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What is a meta-table, and why would you use it?

Asked by

maxpax2009 340

4 years ago

So, i have heard a lot of people talking about meta-methods, and newproxy and stuff.

Why would you use it, and how do you use it? Thanks, for reading.

This is a very large subject that introduces OOP, and is generally too broad to simplify in a short time. I suggest searching existing posts within this website, or researching through different sources. Ziffixture 6913 — 4y

Okay then. maxpax2009 340 — 4y

https://scriptinghelpers.org/questions/102848/what-is-__index-and-what-happens-when-you-set-it-to-a-table94975 JesseSong 3916 — 4y

1 answer

Answered by

Elyzzia 1294

4 years ago

Edited 4 years ago

a metatable allows you to specify special behavior when you do different things to a table with metamethods

a metamethod is just a function that runs when you do that thing to the table

for example, __index is a metamethod that runs whenever you index a table with a nonexistent key

1local metatable = {}
2metatable.__index = function(table, key) -- this has a table parameter since you can give multiple tables the same metatable
3    return "The key " .. key .. " does not exist!"
4end
5 
6local table = {}
7setmetatable(table, metatable)
8 
9print(table.burger) --> The key burger does not exist!

you can also set __index to a table, in which case it will attempt to index that table instead

1local fallbackTable = {}
2fallbackTable.burger = "nom nom nom"
3local metatable = {}
4metatable.__index = fallbackTable
5local table = {}
6setmetatable(table, metatable)
7 
8print(table.burger) --> nom nom nom

there are a lot more metamethods, like __eq

01local metatable = {}
02metatable.__eq = function(a, b)
03    if a.HasBlocks == b.HasBlocks then
04        return true
05    end
06end
07 
08local roblox = {}
09roblox.HasBlocks = true
10roblox.Awesome = true
11local minecraft = {}
12minecraft.HasBlocks = true
13minecraft.Awesome = false
14setmetatable(roblox, metatable)
15setmetatable(minecraft, metatable)
16 
17print(roblox == minecraft) -- true, even though roblox and minecraft are different tables

and also __tostring

01local metatable = {}
02metatable.__tostring = function(tbl)
03    return "This burger is " .. tbl.Awesomeness * 100 .. "% awesome!"
04end
05 
06local awesomeBurger = {}
07awesomeBurger.Awesomeness = 0.6
08setmetatable(awesomeBurger, metatable)
09 
10print(tostring(awesomeBurger)) --> This burger is 60% awesome!

you can find most metamethods and what they do here https://developer.roblox.com/en-us/articles/Metatables

while metatables aren't exclusively used for OOP, OOP is highly reliant on them, so it's good to familiarize yourself with them

What is a meta-table, and why would you use it?

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