lua 2d/3d point class

--
-- define Point class for LUA
--
Point = {}
Point.key_list = {"x","y","z"}

function Point.__index(t,key)
  error("No such Point member or init:"..key ,2)
end

function Point.__newindex(t,key,value)
  for i = 1,#Point.key_list do
    if key == Point.key_list[i] then
       -- t[key] = value
       rawset(t,key,value)
       return
    end 
  end
  error("No such Point member :"..key ,2)
end

function Point.new(...)
 local pt = {}
 args = { ... }
 if #args == 3 then
   pt.x = args[1]
   pt.y = args[2]
   pt.z = args[3]
 elseif #args == 2 then
   pt.x = args[1]
   pt.y = args[2]
 else
    error("2D or 3D only" ,2)  
 end
 setmetatable(pt,Point)
 return pt
end

--
-- Usage
--

p1 = Point.new(1,2)
p2 = Point.new(3,4)

p1.x = 8

x1,y1=p1.x,p1.y
print(string.format("x1,y1=%d,%d",x1,y1))

-- p2 = p1

print(string.format("x2,y2=%d,%d",p2.x,p2.y))
--
local points = {p1,p2}
print("points[1].x=" .. points[1].x)
print("points[2].y=" .. points[2].y)

-- [[ below is two  error test
print("p1.z="..p1.z)  -- : No such Point member or init:z
p1.xxx = 567          -- : No such Point member :xxx
-- ]]

--
--[[ output
--
x1,y1=8,2
x2,y2=3,4
points[1].x=8
points[2].y=4
: No such Point member or init:z
: No such Point member :xxx
-- ]]