How does the .__index metamethod work in this situation?

[AustnBlox (Austin)]

How does the .__index metamethod work in this situation?

local Car = {}
Car.__index = Car

function Car.new(position, driver, model)
    local newcar = {}
    setmetatable(newcar, Car)

    newcar.Position = position
    newcar.Driver = driver
    newcar.Model = model

    return newcar
end

return Car

[Remingling]

Here’s a good explanation on metatables

@C_Sharper

__index is a metamethod, which metamethods are used with metatables. If you’ve ever learned about operator overloading which is a concept in some other languages (like C#, for example) it’s pretty similar to that idea.

There’s actually quite a few threads that were already made that are pretty intuitive tutorials as to how metatables/metamethods function.

A technical definition though:

The __index metamethod will fire when you index something in a table that does not exist. You can assign an __index key to either some value, or a function as a handler. The table, along with the index, will be passed as arguments to the function if you assign it to a function rather than a value.

It’s actually pretty cool. Here’s another example using code:

local myMetamethods = {}

--//Assign metamethod
myMetatable.__index = function(table, index)
print("Index of: "..index.." was not found in "..table)
end

--//Create the metatable
local myMetatable = setmetatable({}, myMetamethods)

myMetatable.apples = 2 --//The __index metamethod will not be invoked, as a new index was created in our table.

print(myMetatables.oranges) --//This index does NOT exist in our table, so our metamethod which is attached to this metatable will invoke, causing the function to execute.

Hope this helps a bit. I can clarify anything that seems confusing. The concept of metatables and metamethods is definitely more complex if you don’t have a strong understanding of tables and table manipulation already.

[SubtotalAnt8185]

When you create a car instance, your new car instance has access to Car.new, without creating duplicate functions.

[Youtub5005 (Boss)]

Anytime you try to index something within newcar that doesn’t exist within it, instead of returning nil right away it will search within the Car table to see if what you index exists within Car instead, and returning whatever that value is (or nil if it also doesn’t exist in there)