attempt to index global 'world' error

Dikkesnoek · August 2012

Hello,

I think that I still don't understand Lua and Classes completely.
In the main.lua I specify a world globally like:

world = b2.World.new(0, 9.8, true)

I have a class called Block (block.lua). In this class I will handle
touches and mouse events. Also in this class I need to handle
physics. When I release the mouse button for example, the block
needs to drop to the floor due the gravity. When I define at the
beginning of block.lua:

Block = Core.class(Sprite)
 
local dummyBody = world:createBody({})

in the Block class I will get the error:

attempt to index global 'world'

I wan't to use one world so I thought that this was the right
approach. Any idea's?

Thanks,

Marc

zvardin · August 2012

In your Block class if you need access to the world, I would create a parameter to pass your game/world object in to keep a reference, so something like:

function Block:init(world)
    self.world = world
end

Dikkesnoek · August 2012

Thanks, I will try this. Is there no way to access global variables
from the main file in your class file(s) and vice versa? Do you
always need to pass these values via function parameters?

Regards,

Marc

petec · August 2012

Try including main.lua in the code dependency of block.lua (right click on the block.lua file in the project panel to access code dependencies). That might do it using world as a global.

ar2rsawseen · August 2012

exactly as @petec said. For now there is a pretty random order (probably order of creation) how lua files are read and processed. So probably Block class file is executed before main file, so you are trying to use variable which is not yet defined.
Usually this is not a problem, because you can define class, but use it only later.
So for example if you'd defined world usage inside call constructor, like:

Block = Core.class(Sprite)
function Block:init()
    local dummyBody = world:createBody({})
end

Then it would be valid, and you could use your class afterwards (for example in the end of main.lua file) like:

local block = Block.new()

Dikkesnoek · August 2012

Thanks, I tried both. The first option seems to work, the second one
will give errors in other classes I'll use.

What I don't get is that:

function Block:init(world)
	-- Get the world.
	self.world = world
	-- Create empty Box2D body for touches and mouse.
	dummyBody = self.world:createBody({})
 
	-- etc --
end

is ok and,

function Block:init(world)
	-- Create empty Box2D body for touches and mouse.
	dummyBody = world:createBody({})
 
	-- etc --
end

is not ok.

I know that it's Lua and not C, but....

Marc

petec · August 2012

As @ar2rsawseen said, I think it's to do with the order in which the lua files are being executed, but I see what you are getting at. Why does 'world' get picked up OK when you use self.world=world in the first example but not in the second example? I'm afraid I have no idea. I just know that I've had to use code dependencies occasionally to get things working.

ar2rsawseen · August 2012

Actually I have no idea either. But there must be some logical explanations. There must be one!!!

Scouser · August 2012

@Dikkesnoek: did you try @petec's suggestion above?

Try including main.lua in the code dependency of block.lua (right click on the block.lua file in the project panel to access code dependencies). That might do it using world as a global.

That should work. You must set the checkbox next to main.lua in the code dependencies list for every class that accesses world.

Dikkesnoek · August 2012

Hi,

Yes I tried that. It gave me errors in other classes. These classes are not
directly dependent of main.lua.

Marc

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