Lua Object-Orientation Changing metamethods of an object

Example

Having

local Class = {}
Class.__meta = {__index=Class}
function Class.new() return setmetatable({}, Class.__meta)

Assuming we want to change the behavior of a single instance object = Class.new() using a metatable,

there are a few mistakes to avoid:

setmetatable(object, {__call = table.concat}) -- WRONG

This exchanges the old metatable with the new one, therefore breaking the class inheritance

getmetatable(object).__call = table.concat -- WRONG AGAIN

Keep in mind that table "values" are only reference; there is, in fact, only one actual table for all the instances of an object unless the constructor is defined as in ¹, so by doing this we modify the behavior of all instances of the class.

---

One correct way of doing this:

Without changing the class:

setmetatable(
    object,
    setmetatable(
        {__call=table.concat},
        {__index=getmetatable(object)}
    )
)

How does this work? - We create a new metatable as in mistake #1, but instead of leaving it empty, we create a soft copy to the original metatable. One could say the new metatable "inherits" from the original one as if it was a class instance itself. We can now override values of the original metatable without modifying them.

Changing the class:

1st (recommended):

local __instance_meta = {__index = Class.__meta}
-- metatable for the metatable
-- As you can see, lua can get very meta very fast
function Class.new()
    return setmetatable({}, setmetatable({}, __instance_meta))
end

2nd (less recommended): see ¹

---

¹ function Class.new() return setmetatable({}, {__index=Class}) end