Confused about how Metatables work?

[E_Esty (cactus)]

I want to make metatables that whenever something gets inserted the game will refresh the table (right now I’m just using print to see if __newindex will actually work but apparently whenever I try to index this table the table keys become nonexistant).
But for some reason when ever I do that the entire table gets destroyed?
No clue how setmetatable.
Still clueless but I have searched everyone for this and I don’t know what comes else.
Metatables in metatables work right?

[Jaycbee05 (Jaycbee)]

What do you mean by refresh the table?, and could you provide the script you are using if possible?

[E_Esty (cactus)]

Lets just say it prints something out for now, what I mean by refreshing the table is that there is something in my game’s script that is suppose to recieve a table and it will reprocess it.

[MetaProx]

What is the specific use case of this Metatable? It sounds like what you’re doing can easily be pulled off without using a Metatable.

[Jaycbee05 (Jaycbee)]

Ok so i think the issue (although i don’t have any reference to the specific code you are using ) is you are not using setmetatable, like if i were to do this:

local Table = {}

setmetatable(Table, Table)


function Table.__newindex()
   print('a') -- a would print 
end

Table[5] = 3

When a table is created it has not metatable and in order to use metamethods you would have to give it a meta table using setmetatable(Table,Table)

This thread goes more in depth into this:

What are metatables for and how to understand them?

Hello, Developers I came here to ask how do metatables function? I am developing on Roblox for a 2 years and I always wanted to learn metatables but sadly I could not because I did not find any clear explanation of them anywhere. Because that DevForum is full with better and more experienced developers than me, I want to hear your guys suggestions how can I start using them.

[E_Esty (cactus)]

i am using metatables correctly btw heres my code

	ThisWindow.CONFIG = {}
setmetatable(ThisWindow.CONFIG, {__newindex = function()
	print("how")
end})
ThisWindow.CONFIG.Icon = ""
ThisWindow.CONFIG.Position = "Preset"
ThisWindow.CONFIG.Name = "Unamed Window"
ThisWindow.CONFIG.Size = UDim2.new(0,250,0,250)
print(table.unpack(ThisWindow.CONFIG))

The thing prints correctly, however all of the tables keys become nil.
so ThisWindow.CONFIG.Size = nil.

note: this is inside a meta table
i have no idea why this doesn’t work?

[MetaProx]

My question still stands, I’m really betting you’re not needing a metatable for this.
What is this for?

[E_Esty (cactus)]

This is for my windows system thing.
I want it to print something (for now) whenever a value is changed.
This is the only way I know how to check if a value is changed in a table.

[MetaProx]

Ah okay, so for debugging? For the most part you can easily just print variables inside of tables without needing to go through all this effort.

Edit: I’m really not getting this.

Edit 2: Oh wait hold on, I kinda see the need for this.

[Jaycbee05 (Jaycbee)]

You have to understand metamethods for this, you would have to use the arguments passed to the __newindex(Table,index, Value) to set to a new/different table. Because when you do this ThisWindow.CONFIG.Position = "Preset" it’s not actually setting ThisWindow.CONFIG.Position to anything instead it’s being “sent” to your new index function

[aschepler]

Note that when Lua finds a metatable function, it uses that INSTEAD of the default behavior. So ThisWindow.CONFIG.Icon = "" calls the __newindex function but doesn’t actually put a value in the ThisWindow.CONFIG table.

To put the value in the table in addition to your print (without causing an infinite recursion back into __newindex), use rawset:

setmetatable(ThisWindow.CONFIG, {__newindex = function(t, k, v)
    print("how")
    rawset(t, k, v)
end})