Inheritance for metamethods in Lua

Question

i very much like how object oriented programming is described in "programming in lua" 16.1, 16.2:

http://www.lua.org/pil/16.1.html

http://www.lua.org/pil/16.2.html

and would like to follow this approach. but i would like to take things a little further: i would like to have a base "class" called "class", which should be the base of all subclasses, because i want to implement some helper methods there (like "instanceof" etc.), but essentially it should be as described in the book:

function class:new(o)
    o = o or {}
    setmetatable(o, self)
    self.__index = self
    return o
end

now to my problem:

i would like to have a class "number", which inherits from "class":

number = class:new()

i would like to define metamethods for operator overloading (__add, __sub, etc.) in this class, so something like:

n1 = number:new()
n2 = number:new()

print(n1 + n2)

works. this is not really a problem. but now i would like to have a third class "money", which inherits from "number":

money = number:new{value=10,currency='EUR'}

i introduce some new properties here and such.

now my problem is, that i don't get things to work, that "money" inherits all methods from "class" and "number" including all metamethods defined in "number".

i've tried several things like overwriting "new" or modifying metatables but i was not able to get things to work, without either loosing the methods of "class" in "money" or loosing the metamethods of "number" in "money"

i know, that there a lot's of class implementations out there, but i would really like to stick with the minimal approach of lua itself.

any help would be very much appreciated!

thanks very much!

Answer 1

I think that the problem you are having is due to the fact the the operator metamethods are looked up using something similar to rawget(getmetatable(obj) or {}, "__add"). Thus the operators are not inherited with along with the other functions.

I have had some success with having the new function copy the operators like this:

function class:new(o)
    o = o or {}
    setmetatable(o, self)
    self.__index = self
    local m=getmetatable(self)
    if m then
        for k,v in pairs(m) do
            if not rawget(self,k) and k:match("^__") then
                self[k] = m[k]
            end
        end
    end
    return o
end