Class Function returns wrong values - Lua

Question

I make program in lua where I create a class that stores 3 values and a function that can return the "x" value. This is the code:

Vector3 = {}
Vector3.__index = Vector3

function Vector3:new(x,y,z)
  local instance = 
  {
    x = x or 0,
    y = y or 0,
    z = z or 0
  }
  setmetatable(instance, Vector3)
  return instance
end

function Vector3:printX()
  return self.x
end

And this is my main.lua code:

require "Vector3"

vec1 = Vector3.new(1,2,3)
vec2 = Vector3.new(5,1,0)

print(vec1:printX())

This what this line does

vec1 = Vector3.new(1,2,3)

Is create a new class and assign 1 to x, 2 to y, and 3 to z. But when I print out the value returned by printX(), it gives me 2. If I change it to print out the y value, it gives me 3. And if I change it to print out the z value, its nil. Is there anything wrong with my code?

I want it to return the correct values.

function Vector3:new( means the first parameter of your constructor is self which you seemingly did not intend. You can just declare Vector3.new. — tehtmi
– tehtmi, Commented May 1, 2023 at 20:46
Not sure what you mean, but you aren't using the self parameter. You don't need : to create the instance and return it as you are doing. You could alternative call new as Vector3:new to pass Vector3 as the self parameter, but this is also unnecessary. — tehtmi
– tehtmi, Commented May 1, 2023 at 21:41
Crap. I'm dumb. I didn't realize that in your first comment you had a "." instead of a ":". Thanks that helped @tehtmi — TheJalfireKnight
– TheJalfireKnight, Commented May 2, 2023 at 3:25

Luatic · Accepted Answer · 2023-05-02 06:52:58Z

As tehtmi has pointed out, function Vector3:new(x,y,z) is equivalent to function Vector3.new(self,x,y,z). The call Vector3.new(1,2,3) thus sets self = 1, x = 2, y = 3 and z = nil; 2 is printed.

There are two ways to fix this:

Fix your constructor to be simply Vector3.new, removing the self parameter.
Fix your call to be Vector3:new, passing self = Vector3. This obsoletes the Vector3 global variable access in the constructor; setmetatable(instance, Vector3) can then be replaced with setmetatable(instance, self).

If you rewrite the constructor slightly, you get the standard "class" template outlined in the "Programming in Lua" book:

function Vector3:new(x,y,z)
  local instance = 
  {
    x = x or 0,
    y = y or 0,
    z = z or 0
  }
  setmetatable(instance, self)
  self.__index = self -- note the addition of this line
  return instance
end

You can then remove the Vector3.__index = Vector3 line since this will be done in the constructor every time you call Vector3:new(...) (arguably questionable performance-wise). The advantage of this approach is how it deals with inheritance: Inheritance is implemented simply as instantiation; you do not need to take care of writing a new constructor. Say you want a class Scale to extend your Vector3 class:

Scale = Vector3:new() -- a scale is a vector ("inherits")
function Scale:scale(vec) -- scale a vector (componentwise multiplication)
    return Vector3:new(self.x * vec.x, self.y * vec.y, self.z * vec.z)
end

Usage:

-- Instantiation:
-- This wouldn't work if you used `.`
-- since then `Scale.new` would be just `Vector3.new`
-- which sets `Vector3` and not `Scale` as the metatable,
-- but since the `:` passes `Scale`/`Vector3` as `self`, everything is fine.
local s = Scale:new(1, 2, 3)
local v = s:scale(Vector3.new(4, 5, 6))
print(v.x, v.y, v.z) -- 4, 10, 18

You ultimately have to decide whether you prefer constructors looking up their class through an upvalue or global variable or having it passed as the self parameter.

As a rule of thumb, if you define a function using :, you should probably call it using :. Such functions are often called "methods". See difference between . and : in Lua.

Besides, I would rename printX to getX or remove it entirely (since it is currently redundant with just directly accessing x). Note also that default values are often implemented by sticking them into the class table (in your case Vector3.x, Vector3.y, Vector3.z = 0, 0, 0 would allow you to eliminate the or 0's in the constructor).

Thank you for this in depth explanation. I now understand this better.

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