GDScript Inner Classes: class Inside a Script

The class keyword defines a class inside a script. It is the right tool for small helper types that belong to one script:

extends Node

class Slot:
	var item: String
	var amount: int

	func _init(slot_item: String, slot_amount: int = 1):
		item = slot_item
		amount = slot_amount

	func _to_string() -> String:
		return "%s x%d" % [item, amount]

var slots: Array[Slot] = []

func _ready():
	slots.append(Slot.new("potion", 3))
	slots.append(Slot.new("key"))
	print(slots)
	print(slots[0].amount)

[potion x3, key x1]
3

An inner class works like a file class: it has variables, methods, _init(), and it can be used as a type (Array[Slot], var s: Slot).

Extending Inside a File

An inner class may extend an engine class or another inner class:

extends Node

class Shape:
	func area() -> float:
		return 0.0

class Square extends Shape:
	var side: float
	func _init(s: float):
		side = s
	func area() -> float:
		return side * side

class Circle extends Shape:
	var radius: float
	func _init(r: float):
		radius = r
	func area() -> float:
		return PI * radius * radius

func _ready():
	var shapes: Array[Shape] = [Square.new(3), Circle.new(1)]
	for shape in shapes:
		print("%.2f" % shape.area())

9.00
3.14

What an Inner Class Can See

An inner class can use the outer script's constants, enums and other inner classes. It cannot use the outer script's member variables or methods, because an inner object is not tied to any outer object:

extends Node

const MAX_STACK = 99
enum Rarity { COMMON, RARE }

class Stack:
	var count := 0
	var rarity := Rarity.COMMON

	func add(n: int) -> void:
		count = mini(count + n, MAX_STACK)

func _ready():
	var stack := Stack.new()
	stack.add(150)
	print(stack.count, " ", Rarity.find_key(stack.rarity))

99 COMMON

Writing a member variable of the outer script inside Stack is a parse error: Identifier "player_name" not declared in the current scope. Pass the outer object in, for example func _init(owner_node: Node), when the inner class needs it.

Inner Classes Can Nest

extends Node

class Level:
	class Room:
		var doors := 2
	var rooms: Array[Room] = []

func _ready():
	var level := Level.new()
	level.rooms.append(Level.Room.new())
	print(level.rooms.size(), " room with ", level.rooms[0].doors, " doors")

1 room with 2 doors

Using an Inner Class from Another Script

Inner classes are reached through the outer class. With class_name Inventory in inventory.gd:

var slot := Inventory.Slot.new("arrow", 20)
var all_slots: Array[Inventory.Slot] = []

Without a class_name, preload the script first: const Inventory = preload("res://inventory.gd"), then Inventory.Slot.

Inner Class or Separate File?

Keep a class inner while it is small and only its outer script uses it. Move it to its own file with class_name when other scripts use it often, when it grows past a screen, or when it should be a node or resource type you can add in the editor.