Tables As Objects

Tables can be used to act much like objects in other object oriented languages. This is closely related to special syntax you can use for functions that are in a table.

Related reading: https://www.lua.org/pil/16.html

Calling Functions

When a table has a function at some index, the function can be called in the normal way, with a dot. But you can also call it using a colon : for extra behavior.

local my_table = {
    func = function()
        print("Func call")
    end
}

-- Normal way
my_table.func()

-- Other way
my_table:func()

When calling these functions with a colon, there is an impliict value passed into the function, being the lefthand side of the colon.

function func(param)
    print(param)
end

local my_table = {}
my_table.func = func

my_table.func()
my_table:func()

The above code outputs

nil
table: 0x55f82a449f40

or similar.

Despite putting nothing in the parentheses, the function call using the colon received the table as input.

self

When using the colon to call a function, you can say the table passes itself in. self is the standard name to use for this first parameter. You might notice it color differently in your code editor.

Having itself passed in means we can safely modify members of the table.

--[[
    Adds `a` to the `num` index
]]
function add_to_num(self, a)
    self.num = self.num + a
end

local my_table = {
    num = 0,
    add_to_num = add_to_num
}

-- 0
print(my_table.num)

my_table:add_to_num(2)

-- 2
print(my_table.num)

-- Notice that the above call is identical to this.
my_table.add_to_num(my_table, 2)

-- 4
print(my_table.num)

Defining Functions

There's special syntax for defining a function at some String index in a table.

local my_table = {
    num = 0
}

-- Normal
my_table.func_1 = function(self, a)
    self.num = self.num + a
end

-- Another way
function my_table.func_2(self, a)
    self.num = self.num + a
end

-- Another way that also implicitly has a "self" parameter
function my_table:func_3(a)
    self.num = self.num + a
end

-- 0
print(my_table.num)

my_table:func_1(1)
my_table:func_2(1)
my_table:func_3(1)

-- 3
print(my_table.num)

In ONB

In the ONB sections, you will often see self in example functions, and you will nearly always call engine functions with a colon. When interacting with the C++ objects the engine provides, you can nearly think of them the same way as these tables.