Open-source News

Trick Lua into becoming an object-oriented language Seth Kenlon Thu, 10/27/2022 - 03:00

Lua isn't an object-oriented programming language, but a scripting language utilizing C functions and a C-like syntax. However, there's a cool hack you can use within Lua code to make Lua act like an object-oriented language when you need it to be. The key is in the Lua table construct, and this article demonstrates how to use a Lua table as a stand-in for an object-oriented class.

What is object-oriented programming?

The term "object-oriented" is a fancy way of describing, essentially, a templating system. Imagine you're programming an application to help users spot and log zombies during a zombie apocalypse. You're using an object-oriented language like C++, Java, or Python. You need to create code objects that represent different types of zombies so the user can drag them around and arrange them on a map of the city. Of course a zombie can be any number of things: dormant, slow, fast, hungry, ravenous, and so on. That's just textual data, which computers are good at tracking, and based on that data you could even assign the virtual "object" a graphic so your user can identify which general type of zombie each widget represents.

You have a few options for how you can resolve this requirement for your application:

• Force your users to learn how to code so they can program their own zombies into your application

• Spend the rest of your life programming every possible type of zombie into your application

• Use a code template to define the attributes of a zombie object, allowing your users to create just the items they need, based on what zombie they've actually spotted

Obviously, the only realistic option is the final one, and it's done with a programming construct called a class. Here's what a class might look like (this example happens to be Java code, but the concept is the same across all object-oriented languages):

public class Zombie {
  int height;
  int weight;
  String speed;
  String location;
}

Programming and development Red Hat Developers Blog Programming cheat sheets Try for free: Red Hat Learning Subscription eBook: An introduction to programming with Bash Bash shell scripting cheat sheet eBook: Modernizing Enterprise Java An open source developer's guide to building applications

Whether or not you understand the code, you can probably tell that this is essentially a template. It's declaring that when a virtual "object" is created to represent a zombie, the programming language assigns that object four attributes (two integers representing height and weight, and two words representing the movement speed and physical location). When the user clicks the (imaginary) Add item button in the (imaginary) application, this class is used as a template (in programming, they say "a new instance" of the class has been created) to assign values entered by the user. Infinite zombies for the price of just one class. That's one of the powers of object-oriented programming.

Lua tables

In Lua, the table is a data type that implements an associative array. You can think of a table in Lua as a database. It's a store of indexed information that you can recall by using a special syntax. Here's a very simple example:

example = {} example.greeting = "hello" example.space = " " example.subject = "world" print(example.greeting .. example.space .. example.subject)

Run the example code to see the results:

$ lua ./example.lua hello world

As you can tell from the sample code, a table is essentially a bunch of keys and values kept within a single collection (a "table").

Lua metatable

A metatable is a table that serves as a template for a table. You can designate any table as a metatable, and then treat it much as you would a class in any other language.

Here's a metatable to define a zombie:

Zombie = {} function Zombie.init(h,w,s,l) local self = setmetatable({}, Zombie) self.height = h self.weight = w self.speed = s self.location = l return self end -- use the metatable function setup() z1 = Zombie.init(176,50,'slow','Forbes & Murray Avenue') end function run() print(z1.location .. ": " .. z1.height .. " cm, " .. z1.weight .. " kg, " .. z1.speed) end setup() run()

To differentiate my metatable from a normal table, I capitalize the first letter of its name. That's not required, but I find it a useful convention.

Here's the results of the sample code:

$ lua ./zombie.lua Forbes & Murray Avenue: 176 cm, 50 kg, slow

This demonstration doesn't entirely do metatables justice, because the sample code creates just a single object with no interaction from the user. To use a metatable to satisfy the issue of creating infinite items from just one metatable, you would instead code a user input function (using Lua's io.read() function) asking the user to provide the details of the zombie they spotted. You'd probably even code a user interface with a "New sighting" button, or something like that. That's beyond the scope of this article, though, and would only complicate the example code.

Creating a metatable

The important thing to remember is that to create a metatable, you use this syntax:

Example = {} function Example.init(args) local self = setmetatable({}, Example) self.key = value return self endUsing a metatable

To use a metatable once it's been created, use this syntax:

my_instance = Example.init("required args")Object-oriented Lua

Lua isn't object-oriented, but its table mechanism makes handling and tracking and sorting data as simple as it possibly can be. It's no exaggeration to say that once you're comfortable with tables in Lua, you can be confident that you know at least half of the language. You can use tables (and, by extension, metatables) as arrays, maps, convenient variable organization, close-enough classes, and more.

Lua isn't an object-oriented programming language. Here's a hack to use a Lua table as a stand-in for an object-oriented class.

Image by:

kris krüg

Programming What to read next This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License. Register or Login to post a comment.

Cat9 And LASH Want To Change Your Linux Command Line  Hackaday

How to create and manage container volumes with Podman  TechRepublic

How to Play Windows Games on Linux  MUO - MakeUseOf

Counting individual characters on Linux  Network World

AMD Ryzen 7000 CPUs are Mostly Faster than Intel's 13th Gen Raptor Lake Chips in Linux Gaming  Hardware Times

Work carried out by Google engineers and others around the Linux kernel's read-copy update (RCU) synchronization mechanism to make it "lazier" is helping with 5~10% power-savings for idle or lightly-loaded systems. This "Lazy RCU" work is likely to be merged for the Linux 6.2 kernel merge window in December...

Linux 6.2 Likely To Enjoy Measurable Power-Savings While Idle Or Lightly Loaded  Phoronix

Rakuten Symphony Telecom and CIQ Bring Open Source and Open Community to Open RAN Deployments with Rocky Linux - High-Performance Computing News Analysis | insideHPC  insideHPC

Microsoft's Lennart Poettering proposes tightening up Linux boot process  The Register

Vanilla OS is a brilliant take on the Linux desktop  TechRepublic

CD PROJEKT RED announce The Witcher Remake in Unreal Engine 5  GamingOnLinux

Teen dream team reboots Rolling Rhino into Rhino Linux  The Register

JSAUX cancels current Steam Deck Dock with RGB, after competitor pinched it  GamingOnLinux

Last week Intel began shipping their 13th Gen Core "Raptor Lake" processors. As noted in that launch-day article last week, unfortunately I wasn't able to deliver my usual Linux performance review in time due to hitting some DDR5 issues with my test platform, but those have now been resolved with a new Z790 motherboard. So as the first of my Raptor Lake Linux benchmarks, here is a look at how the top-end Core i9 13900K performs against the AMD Ryzen 7000 series on Ubuntu Linux along with the older Intel/AMD processors.

Intel Core i9 13900K Linux Benchmarks - Performing Very Well On Ubuntu Review  Phoronix

Lemonduck Cryptojacking Botnet Reveals Ongoing API  Security Intelligence

Debian-Based SparkyLinux 2022.10 Rolling ISOs Updated with Linux Kernel 6.0 by Default  9to5Linux

How to Use Your Android Phone as a Speaker for Your Windows, Mac, or Linux Computer  MUO - MakeUseOf

How to Use Your Android Phone as a Speaker for Your Windows, Mac, or Linux Computer  MUO - MakeUseOf