Metatables

The metatables for Strings and all ROBLOX types are locked; however, in normal Lua (not RBX.lua) you can set the metatables of these objects using the debug library.[1]

What is a Metatable?

Metatables allow tables to become more powerful than before. They are attached to data and contain values called Metamethods. Metamethods are fired when a certain action is used with the datum that it is attached to. You may think that if you have code like this:

local list = {1, 2}
print(list[3])

The code will search through the list for the third index in list, realize it's not there, and return nil. That's totally wrong. What really happens is the code will search through the list for the third index, realize it's not there, and then try to see if there's a metatable attached to the table, returning nil if there isn't one.

getmetatable and setmetatable

There are two main functions when dealing with metatables: getmetatable and setmetatable. You use setmetatable to give a table a metatable, and you use getmetatable to retrieve the metatable of an object. Here's an example:

local x = {}
local metaTable = {}       -- metaTables are tables, too!
setmetatable(x, metaTable) -- Give x a metatable called metaTable!
print(getmetatable(x))

Output:

table: [hexadecimal digits]

Metatable Demonstration

local list = {1, 2}
print("In table List, key \"z\" is "..tostring(list.z))

Output:

In table List, key "z" is nil

Now, look at what happens with metatables:

local list = {1, 2}       -- A normal table
local metatable = {       -- A metatable
    __index = function(t, key) 
        rawset(t, key, 0) -- Set t[key] to 0
        return t[key]     -- return t[key] (which is now 0)
    end
}
setmetatable(list, metatable) -- Now list has the metatable metatable
print("In table List, key \"z\" is "..tostring(list.z))

Output:

In table List, key "z" is 0

What happened there? list is a normal table, with nothing unusual about it. metatable is also a table, but it has something special: the __index metamethod. The __index metamethod is fired when t[key] is nil, or in this case, list.z is nil. Now, nothing would happen because the two tables (list and metatable) aren't linked. That's what the third line does: sets list's metatable to metatable, which means that when list is indexed (__index) at an index that's nil (list.z), the function associated with __index in Metatable is run. The function at __index in metatable uses rawset to make a new value in the Table (or list). Then, that value is returned. So, list.z is set to 0, and then list.z is returned (which is 0).

Metamethods

Metamethods are the functions that are stored inside a metatable. They can go from calling a table, to adding a table, to even dividing tables as well. Here's a list of metamethods that can be used:

• __index(Table, Index) — Fires when Table[Index] is nil.
• __newindex(Table, Index, Value) — Fires when Table[Index] = Value when Table[Index] is nil.
• __call(Table, arg) — Allows the table to be used as a function, arg is the arguments passed, Table(arg).
• __concat(Table, Object) — The .. concatenation operator.
• __unm(Table) — The unary – operator.
• __add(Table, Object) — The + addition operator.
• __sub(Table, Object) — The – subtraction operator.
• __mul(Table, Object) — The * mulitplication operator.
• __div(Table, Object) — The / division operator.
• __mod(Table, Object) — The % modulus operator.
• __pow(Table, Object) — The ^ exponentiation operator.
• __tostring(Table) — Fired when tostring is called on the table.
• __metatable — if present, locks the metatable so getmetatable will return this instead of the metatable and setmetatable will error. Non-function value.
• __eq(Table, Table2) — The == equal to operator˚
• __lt(Table, Table2) — The < less than operator˚
• __le(Table, Table2) — The <= operator˚
• __mode — Used in Weak Tables, declaring whether the keys and/or values of a table are weak.
• __gc(Object) — Fired when the Object is garbage-collected.
• __len(Object) — Fired when the # operator is used on the Object.

˚ Requires two tables; does not work with a table and a number, string, etc. The tables must have the same metatable.

Using Metatables

There are many ways to use metatables, for example the __unm metamethod (to make a table negative):

local table1 = {10,11,12}

local metatable = {
    __unm = function(t) -- __unm is for the unary operator -
        local negated = {} -- the table to return
        for key, value in pairs(t) do
            negated[key] = -value 
        end
        return negated -- return the negative Table!
    end
}
setmetatable(table1, metatable)
print(table.concat(-table1, "; "))

Output:

-10; -11; -12

Here's an interesting way to declare things using __index:

local t = {}

local metatable = {
    __index = {x = 1}
}

setmetatable(t, metatable)
print(t.x)

Output:

1

__index was fired when x was accessed from the table. __index then defined x as 1 instead of nil; therefore, 1 was returned.

Now you can easily do that with a simple function, but there's a lot more where that came from. Take this for example:

local table = {10, 20, 30}
print(table(5))

Now, obviously you can't call a table. That's just crazy, but (surprise, surprise!) with metatables you can.

local table = {10, 20, 30}

local metatable = {
    __call = function(table, param)
        local sum = {}
        for i, value in ipairs(table) do
            sum[i] = v + param -- Add the argument (5) to the value, then place it in the new table (t).
        end
        return unpack(sum) -- Return the individual table values
    end
}

setmetatable(table, metatable)
print(table(5))

Output:

15	25	35

You can do a lot more as well, such as adding tables!

local table1 = {10, 11, 12}
local table2 = {13, 14, 15}

for k, v in pairs(table1 + table2) do
    print(k, v)
end

This will error saying that you're attempting to perform arithmetic on a table. Let's try this with a metatable.

local table1 = {10, 11, 12}
local table2 = {13, 14, 15}

local metatable = {
    __add = function(table1, table2)
        local sum = {}
        for key, value in pairs(table1) do
            if table2[key] ~= nil then -- Does this key exist in that table?
                sum[key] = value + table2[key]
            else                       -- If not, add 0.
                sum[key] = value
            end
        end
        
        -- Add all the keys in table2 that aren't in table 1
        for key, value in pairs(table2) do
            if sum[key] == nil then
                sum[key] = value
            end
        end
        return sum
    end
}

setmetatable(table1, metatable)
setmetatable(table2, metatable)

for k, v in pairs(table1 + table2) do
    print(k, v)
end

Output:

1 23
2 25
3 27

If the two tables have two different __add functions, then Lua will go to table1 first and if it doesn't have an __add function, then it'll go to the second one. That means that you really only have to set the metatable of Table1 or Table2, but it's nicer and more readable to set the metatable of both.

Here is one last example breaking away from using separate variables when it isn't necessary.

local t = setmetatable({
    10, 20, 30
}, {
    __call = function(a, b) 
        return table.concat(a, b..' ')..b 
    end
})
print('Tables contains '..t(1)) --> Tables contains 101 201 301

Use Cases

Now, I am well aware that you can do all of these as a simple function yourself, but there's a lot more than what you think it can do. Let's try a simple program that will memorize a number when a possibly laggy math problem is put into it.

For this one we will be using the __index metamethod just to make it simple:

Example

local Table = {}

local function mathProblem(num)
	for i = 1, 20 do
		num = math.floor(num * 10 + 65)
	end
	for i = 1, 10 do
		num = num + i - 1
	end
	return num
end

local Metatable = {
	__index = function (object, key)
		local num = mathProblem(key)
		object[key] = num
		return num
	end
}

local setmetatable(Table, Metatable)

print(Table[1]) -- Will be slow because it's the first time using this number.
print(Table[2]) -- will be slow because it's the first time using this number.
print(Table[1]) -- will be fast because it's just grabbing the number from the table.

See Also

http://www.lua.org/manual/5.1/manual.html#2.8

http://www.lua.org/pil