Lua - trying to create a good Vec2 class - oop

Question

I'm learning how to use Lua and Love2d and I want to create a Vec2 class using metamethods and metatables. This is what I have so far:
class.lua: (The base class files)
local Class = {}
Class.__index = Class
-- Constructor
function Class:new() end
-- Inherite from Class
-- type = The name of the new class
function Class:derive(type)
print("Class:", self)
local cls = {}
cls["__call"] = Class.__call
cls.type = type
cls.__index = cls
cls.super = self
setmetatable(cls, self)
return cls
end
function Class:__call(...)
local inst = setmetatable({}, self)
inst:new(...)
return inst
end
function Class:getType()
return self.type
end
return Class
vec2.lua
local class = require "class"
local Vec2 = class:derive("Vec2")
function Vec2:new(x, y)
self.x = x or 0
self.y = y or 0
getmetatable(self).__add = Vec2.add
end
function Vec2.add(a, b)
local nx, ny
nx = a.x + b.x
ny = a.y + b.y
return Vec2:new(nx, ny)
end
return Vec2
and in my main.lua I have:
local v1 = Vec2:new(10, 10)
local v2 = Vec2:new(5, 3)
local v3 = v1 + v2
print("v3:", v3.x, v3.y)
and I get this error:
Error: main.lua:12: attempt to perform arithmetic on local 'v1' (a nil
value)

Answer 1

Vec2.new does not return a value. Hence the assignment local v1 = Vec2:new(10,10) results in v1 being nil
Try local v1 = Vec2(10,10) instead. Same mistake in Vec2.add
The instance is created in the __call metamethod which calls new with your parameters. You're not supposed to call new directly unless you want to re-initialize an existing instance.
function Class:__call(...)
local inst = setmetatable({}, self)
inst:new(...)
return inst
end