OOP in Lua

This is a scripting tutorial teaches you the basics about how to start using an Object-Oriented developing interface with Lua. Originally created by novo @adam.mta. - Forum topic.

Glossary

• environment: either a table or an array containing values.
• self: predefined variable referring to the environment within which we are executing the code.

Initialising

There is a basic and simple predefined variables we should recognize and know: self.

Our first environment

local array = {}
function array:example (argument)
    return "Hello" .. argument
end

-- The above is syntactic sugar for:
function array.example(self, argument)
    return "Hello" .. argument
end

What we do upon above is defining a local environment and then declaring the function example as part of it. So how do we call example? This is how:

array:example(", world!")

-- The above is syntactic sugar for the
-- following line of code. This makes it
-- easier and faster to use OOP in Lua.
array.example(array, ", world!")

We're able to call a function using two ways: ":" and ".". As you can see on the example above, if we use a dot we're directly accessing the function. We need to manually send the self value . But if use a colon, self will be the environment the function is under, i.e. array. We can send the self value in case we want a function to override the object, read ahead to advanced examples for more information about this.

Variables

local array = {text = "none"}
function array:setKey (key, value)
    self[key] = value
end
function array:getKey (key)
    return self[key]
end

array:getKey("text") -- returns "none"
array:setKey("text", "something") -- sets "text"'s value to 'something'
array:getKey("text") -- returns "something"

Since we are doing array:setKey(..), the "secret" self variable set in both functions are set to array. This means that self[key] = value is actually setting array[key] = value.

Metatables

Content's provenance: NovaFusion

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Knowledge of the usage of metatables will allow you to create much more powerful scripts. Every table can have a metatable attached to it. A metatable is a table that can change the behaviour of the table it's attached to. Let's see an example.

local t = {} -- our normal table
local mt = {} -- our metatable, which contains nothing right now
setmetatable(t, mt) -- set the metatable of *t* to *mt*
getmetatable(t) -- this will return mt

As you can see, getmetatable and setmetatable are the main functions here. This is pretty much self explanatory. A shortened version of this code is this:

local t = setmetatable({}, {})

The function setmetatable returns its first argument, therefore we can use this shorter form.

Now, what do we put in these metatables? Metatables can contain anything, like a regular table, but certain keys that always start with __ (two underscores in a row), such as __index and __newindex help modify the behaviour of the target table. The values corresponding to these keys will usually be functions or other tables. Here's an example:

local t = setmetatable({}, {
    __index = function(tab, key)
        if key == "foo" then
            return 0
        else
            return rawget(tab, key)
        end
    end
})

So as you can see, we assign a function to the __index key. Now let's have a look at what this key is all about.

__index

The most used metatable key is most likely __index; it can contain either a function or table.

When you look up a table with a key, regardless of what the key is (t[4], t.foo, and t["foo"], for example), and a value hasn't been assigned for that key, Lua will look for an __index key in the table's metatable (if it has a metatable). If __index contains a table, Lua will look up the key originally used in the table belonging to __index. This probably sounds confusing, so here's an example:

other = { foo = 3 }
t = setmetatable({hello = "world"}, { __index = other })
t.foo -- 3
t.bar -- nil
t.hello -- "world"

If __index contains a function, then it's called, with the table that is being looked up and the key used as parameters. By default, this function is "rawget". As we saw in the code example above the last one, this allows us to use conditionals on the key, and basically anything else that Lua code can do. Therefore, in that example, if the key was equal to the string "foo" we would return 0, otherwise we look up the table table with the key that was used; this makes t an alias of table that returns 0 when the key "foo" is used.

You may be wondering why the table is passed as a first parameter to the __index function. This comes in handy if you use the same metatable for multiple tables, supporting code re-use and saving computer resources. We'll see an example of this when we take a look at the Vector class.

__newindex

Next in line is __newindex, which is similar to __index. Like __index, it can contain either a function or table.

When you try to set a value in a table that is not already present, Lua will look for a __newindex key in the metatable. It's the same sort of situation as __index; if __newindex is a table, the key and value will be set in the table specified:

other = {}
t = setmetatable({}, { __newindex = other })
t.foo = 3
other.foo -- 3
t.foo -- nil

As would be expected, if __newindex is a function, it will be called with the table, key, and value passed as parameters:

t = setmetatable({}, {
    __newindex = function(t, key, value)
        if type(value) == "number" then
            rawset(t, key, value * value)
        else
            rawset(t, key, value)
        end
    end
})

t.foo = "foo"
print(t.foo) -- "foo"

t.bar = 4
print(t.bar) -- 16

t.la = 10
print(t.la) -- 100

When creating a new key in t, if the value is a number it will be squared, otherwise it will just be set using the default "__newindex" function, rawset.. This introduces us to our friends, rawget and rawset.

rawget and rawset

There are times when you need get and set a table's keys without dealing with metatables. Rawget and rawset bypass metatables, if you didn't use these when actually getting a value from a table inside a metatable function, an infinite loop will occur. Rawset and rawget do not provide performance enhancements over regular methods, so people don't overuse it to try to gain a performance boost. You may have noticed that the __index arguments are exactly the same as the possible arguments for rawget - this is because rawget/rawset are actually the default metamethods for regular tables.

Operators

Many of the metatable keys available have to do with operators (as in, +, -, etc.), allowing you to make tables support the use of operators on them. For example, say we wanted a table to support the multiplication operator (*), this is how we would do it:

t = setmetatable({}, {
    __mul = function(t, other)
        new = {}
    
        for i = 1, other do
            for _, v in ipairs(t) do table.insert(new, v) end
        end
    
        return new
    end
})
t[1] = "M"
t[2] = "T"
t[3] = "A"
t[4] = 1.4
t = t * 2 -- { "M", "T", "A", 1.4, "M", "T", "A", 1.4 }

This allows us to create a new table with the original replicated a certain amount of times using the multiplication operator. The corresponding key for multiplication is __mul; unlike __index and __newindex the keys for operators can only contain functions. The first parameter these functions always receive is the target table, and then the value on the right hand side (except for the unary - which has the key of __unm). Here's a list of the supported operators:

• __add: Addition (+)
• __sub: Subtraction (-)
• __mul: Multiplication (*)
• __div: Division (/)
• __mod: Modulos (%)
• __unm: Unary -, used for negation on numbers
• __concat: Concatenation (..)
• __eq: Equality (==)
• __lt: Less than (<)
• __le: Less than or equal to (<=)

The operators >, >= and ~= do not exist because these can be calculated by flipping the right values. Lua does this:

• >: More than - not __le
• ~=: Not equal to -- not __eq
• >=: More than or equal to - not __lt

__call

Next comes the __call key, which allows you to call tables as functions. A code example:

t = setmetatable({}, {
    __call = function(t, a, b, c, multiplier)
        return (a + b + c) * multiplier
    end
})

t(1, 2, 3, 4) -- 24

The function in __call passes the target table as usual, followed by the arguments we passed to it. __call is very useful for many things, it is often used to create new objects.

__tostring

Last of all is __tostring. If implemented, it's used by tostring to convert a table into a string, most handy when using a function like print. Normally, when you try to convert a table to a string, you something in the format of "table: 0x<hex-code-here>", but you can change that using __tostring. An example:

t = setmetatable({ 1, 2, 3 }, {
    __tostring = function(t)
        sum = 0
        for _, v in pairs(t) do sum = sum + v end
        return "Sum: " .. sum
    end
})

print(t) -- prints out "Sum: 6"

Building the Vector Class

[[{{{image}}}|link=|]]	Note: This section is just an example, use the official Vector functions instead.

To wrap everything up, we'll write a class encapsulating a 2D vector. It's too large to put here, but you can see the full code at gist #1055480. I've positioned all the stuff to do with metatables first in the file, as that's the most important stuff. (Be warned, this may be a bit confusing if you've never encountered Object-Oriented Programming before.)

Vector = {}
Vector.__index = Vector

This code sets up the table for the Vector class, and sets the __index key to point back at itself. Now, what's going on here? You've probably noticed that we've put all the metatable methods inside the Vector class. What you're seeing is the simplest way to achieve OOP (Object-Oriented Programming) in Lua. The Vector table represents the class, which contains all the functions that instances can use. Vector.new (shown below) creates a new instance of this class.

function Vector.new(x, y)
  return setmetatable({ x = x or 0, y = y or 0 }, Vector)
end

It creates a new table with x and y properties, and then sets the metatable to the Vector class. As we know, Vector contains all the metamethods and especially the __index key. This means that we can use all the functions we define in Vector through this new table. We'll come back to this in a moment.

Another important thing is the last line:

setmetatable(Vector, { __call = function(_, ...) return Vector.new(...) end })

This means that we can create a new Vector instance by either calling Vector.new or just Vector.

The last important thing that you may not be aware of is the colon syntax. When we define a function with a colon, like this:

function t:method(a, b, c)
  -- ...
end

What we are really doing is defining this function:

function t.method(self, a, b, c)
  -- ...
end

This is syntactic sugar to help with OOP. We can then use the colon syntax when calling functions:

-- these are the same
t:method(1, 2, 3)
t.method(t, 1, 2, 3)

Now, how do we use this Vector class? Here's a final example:

a = Vector.new(10, 10)
b = Vector(20, 11)
c = a + b
print(a:len()) -- 14.142135623731
print(a) -- (10, 10)
print(c) -- (30, 21)
print(a < c) -- true
print(a == b) -- false

Because of the __index in Vector, we can use all the methods defined in the class through the instances.

Advanced examples

Overriding self's value:

local array = {text = "none"}
local array2 = {text = "none2"}
function array:setKey (key, value)
	self[key] = value
end
function array:getKey (key)
	return self[key]
end
array.getKey(array2, "text") -- returns "none2"
array.setKey(array2, "text", "something2") -- sets array2's "text" value to 'something2'
array.getKey(array2, "text") -- returns "something"
array:getKey("text") -- returns "none"
array.getKey(array, "text") -- same as above

A simple backpacks example:

local backpack = {list = {}}
function backpack:create (owner, slots)
	if self.list[owner] then return end -- return false in case this player already owns a backpack
		local new = {
		items = {},
		slots = slots or 100,
		owner = owner
	}
	setmetatable(new.items, 
	{
		__newindex = function(table, key, value) -- this is called once a new value/item is added into the player's table/backpack
			if #table >= new.slots then return end -- return false in case there isn't any free slot left
			return rawset(table, key, value)
		end
	}
	)
	self.list[owner] = new
end

function backpack:addItem (player, item, value)
	if not self.list[player] then return end
	self.list[player][item] = value
end

Useful links

• Official MTA OOP Introduction
• OOP