

...

Any idea why the inheritance is not working?

OZApps · November 2012

@gyrosp,
when you call the

 local slowZombie = SlowZombie.new()

you are not returning anything in your

function ZombieBase:init()

just add the line

return self

and it shall work for you

cheers,

Mells · November 2012

Edit : my mistake

gyrosp · November 2012

@OZApps + @Mells: Neither is working

Mells · November 2012

@gyrosp
Can you create a test file? No doubt that your problem will be solved in no time.
Looking at all the answers you got above, isn't this community amazing?

gyrosp · November 2012

@Mells: Yes, it is

.

I'm sorry but I installed Gideros just today. What do you mean with test file? My project source files?

Mells · November 2012

@gyrosp
you create a very simplified version of your two classes + main.lua so we can see better where the problem is.
You just have to duplicate your project and remove unnecessary files.

gyrosp · November 2012

Ok, these are my classes and the main.lua file. It's basically the same code I've posted above.

vitalitymobile · November 2012

https://docs.google.com/document/pub?id=149g_P1YlE4_6v_hHbFx3YAaYpdU0HoM70XJSLTE38ww#h.vwgiperksgpn

Right click to SlowZombie and add Code Dependencies (Check on ZombieBase)

And ... 'Gideros-Magic' will happen :P

Mells · November 2012

"Voilà"

gyrosp · November 2012

:O ... thanks a lot, this is one of those annoying beginner problems you'll face only once

techdojo · November 2012

Glad you got it working

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