Inheritance with hump in love2D

Question

I'm trying to do OOP using the hump library in Lua for a game coded in löve 2D. Everything is working fine. However, when I try to play with my code the way bellow, a message error tells me that "self" is a nill value. Can someone tell me what I did wrong please?

Item=Class{

  init=function(x,y,size)
    self.x=x
    self.y=y
    self.size=size

    self.dx=dx
    self.dy=dy
    self.dx2=dx2
    self.dy2=dy2
  end;

  update=function(dt)
    self.dx=self.dx+self.dx2
    self.x=self.x+self.dx*dt
    self.dy=self.dy+self.dy2
    self.y=self.y+self.dy*dt
  end;

  coliide=function(ball)
    return math.sqrt((self.x-ball.x)^2+(self.y-ball.y)^2)<self.size
  end;

  reset=function()
    self.x=love.graphics.getWidth()/2
    self.y=love.graphics.getHeight()/2
    self.dy=0
    self.dx=0
    self.dy2=0
    self.dx2=0
  end

}

Thank you and regards

Answer 1

In the given snippet

Item = Class{}
Item.init=function(x,y,size)
    self.x = x
end

self is nil because you did not define it.

In order to do what you want you have to define the function like that:

Item.init = function(self, x, y, size)
   self.x = x
end

and call it like that

Item.init(Item, x, y, size)

Then self equals Item and you may index it without an error.

To make this a bit more convenient we can use something called Syntactic Sugar

Let's have a look into the Lua 5.3 Reference Manual:

3.4.10 - Function Calls

A call v:name(args) is syntactic sugar for v.name(v,args), except that v is evaluated only once.

3.4.11 - Function Definitions

The colon syntax is used for defining methods, that is, functions that have an implicit extra parameter self. Thus, the statement

 function t.a.b.c:f (params) body end 

is syntactic sugar for

 t.a.b.c.f = function (self, params) body end

Using this knowledge we can simply write:

function Item:init(x,y,size)
  self.x = x
end

and call it like so:

Item:init(x,y)

Answer 2

The implicit self argument is available to function when it was declared using colon syntax. E.g.:

Item=Class{}

function Item:init(x,y,size)
    self.x = x
    self.y = y
    -- ...
end

Alternatively you could just add self argument explicitly in your existing code. Just make sure you're calling it with colon syntax.