Object-Oriented Programming

Object-Oriented Programming (shortened to OOP) is a programming paradigm to wrap up a set of variables and functions (called methods) inside of a single object. A common example of this is a "Player" in a game, each player has the same set of variable names but different values, such as health, name, etc. A class is the definition of the variables and methods which are used by objects created using that class.

Since Lua has no true OOP implementation such as classes, we can get quite close to OOP and still be able to create unique objects and play around with them. The way we wrap all variables and functions inside a "class" is by holding all of that data in a table. Here's an example of creating a Player class:

Player = {}

setmetatable(Player, {
	__call = function(self, name, health)
		local playerData = {
			name = name,
			health = health
		}
		return setmetatable(playerData, {__index = Player})
	end
})

function Player.getName(self)
	return self.name
end

function Player.getHealth(self)
	return self.health
end

function Player.print(self)
	print('Player "'.. self:getName() ..'": (Health: '.. self:getHealth() ..')')
end

local playerOne = Player('John', 100)
local playerTwo = Player('Jack', 150)
playerOne:print()
playerTwo:print()

The output of the above example results in:

Player "John": (Health: 100)
Player "Jack": (Health: 150)

Here we use the __callmetamethod to create a definition on what happens when you call the table like it's a function. Player is defined at the top as a table, {}, but with the __call metamethod, doing Player(name, health)is valid. Once the __callmetamethod is invoked, we simply take the arguments passed to it and insert it into a table. After which, we just simply return the table after using setmetatable to define our object's __index, which is used whenever we try to access the player object's table. The __index will hold everything inside of the Player table, but out object only holds name and health. Using __index, if what we're trying to access doesn't exist in the player object such as doing playerOne.xyz, it uses the table defined in __index to look in as well, if xyz is a key inside of Player, then that is returned.

We can see that here, there are 3 methods defined inside the Player table, which are getName, getHealth, and print. When calling playerOne:print(), it searches the player object which only holds {name = 'John', health = 100}, since print is not a key in our player object, it will search the Player table for it, and since we did define a print method, that will be used.

Again, playerOne:print() is the equivalent to Player.print(playerOne), just shorter and using the : syntax sugar.