Love 2D(Lua) unexpected nil in object

Question

Trying to get my feet wet with LUA and Love2D, and I am having an issue with object instantiation / access in Lua.

Source with the bug can be found here: https://bitbucket.org/dannsee/love_scrollingshooter

I am In my main, I create an object, Enemies

enemies = Enemies:new()

and inside of the enemies object, i create an object to hold peristant values, which I am calling Timers.

timers = Timers:new()

So the enemies 'constructor' method looks (basically) like this

Enemies = {} -- so that Enemies will not be nil when new() is called
timers = {} -- so that timers will be accessible in the class scope

function Enemies:new(enemies)
  enemies = enemies or {}
  timers = Timers:new()
  setmetatable(enemies, self)
  self.__index = self
  return enemies
end

while the Timers being created are looking as such

Timers = {} -- so that Timers will not be nil when new() is called

function Timers:new(timers)
    timers = timers or {
      miniBombTimerMax = 0.2,
      miniBombTimer = minibombTimerMax
    } 
    setmetatable(timers, self)
    self.__index = self
    return timers
end

But when I try to refrence one of the timers ( from inside the enemies object) , I am getting a nil value exception.

timers.miniBombTimer -- Produces nil exception

It seems to me that this should both 1. be in scope, since it is an object created inside this class, and is instantiated locally as timers = {} before it is assigned a value, and 2. not nil becuase it is being given a value in the 'constructor'. But it seems there is more going on here that I am not grasping.

I am new to Lua, which may be obvious at this point, but from what I have read about variable scope it seems that this should be valid. I don't understand why the timers are not being created with values.

Read through lua-users.org/wiki/ObjectOrientationTutorial? You didn't show the part of the code with the problem, but presumably you're not using self? — user94559
– user94559, Commented Aug 21, 2017 at 16:24
Your constructors are also not using self. I'm assuming you want timers to be a member of the Enemies class, but maybe not? Perhaps you can share a minimal example of how you intend to use this class. — user94559
– user94559, Commented Aug 21, 2017 at 16:36

Sophie Swett · Accepted Answer · 2017-08-21 21:19:03Z

Careful with your globals! In Lua, it's very easy to accidentally set a global variable when you don't mean to, and it looks like that's exactly what's happening.

function Enemies:new(enemies)
  enemies = enemies or {}
  timers = Timers:new()
  setmetatable(enemies, self)
  self.__index = self
  return enemies
end

On the third line here, since timers doesn't exist as a local variable here, this value ends up getting put into a global variable called timers instead. If you want to set a property of enemies, you need to mention enemies explicitly:

function Enemies:new(enemies)
  enemies = enemies or {}
  enemies.timers = Timers:new()
  setmetatable(enemies, self)
  self.__index = self
  return enemies
end

Now, you write:

But when I try to refrence one of the timers ( from inside the enemies object) , I am getting a nil value exception.

Lua doesn't really have any concept of being "inside an object" or "inside a class". In some languages, when you're writing code inside of a class, all of the class's members are in scope and you can refer to them "bare". Lua is not one of those languages; in Lua, if you want to refer to a "class member", you need to use the dot notation, explicitly stating which object you're accessing. (Or you can do the "advanced method", using _ENV.)

By the way...

timers = {} -- so that timers will be accessible in the class scope

From what I see in the question, this line doesn't do much; it just creates a global variable which is never used.

Also, this line in Enemies:new:

self.__index = self

This just sets Enemies.__index every time Enemies:new is called. This is fine, but you may as well just set it once:

function Enemies:new(enemies)
  enemies = enemies or {}
  enemies.timers = Timers:new()
  setmetatable(enemies, self)
  return enemies
end

Enemies.__index = Enemies

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