Attempt to index local 'self' (a nil value)

Question

I have a problem with classes. I got below error: Attempt to index local 'self' (a nil value) When I call the getter method of below class. Item.lua file:

require "classlib"
Item = class("Item")

function Item:__init()
    self.interval = 1
end

function Item:getInterval()
    return self.interval
end

I'm calling this getter function like this:

dofile("../src/item.lua")

item = Item()

function test_item()
    assert_equal(1, item.getInterval())
end

What's the problem here?

Kind regards...

Call item:getInterval() instead of item.getInterval() inside test_item(). — Omri Barel
– Omri Barel, Commented Dec 28, 2012 at 12:14
Might be useful to tell what is this "classlib". Notice you have unmatching quotes (single vs. double) in the class() call. And @OmriBarel should probably post an answer, instead of a comment... :-) — PhiLho
– PhiLho, Commented Dec 28, 2012 at 12:35

Bartek Banachewicz · Accepted Answer · 2017-01-16 07:23:17Z

46

In general, you should call member functions by :.

In Lua, colon (:) represents a call of a function, supplying self as the first parameter.

Thus

A:foo()

Is roughly equal to

A.foo(A)

If you don't specify A as in A.foo(), the body of the function will try to reference self parameter, which hasn't been filled neither explicitly nor implicitly.

Note that if you call it from inside of the member function, self will be already available:

-- inside foo()
-- these two are analogous
self:bar()
self.bar(self)

All of this information you'll find in any good Lua book/tutorial.

edited Jan 16, 2017 at 7:23

answered Dec 28, 2012 at 12:46

Bartek Banachewicz

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Comments

Alar · Accepted Answer · 2012-12-29 09:42:30Z

1

the obj:method is just syntactictal sugar for:

definition:

function obj:method(alpha) is equivalent to obj.method(self,alpha)

execution:

obj:method("somevalue") is equivalent to obj.method(obj,"somevalue") Regards

answered Dec 29, 2012 at 9:42

Alar

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Comments

coanor · Accepted Answer · 2013-03-31 17:07:10Z

-2

Change:

assert_equal(1, item.getInterval())

to:

assert_equal(1, item:getInterval())

In Lua, it was some ridiculous for error reporting. From class point of view, the .getInterval() method should called with a self parameter, while the :getInterval() method is implicitly included the self parameter. And the syntax error should labeled in the called point, not the definition-body of getInterval().

In traditional, while you miscalled a method, it was not the method's fault, but the caller.

answered Mar 31, 2013 at 17:07

coanor

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1 Comment

Bartek Banachewicz Over a year ago

Your answer doesn't really bring any new information in, and it's partially incorrect. the syntax error should labeled in the called point - it would be, but it's not a syntax error. o.f() is still valid Lua code, it just does something different from the colon call.

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