Lua error attempt to index global nil value

Question

For a game using CoronaSDK I'm trying to implement some OOP in Lua.

If I run the following code in the Corona simulator:

-- Terrain Sprites base

local TerrBase = {}
local TerrBase_mt = {_index = TerrBase}

function TerrGrass_mt.baseAdd(a,b)
    print(a+b)
end



-- Terrain Sprites - Grass

local TerrGrass = {}

function TerrGrass.new()
    local grass = {}
    setmetatable(grass,TerrBase_mt)
    return grass
end

function TerrGrass.add(a,b)
    print(a+b)
end

function TerrGrass.sub(a,b)
    print(a-b)
end

function TerrGrass.mul(a,b)
    print(a*b)
end

function TerrGrass.div(a,b)
    print(a/b)
end

--

local grass = TerrGrass.new()

grass.add(5,7)

I get this message:

What is the reason for this error?

Piglet · Accepted Answer · 2019-03-31 15:10:41Z

2

local TerrBase = {}
local TerrBase_mt = {_index = TerrBase}

function TerrGrass_mt.baseAdd(a,b)  -- <---- ERROR
    print(a+b)
end

You're attempting to index a table named TerrGrass_mt. But the table you defined is named TerrBase_mt.

TerrGrass_mt is a nil value, hence the error.

Let's have a look at the error message:

main.lua 12: attempt to index global 'TerrGrass_mt' (a nil value).

This tells you that the error occured in line 12 of the file main.lua.

You attempted to index (use the index operator .) on a global named TerrGrass_mt. This tells you that the problem is is TerrGrass_mt.and that TerrGrass_mt is nil.

Something like

function a(b) c = b.d end
a()

would give you an error for indexing a local nil value b because here b's scope is local.

Whenever you get an error for using nil values in any way you have to find out why the value is nil and either fix that or in some cases replace it with a default value.

edited Mar 31, 2019 at 15:10

answered Mar 31, 2019 at 14:56

Piglet

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4 Comments

Locohost Over a year ago

Thank you for the detailed reply! I googled a bit more and found a good Lua tutorial that I got working. I edited/added the working code in the 4 Lua scripts to OP. I greatly appreciate your reply and am accepting it as the answer. Thanks again :-)

Piglet Over a year ago

@Locohost well your problem was a typo. Adding different code as a workaround doesn't help anyone.

Locohost Over a year ago

You removed all the important information (mainly the properly formatted and working OOP code!) and edited this post to death. This isn't even my post any more.

Piglet Over a year ago

@Locohost well I removed anything that was irrelevant to the problem you wanted help on. your question was not how to implement OOP in Lua but why you get that error message and what you were doing wrong. That quesiton is still there and so is your errorneous code. please read How to Ask. it's all about focus! I didn't mean to hurt your feelings by improving your post according to the guidelines. the next guy who faces that error doesn't care about how long you've been into Lua, how much JavaScript you know or how to implement OOP in Lua. they want examples for indexding global nil values.

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