Can LUA meta tables assist in detection of nilling object?

Question

I was wondering if you can detect the nilling of an object through meta tables ?

foo = {}
foo_mt = {
    __newindex = function (t, k, v)
        print (k, v)
        rawset (t, k, v)
      end
}   

setmetatable (foo, foo_mt) 

foo ['oof'] = 3

outputs: oof  3

foo ['oof'] = nil

__newindex will not be called, so is there another meltable method ?

You need to create a [proxy object](https://www.lua.org/pil/13.4.4.html) — Egor Skriptunoff, Mar 23 '19 at 07:22
Thanks, I had gone there but had messed the syntax. Good reference. — Aardvark, Mar 23 '19 at 08:23

2dengine · Answer 1 · 2020-08-29T05:45:45.923

Just to clarify, __newindex is only triggered when the previous value is nil. So the following code will trigger __newindex twice:

foo = {}
foo_mt = {
    __newindex = function (t, k, v)
        print (k, v)
        rawset (t, k, v)
      end
}   

setmetatable (foo, foo_mt) 

foo['oof'] = nil
foo['oof'] = nil

If foo['oof'] already has a non-nil value then __newindex will not be called (because the index already exists).

But yea, you need something like an empty proxy table if you want to catch all modifications to your table like Egor pointed out.

proxy = {}
foo = {}
foo_mt = {
  __newindex = function (t, k, v)
    print (k, v)
    rawset (proxy, k, v)
  end,
  __index = function(t, k)
    return rawget(proxy, k)
  end
}
setmetatable (foo, foo_mt) 

foo['ok'] = true
foo['ok'] = nil

Can LUA meta tables assist in detection of nilling object?

1 Answers1