Instance vs Static Variables

Part of the Object Oriented Programming section of Coddy's GDScript journey. Lesson 9 of 65.

A normal var in a class is an instance variable: each object has its own. A static var belongs to the class instead: there is one value, shared by every object.

# student.gd
class_name Student
extends RefCounted

static var school: String = "Godot High"
static var count: int = 0
var student_name: String

func _init(new_name: String) -> void:
	student_name = new_name
	count += 1

var ada := Student.new("Ada")
var bo := Student.new("Bo")
print(Student.count) # 2
print(ada.student_name, " and ", bo.student_name) # Ada and Bo
Student.school = "Godot Academy"
print(ada.school) # Godot Academy
print(bo.school) # Godot Academy

Read a static variable through the class name, as in Student.count, with no object needed. Inside the class, use its name: count += 1 in _init() counts every student created so far.

Changing Student.school changes it for every student at once. ada.school = "Night School" would do the same, because there is only one school; writing the class name makes that clear.

A const is shared the same way but can never change. Use a static var for shared data that changes while the program runs, such as a counter or a setting all objects follow.

Challenge

Easy

Complete the class BankAccount in bank_account.gd:

a static int variable interest_percent that starts at 2, shared by all accounts
a static int variable account_count that starts at 0; the constructor adds 1 to it for every new account
describe() returns HOLDER: $BALANCE at PERCENT%, using the account's holder and balance and the shared interest_percent

The supplied code reads how many accounts follow and one holder balance line each, then a new interest rate. It prints BankAccount.account_count and every account, sets BankAccount.interest_percent to the new rate, and prints every account again. For Alice 1000, Bob 2000 and the rate 3 it prints:

Accounts opened: 2
Alice: $1000 at 2%
Bob: $2000 at 2%
After the rate change:
Alice: $1000 at 3%
Bob: $2000 at 3%

Your code goes in bank_account.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 accounts: Array[BankAccount] = []
	var count := int(OS.read_string_from_stdin())
	for i in count:
		var parts := OS.read_string_from_stdin().split(" ")
		accounts.append(BankAccount.new(parts[0], int(parts[1])))
	print("Accounts opened: %d" % BankAccount.account_count)
	for account in accounts:
		print(account.describe())
	BankAccount.interest_percent = int(OS.read_string_from_stdin())
	print("After the rate change:")
	for account in accounts:
		print(account.describe())