Introduction to OOP

Part of the Object Oriented Programming section of Coddy's GDScript journey. Lesson 2 of 65.

Object-oriented programming (OOP) organizes a program around objects. An object bundles data (variables) with the functions that work on that data. A class describes what its objects hold and do, and you create as many objects from it as you need.

In GDScript every script file is a class. Name it with class_name and declare the variables each object holds:

# car.gd
class_name Car
extends RefCounted

var brand: String = "Generic"
var wheels: int = 4

extends RefCounted makes Godot free a Car object by itself once no variable refers to it any more. Use it for every class that holds data and is not a node. A script without an extends line extends RefCounted too.

Call new() on the class to create an object, then reach its variables with a dot:

# main.gd
var my_car := Car.new()
print(my_car.wheels) # 4
print(my_car is Car) # true

The is operator checks which class an object was created from. Printing the object itself shows only an id such as <RefCounted#-9223372009860233781>, so print its variables instead.

The class is the blueprint and each object is a separate thing built from it: calling Car.new() twice gives you two cars.

Challenge

Easy

Create the class Car in car.gd:

its class name is Car and it extends RefCounted
a String variable brand that starts as "Generic"
an int variable wheels that starts as 4

The supplied code reads how many cars to build and creates each one with Car.new(). It prints the number of cars and their wheels in total. When it built at least one car, it reads a brand for the first car and prints the brands of the first and the last car. For the input 3 and Tesla it prints:

Cars: 3
Wheels: 12
First car: Tesla
Last car: Generic

Your code goes in car.gd. main.gd holds the supplied input/output code and cannot be edited.

Try it yourself

extends Node

func _ready():
	# Supplied input/output code: keep it as it is
	var count := int(OS.read_string_from_stdin())
	var garage: Array[Car] = []
	for i in count:
		garage.append(Car.new())
	var total := 0
	for car in garage:
		total += car.wheels
	print("Cars: %d" % garage.size())
	print("Wheels: %d" % total)
	if count > 0:
		garage[0].brand = OS.read_string_from_stdin()
		print("First car: %s" % garage[0].brand)
		print("Last car: %s" % garage[-1].brand)