metatables lua code example

Example 1: lua metatable

--[[
	Metatables allow you to, among other things, redefine operators.
	Let's demonstrate with a simple vector "class".
--]]

Vector = {}
Vector.mt = {} -- The metatable we will be using

function Vector.new(x, y)
	local vec = {}
	setmetatable(vec, Vector.mt) -- "Link" the vector with the metatable
	vec.x = x
	vec.y = y
	return vec

function Vector.add(u, v) -- Return the resulting vector of u+v
	local result = Vector.new(u.x, u.y)
	result.x = result.x + v.x
	result.y = result.y + v.y
	return result
    
--[[
    This is where metatables get interesting, as we redefine the + operator
    for vectors.
--]]
Vector.mt.__add = Vector.add
    
--[[
    There are multiple predefined operators you can replace, such as:
    __add(u+v), __sub(u-v), __mul(u*v), __div(u/v), __mod(u%v), 
    __unm(-u), __concat(u..v), __eq(u==v), __lt(u<v), __le(u<=v),
    __len(#u), etc...
--]]

Example 2: lua setmetatable

table = {}
metatable = {}
setmetatable(table, metatable)