Need help understanding OOP

[n_cy]

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

This is my understanding so far, first when you call Car.New it runs the function thats inside the Car table.

Second it creates a new car table and creates a metatable with the __index metamethod

third it creates new properties for newcar which triggers the metamethod

Lastly the metamethod runs the function Car ? This part is confusing

[TheCarbyneUniverse (Carbyne)]

So, .__index is used to redirect the script to the metatable if the table has a nil value that you’re trying to index.

First, let’s take a look at the code, and form some pseudocode to understand it better. The Car.new() function returns a table called newcar. But all of the functions/constructors we are creating, such as Car.new() are for the metatable Car.

Say you add a function:

function Car:Brakes(speed) --slow down the car

   self.Speed  = speed --self will be the newcar table, a particular object
 
end

Now if we use the code in another script:

local car = Car.new(pos, dr, model) --returns the newcar table

car:Brakes(15) --we've called :Brakes on newcar, but the function is defined for the metatable Car

So, this is where the index metamethod comes in. It looks if :Brakes exists in newcar, which it doesn’t, meaning the .__index will fire. The metamethod will make Lua look inside of Car, in which the Brakes function does exist.

You can also think about it this way: “Car” is the class, not a particular object. newcar is the particular object, so instead of having to pass newcar as a parameter each time a function is called, you can redirect Lua so that the class functions are applied to the object. Essentially it’d be like:

function newcar:Brakes()

end

If you want to learn more, check out these awesome resources:

All about Object Oriented Programming

ALL ABOUT OOP! Prerequisites An understanding of meta-tables (although the required code will be explained) How tables work and a competent grasp of the Lua syntax Parts What is OOP? How does it help me? How do I make this work in Lua? Integrating with module scripts What about inheritance? What is OOP? OOP stands for Object Orientated Programming and is a way of laying out code in a more friendly way whilst also keeping large projects organised. You have used objects in pro…

All you need to know about Metatables and Metamethods

Metatables! Truly an interesting subject. Through my activity in different forums and discord servers, I have seen many people get confused, on what they are. Main reason being, not a super wide collection of resources exist explaining them, and in general the idea of metatables is different and unique for someone who is still baby-sitted by how straight forward Luau is. I. Metatables There isn’t really a definition to what a metatable is. It’s just that, any table can have a metatable. Stick …

[n_cy]

Thanks for the detailed post, it helped clear out some stuff

one question though, on the all about oop tutorial does the metamethod just change newcar into car, so it can get the function?

[0Shank]

No it doesn’t this is how it works. When you index an table in lua and the value of the index is nil lua checks if that table has an metatable, if it does have a metatable it checks if the metatable has the .__index metamethod, if it does it tries to find the index in the table attached to this metamethod in the case you showed it is attached to the metatable itself. , if it finds the index is the table then it returns that value aissgned to the key. This is if you set the .__index of a metamethod as a table, you can also set it as a function, but I won’t be telling it here.