Flyweight pattern

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In computer programming, flyweight is a software design pattern. A flyweight is an object that minimizes memory usage by sharing as much data as possible with other similar objects; it is a way to use objects in large numbers when a simple repeated representation would use an unacceptable amount of memory. Often some parts of the object state can be shared, and it is common practice to hold them in external data structures and pass them to the objects temporarily when they are used.

A classic example usage of the flyweight pattern is the data structures for graphical representation of characters in a word processor. It might be desirable to have, for each character in a document, a glyph object containing its font outline, font metrics, and other formatting data, but this would amount to hundreds or thousands of bytes for each character. Instead, for every character there might be a reference to a flyweight glyph object shared by every instance of the same character in the document; only the position of each character (in the document and/or the page) would need to be stored internally.

Another example is string interning.

In other contexts the idea of sharing identical data structures is called hash consing.


The Flyweight [1] design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

What problems can the Flyweight design pattern solve? [2]

  • Large numbers of objects should be supported efficiently.
  • Creating large numbers of objects should be avoided.

When representing large text documents, for example, creating an object for each character in the document would result in a huge number of objects that could not be processed efficiently.

What solution does the Flyweight design pattern describe?

Define Flyweight objects that

  • store intrinsic (invariant) state that can be shared and
  • provide an interface through which extrinsic (variant) state can be passed in.

This enables clients to (1) reuse (share) Flyweight objects (instead of creating a new object each time) and (2) pass in extrinsic state when they invoke a Flyweight operation.
This greatly reduces the number of physically created objects.
Intrinsic state is invariant (context independent) and therefore can be shared (for example, the code of character 'A' in a given character set).
Extrinsic state is variant (context dependent) and therefore can not be shared and must be passed in (for example, the position of character 'A' in a text document).
See also the UML class and sequence diagram below.


According to the textbook Design Patterns: Elements of Reusable Object-Oriented Software,[3] the flyweight pattern was first coined and extensively explored by Paul Calder and Mark Linton in 1990 to efficiently handle glyph information in a WYSIWYG document editor,[4] although similar techniques were already used in other systems, e.g., an application framework by Weinand et al. (1988).[5]


UML class and sequence diagram[edit]

A sample UML class and sequence diagram for the Flyweight design pattern. [6]

In the above UML class diagram, the Client class refers (1) to the FlyweightFactory class to create/share Flyweight objects and (2) to the Flyweight interface to perform an operation by passing in extrinsic (variant) state (flyweight.operation(extrinsicState)). The Flyweight1 class implements the Flyweight interface and stores intrinsic (invariant) state that can be shared.
The sequence diagram shows the run-time interactions: The Client object calls getFlyweight(key) on the FlyweightFactory that creates and returns a Flyweight1 object. After calling operation(extrinsicState) on the returned Flyweight1 object, the Client again calls getFlyweight(key) on the FlyweightFactory, which now shares and returns the already existing Flyweight1 object.

Immutability and equality[edit]

To enable safe sharing, between clients and threads, Flyweight objects must be immutable. Flyweight objects are by definition value objects. The identity of the object instance is of no consequence therefore two Flyweight instances of the same value are considered equal.

Example in C# (note Equals and GetHashCode overrides as well as == and != operator overloads):

public class CoffeeFlavour {
    private readonly string _flavour;

    public CoffeeFlavour(string flavour) {
        _flavour = flavour;

    public string Flavour {
        get { return _flavour; }

    public override bool Equals(object obj) {
        if (ReferenceEquals(null, obj)) return false;
        return obj is CoffeeFlavour && Equals((CoffeeFlavour)obj);

    public bool Equals(CoffeeFlavour other) {
        return string.Equals(_flavour, other._flavour);

    public override int GetHashCode() {
        return (_flavour != null ? _flavour.GetHashCode() : 0);

    public static bool operator ==(CoffeeFlavour a, CoffeeFlavour b) {
        return Equals(a, b);

    public static bool operator !=(CoffeeFlavour a, CoffeeFlavour b) {
        return !Equals(a, b);


Special consideration must be made in scenarios where Flyweight objects are created on multiple threads. If the list of values is finite and known in advance the Flyweights can be instantiated ahead of time and retrieved from a container on multiple threads with no contention. If Flyweights are instantiated on multiple threads there are two options:

  1. Make Flyweight instantiation single threaded thus introducing contention and ensuring one instance per value.
  2. Allow concurrent threads to create multiple Flyweight instances thus eliminating contention and allowing multiple instances per value. This option is only viable if the equality criterion is met.

Example in C#[edit]

using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;

public interface ICoffeeFlavourFactory {
    CoffeeFlavour GetFlavour(string flavour);

public class ReducedMemoryFootprint : ICoffeeFlavourFactory {
    private readonly object _cacheLock = new object();
    private readonly IDictionary<string, CoffeeFlavour> _cache = new Dictionary<string, CoffeeFlavour>();

    public CoffeeFlavour GetFlavour(string flavour) {
        if (_cache.ContainsKey(flavour)) return _cache[flavour];
        var coffeeFlavour = new CoffeeFlavour(flavour);
        ThreadPool.QueueUserWorkItem(AddFlavourToCache, coffeeFlavour);
        return coffeeFlavour;

    private void AddFlavourToCache(object state) {
        var coffeeFlavour = (CoffeeFlavour)state;
        if (!_cache.ContainsKey(coffeeFlavour.Flavour)) {
            lock (_cacheLock) {
                if (!_cache.ContainsKey(coffeeFlavour.Flavour)) _cache.Add(coffeeFlavour.Flavour, coffeeFlavour);

public class MinimumMemoryFootprint : ICoffeeFlavourFactory {
    private readonly ConcurrentDictionary<string, CoffeeFlavour> _cache = new ConcurrentDictionary<string, CoffeeFlavour>();

    public CoffeeFlavour GetFlavour(string flavour) {
        return _cache.GetOrAdd(flavour, flv => new CoffeeFlavour(flv));

Simple implementation[edit]

Flyweight allows you to share bulky data that are common to each object. In other words, if you think that same data is repeating for every object, you can use this pattern to point to the single object and hence can easily save space. Here the FlyweightPointer creates a static member Company, which is used for every object of MyObject.

// Defines Flyweight object that repeats itself.
public class FlyWeight
    public string CompanyName { get; set; }
    public string CompanyLocation { get; set; }
    public string CompanyWebSite { get; set; }
    //Bulky Data
    public byte[] CompanyLogo { get; set; }

public static class FlyWeightPointer
    public static readonly FlyWeight Company = new FlyWeight
        CompanyName = "Abc",
        CompanyLocation = "XYZ",
        CompanyWebSite = ""
        // Load CompanyLogo here

public class MyObject
    public string Name { get; set; }
    public string Company
            return FlyWeightPointer.Company.CompanyName;

Example in Java[edit]

import java.util.HashMap;
import java.util.Map;

interface BMWCarCustomisation {
	// customize Tire size
	int getTireSize();
	String getLaserSignature();
	// a lot of customisation attributes can be in there for a BMW car
	void printCustomisations();

interface BMWCar {
	double calculatePrice(BMWCarCustomisation custom);
	void printFullCharacteristics(BMWCarCustomisation custom);

interface BMWCarFactory {
	BMWCar createCar();

interface BMWCarFlyWeightFactory {
	enum Model {Serie1, Serie2, Serie3};
	BMWCar getBWMModel(Model m);

class BMWSerie1 implements BMWCar {
	private final static double BASE_PRICE = 25000;

	public double calculatePrice(BMWCarCustomisation custom) {
		return BASE_PRICE + getSpecificSerie1PriceBasedOnCustom(custom) + getExportationTaxe(custom);

	public void printFullCharacteristics(BMWCarCustomisation custom) {
		// print all BMW 1 Series specific characteristics
		// (codes in there)
		custom.printCustomisations(); // print details based on these customisations

	private double getSpecificSerie1PriceBasedOnCustom(BMWCarCustomisation custom) {
		// (e.g., calculation based on custom specific to Series 1)
		double sum = 0;
		if (custom.getTireSize() == 19) {
			sum += 1200;
		} else {
			sum += 2100;
		return sum;

	private double getExportationTaxe(BMWCarCustomisation custom) {
		// (calculation based on custom exportation taxes only for this model)
		double sum = 0;
		if (!custom.getLaserSignature().isEmpty()) {
			sum += 987;
		return sum;

class BMWSerie2 implements BMWCar {
	private final static double BASE_PRICE = 28000;

	public double calculatePrice(BMWCarCustomisation custom) {
		return BASE_PRICE + getSpecificSerie2PriceBasedOnCustom(custom);

	public void printFullCharacteristics(BMWCarCustomisation custom) {
		// print all BMW 2 Series specific characteristics
		// (codes in there)
		custom.printCustomisations(); // print details based on these customisations

	private double getSpecificSerie2PriceBasedOnCustom(BMWCarCustomisation custom) {
		// (e.g., calculation based on custom specific to Series 2)
		double sum = 0;
		if (custom.getTireSize() == 19) {
			sum += 2000;
		} else {
			sum += 3000;
		if (!custom.getLaserSignature().isEmpty()) {
			if (custom.getLaserSignature().length() > 10) {
				sum += 1200;
			} else {
				sum += 400;
		return sum;

class BMWSerie2Factory implements BMWCarFactory {
	public BMWCar createCar() {
		return new BMWSerie2();

class BMWSerie1Factory implements BMWCarFactory {
	public BMWCar createCar() {
		return new BMWSerie1();

class BMWSerieFlyWeightFactory implements BMWCarFlyWeightFactory {
	private Map<Model, BMWCar> cache = new HashMap<>();

	public synchronized BMWCar getBWMModel(Model m) {
		if (!cache.containsKey(m)) {
			BMWCarFactory concreteFactory;
			switch (m) {
			case Serie2:
				concreteFactory = new BMWSerie2Factory();
			case Serie3:
				concreteFactory = new BMWSerie3Factory();
			 * Just code to have a hint !
				concreteFactory = new BMWSerie1Factory();
			cache.put(m, concreteFactory.createCar());
		return cache.get(m);

class BMWSerieCarCustomisation implements BMWCarCustomisation {

	private int tireSize;
	private String laserSignature;

	public BMWSerieCarCustomisation(int tireSize, String laserSignature) {
		this.tireSize = tireSize;
		this.laserSignature = laserSignature;

	public int getTireSize() {
		return tireSize;

	public String getLaserSignature() {
		return laserSignature;

	public void printCustomisations() {
		System.out.println("Tire Size:" + getTireSize());
		System.out.println("LaserSignature:" + getLaserSignature());
		System.out.println("LaserSignature Size:" + getLaserSignature().length());

public class MainTest {

	public static void main(String[] args) {
		BMWCarFlyWeightFactory factory = new BMWSerieFlyWeightFactory();
		BMWCar serie1Car = factory.getBWMModel(BMWCarFlyWeightFactory.Model.Serie1);
		BMWCar serie1Car2 = factory.getBWMModel(BMWCarFlyWeightFactory.Model.Serie1);
		System.out.println("check for Object for Serie1:" + (serie1Car == serie1Car2));
		BMWCar serie2Car = factory.getBWMModel(BMWCarFlyWeightFactory.Model.Serie2);
		BMWCar serie2Car2 = factory.getBWMModel(BMWCarFlyWeightFactory.Model.Serie2);
		System.out.println("check for Object for Serie2:" + (serie2Car == serie2Car2));
		BMWCarCustomisation custom1 = new BMWSerieCarCustomisation(19, "Oh yeah");
		BMWCarCustomisation custom2 = new BMWSerieCarCustomisation(21, "For bob");
		BMWCarCustomisation custom3 = new BMWSerieCarCustomisation(26, "give it a ride !!");
		// BMW 1 Series
		System.out.println("BMW Serie1 + Custom1 Price:\nFull Price:" + serie1Car.calculatePrice(custom1));
		System.out.println("BMW Serie1 + Custom2 Price:\nFull Price:" + serie1Car.calculatePrice(custom2));
		System.out.println("BMW Serie1 + Custom3 Price:\nFull Price:" + serie1Car.calculatePrice(custom3));
		/// It's the samed BMW 1 Series Flyweight instance; the variant part is provided
		/// by the operation and customs
		// BMW 2 Series
		System.out.println("BMW Serie2 + Custom1 Price:\nFull Price:" + serie2Car.calculatePrice(custom1));
		System.out.println("BMW Serie2 + Custom2 Price:\nFull Price:" + serie2Car.calculatePrice(custom2));
		System.out.println("BMW Serie2 + Custom3 Price:\nFull Price:" + serie2Car.calculatePrice(custom3));
		/// It's the samed BMW 2 Series Flyweight instance; the variant part is provided
		/// by the operation and customs

The execution of this code will give the following :

check for Object for Serie1:true
check for Object for Serie2:true
BMW Serie1 + Custom1 Price:
Full Price:27187.0
Tire Size:19
LaserSignature:Oh yeah
LaserSignature Size:7
BMW Serie1 + Custom2 Price:
Full Price:28087.0
Tire Size:21
LaserSignature:For bob
LaserSignature Size:7
BMW Serie1 + Custom3 Price:
Full Price:28087.0
Tire Size:26
LaserSignature:give it a ride !!
LaserSignature Size:17
BMW Serie2 + Custom1 Price:
Full Price:30400.0
Tire Size:19
LaserSignature:Oh yeah
LaserSignature Size:7
BMW Serie2 + Custom2 Price:
Full Price:31400.0
Tire Size:21
LaserSignature:For bob
LaserSignature Size:7
BMW Serie2 + Custom3 Price:
Full Price:32200.0
Tire Size:26
LaserSignature:give it a ride !!
LaserSignature Size:17

For this example of code the invariant part are the BMW Series 1 & 2 cars which are Flyweight objects; the variant parts are passed through an operation. For this example we do not need to create a car for each customer need and specification which can be troublesome for all customization possibilities--the customization which is the variant part has an object that is only needed for calculation and printing.

Example in Scala[edit]

run as a script using `scala flyweight.scala`
expected output:
  Serving CoffeeFlavour(Espresso) to table 121
  Serving CoffeeFlavour(Cappuccino) to table 121
  Serving CoffeeFlavour(Frappe) to table 552
  Serving CoffeeFlavour(Espresso) to table 96
  Serving CoffeeFlavour(Cappuccino) to table 3
  Serving CoffeeFlavour(Espresso) to table 3
  Serving CoffeeFlavour(Frappe) to table 3
  Serving CoffeeFlavour(Cappuccino) to table 97
  Serving CoffeeFlavour(Frappe) to table 897
  Serving CoffeeFlavour(Espresso) to table 1
  Serving CoffeeFlavour(Frappe) to table 1
  Serving CoffeeFlavour(Cappuccino) to table 2
  total CoffeeFlavour objects made: 3

/* the `private` constructor ensures that only interned
 * values of type `CoffeeFlavour` can be obtained. */
class CoffeeFlavour private (val name: String){
    override def toString = s"CoffeeFlavour($name)"

object CoffeeFlavour {
  import scala.collection.mutable
  import scala.ref.WeakReference
  private val cache = mutable.Map.empty[String, ref.WeakReference[CoffeeFlavour]]

  def apply(name: String): CoffeeFlavour = synchronized {    
    cache.get(name) match {
      case Some(WeakReference(flavour)) => flavour
      case _ => 
        val newFlavour = new CoffeeFlavour(name)
        cache.put(name, WeakReference(newFlavour))

  def totalCoffeeFlavoursMade = cache.size

case class Order(tableNumber: Int, flavour: CoffeeFlavour){

  def serve: Unit =
    println(s"Serving $flavour to table $tableNumber")

object CoffeeShop {
  var orders = List.empty[Order]

  def takeOrder(flavourName: String, table: Int) {
    val flavour = CoffeeFlavour(flavourName)
    val order = Order(table, flavour)
    orders = order :: orders

  def service: Unit = orders.foreach(_.serve)

  def report =
    s"total CoffeeFlavour objects made: ${CoffeeFlavour.totalCoffeeFlavoursMade}"

CoffeeShop.takeOrder("Cappuccino", 2)
CoffeeShop.takeOrder("Frappe", 1)
CoffeeShop.takeOrder("Espresso", 1)
CoffeeShop.takeOrder("Frappe", 897)
CoffeeShop.takeOrder("Cappuccino", 97)
CoffeeShop.takeOrder("Frappe", 3)
CoffeeShop.takeOrder("Espresso", 3)
CoffeeShop.takeOrder("Cappuccino", 3)
CoffeeShop.takeOrder("Espresso", 96)
CoffeeShop.takeOrder("Frappe", 552)
CoffeeShop.takeOrder("Cappuccino", 121)
CoffeeShop.takeOrder("Espresso", 121)


Example in Ruby[edit]

# Flyweight Object
class Lamp
  attr_reader :color
  #attr_reader makes color attribute available outside
  #of the class by calling .color on a Lamp instance

  def initialize(color)
    @color = color

class TreeBranch
  def initialize(branch_number)
    @branch_number = branch_number

  def hang(lamp)
    puts "Hang #{lamp.color} lamp on branch #{@branch_number}"

# Flyweight Factory
class LampFactory
  def initialize
    @lamps = {}

  def find_lamp(color)
    if @lamps.has_key?(color)
      # if the lamp already exists, reference it instead of creating a new one
      lamp = @lamps[color]
      lamp =
      @lamps[color] = lamp

  def total_number_of_lamps_made

class ChristmasTree
  def initialize
    @lamp_factory =
    @lamps_hung = 0


  def hang_lamp(color, branch_number)
    @lamps_hung += 1

  def dress_up_the_tree
    hang_lamp('red', 1)
    hang_lamp('blue', 1)
    hang_lamp('yellow', 1)
    hang_lamp('red', 2)
    hang_lamp('blue', 2)
    hang_lamp('yellow', 2)
    hang_lamp('red', 3)
    hang_lamp('blue', 3)
    hang_lamp('yellow', 3)
    hang_lamp('red', 4)
    hang_lamp('blue', 4)
    hang_lamp('yellow', 4)
    hang_lamp('red', 5)
    hang_lamp('blue', 5)
    hang_lamp('yellow', 5)
    hang_lamp('red', 6)
    hang_lamp('blue', 6)
    hang_lamp('yellow', 6)
    hang_lamp('red', 7)
    hang_lamp('blue', 7)
    hang_lamp('yellow', 7)
    puts "Made #{@lamp_factory.total_number_of_lamps_made} total lamps"
    puts "Hung #{@lamps_hung} total lamps"

Example in Python[edit]

Attributes can be defined at the class-level instead of only for instances in Python because classes are first-class objects in the language—meaning there are no restrictions on their use as they are the same as any other object. New-style class instances store instance data in a special attribute dictionary instance.__dict__. By default, accessed attributes are first looked-up in this __dict__, and then fallback to the instance's class attributes next. [7] In this way, a class can effectively be a kind of Flyweight container for its instances.

Although Python classes are mutable by default, immutability can be emulated by overriding the class's __setattr__ method so that it disallows changes to any Flyweight attributes.

# Instances of CheeseBrand will be the Flyweights
class CheeseBrand(object):
    def __init__(self, brand, cost):
        self.brand = brand
        self.cost = cost
        self._immutable = True   # Disables future attributions

    def __setattr__(self, name, value):
        if getattr(self, '_immutable', False):  # Allow initial attribution
            raise RuntimeError('This object is immutable')
            super(CheeseBrand, self).__setattr__(name, value)

class CheeseShop(object):
    menu = {}  # Shared container to access the Flyweights
    def __init__(self):
        self.orders = {}  # per-instance container with private attributes

    def stock_cheese(self, brand, cost):
        cheese = CheeseBrand(brand, cost)[brand] = cheese   # Shared Flyweight

    def sell_cheese(self, brand, units):
        self.orders.setdefault(brand, 0)
        self.orders[brand] += units   # Instance attribute

    def total_units_sold(self):
        return sum(self.orders.values())
    def total_income(self):
        income = 0
        for brand, units in self.orders.items():
            income +=[brand].cost * units
        return income

shop1 = CheeseShop()
shop2 = CheeseShop()

shop1.stock_cheese('white', 1.25)
shop1.stock_cheese('blue', 3.75)
# Now every CheeseShop have 'white' and 'blue' on the inventory
# The SAME 'white' and 'blue' CheeseBrand

shop1.sell_cheese('blue', 3)    # Both can sell
shop2.sell_cheese('blue', 8)    # But the units sold are stored per-instance

assert shop1.total_units_sold() == 3
assert shop1.total_income() == 3.75 * 3

assert shop2.total_units_sold() == 8
assert shop2.total_income() == 3.75 * 8

Example in Swift[edit]

import Foundation

// Instances of CoffeeFlavour will be the Flyweights
struct CoffeeFlavor : CustomStringConvertible {
    var flavor: String
    var description: String { return flavor }

// Menu acts as a factory and cache for CoffeeFlavour flyweight objects
struct Menu {
    private var flavors = [String: CoffeeFlavor]()

    mutating func lookup(flavor: String) -> CoffeeFlavor {
        if let f = flavors[flavor] { return f }
        else {
            let cFlavor = CoffeeFlavor(flavor: flavor)
            flavors[flavor] = cFlavor
            return cFlavor

struct CoffeeShop {
    private var orders = [Int: CoffeeFlavor]()
    private var menu = Menu()

    mutating func takeOrder( flavor: String, table: Int) {
        orders[table] = menu.lookup(flavor: flavor)

    func serve() {
        for (table, flavor) in orders {
            print("Serving \(flavor) to table \(table)")

var coffeeShop = CoffeeShop()
coffeeShop.takeOrder(flavor: "Cappuccino", table: 1)
coffeeShop.takeOrder(flavor: "Frappe", table: 3);
coffeeShop.takeOrder(flavor: "Espresso", table: 2);
coffeeShop.takeOrder(flavor: "Frappe", table: 15);
coffeeShop.takeOrder(flavor: "Cappuccino", table: 10);
coffeeShop.takeOrder(flavor: "Frappe", table: 8);
coffeeShop.takeOrder(flavor: "Espresso", table: 7);
coffeeShop.takeOrder(flavor: "Cappuccino", table: 4);
coffeeShop.takeOrder(flavor: "Espresso", table: 9);
coffeeShop.takeOrder(flavor: "Frappe", table: 12);
coffeeShop.takeOrder(flavor: "Cappuccino", table: 13);
coffeeShop.takeOrder(flavor: "Espresso", table: 5);

Example in Crystal[edit]

# Instances of CoffeeFlavor will be the Flyweights
class CoffeeFlavor
  def initialize(new_flavor : String)
    @name = new_flavor

  def to_s(io)
    io << @name

# Menu acts as a factory and cache for CoffeeFlavor flyweight objects
class Menu
  def initialize
    @flavors = {} of String => CoffeeFlavor

  def lookup(flavor_name : String)
    @flavors[flavor_name] ||=

  def total_flavors_made

# Order is the context of the CoffeeFlavor flyweight.
class Order
  private getter table_number : Int32, flavor : CoffeeFlavor

  def initialize(@table_number, @flavor)

  def serve
    puts "Serving #{flavor} to table #{table_number}"

class CoffeeShop
  private getter orders
  private getter menu

  def initialize
    @orders = [] of Order
    @menu =

  def take_order(flavor_name : String, table : Int32)
    flavor = menu.lookup(flavor_name)
    order =, flavor)
    @orders << order

  def service
    orders.each do |order|

  def report
    "Total CoffeeFlavor made: #{menu.total_flavors_made}"

# Program
shop =
shop.take_order("Cappuchino", 2)
shop.take_order("Frappe", 1)
shop.take_order("Espresso", 1)
shop.take_order("Frappe", 897)
shop.take_order("Cappuccino", 97)
shop.take_order("Frappe", 3)
shop.take_order("Espresso", 3)
shop.take_order("Cappuccino", 3)
shop.take_order("Espresso", 96)
shop.take_order("Frappe", 552)
shop.take_order("Cappuccino", 121)
shop.take_order("Espresso", 121)



Serving Cappuchino to table 2
Serving Frappe to table 1
Serving Espresso to table 1
Serving Frappe to table 897
Serving Cappuccino to table 97
Serving Frappe to table 3
Serving Espresso to table 3
Serving Cappuccino to table 3
Serving Espresso to table 96
Serving Frappe to table 552
Serving Cappuccino to table 121
Serving Espresso to table 121
Total CoffeeFlavor made: 4

See also[edit]


  1. ^ Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. pp. 195ff. ISBN 0-201-63361-2.
  2. ^ "The Flyweight design pattern - Problem, Solution, and Applicability". Retrieved 2017-08-12.
  3. ^ Gamma, Erich; Richard Helm; Ralph Johnson; John Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. pp. 205–206. ISBN 0-201-63361-2.
  4. ^ Calder, Paul R.; Linton, Mark A. (October 1990). Glyphs: Flyweight Objects for User Interfaces. The 3rd Annual ACM SIGGRAPH Symposium on User Interface Software and Technology. Snowbird, Utah, United States. pp. 92–101. doi:10.1145/97924.97935. ISBN 0-89791-410-4.
  5. ^ Weinand, Andre; Gamma, Erich; Marty, Rudolf (1988). ET++—an object oriented application framework in C++. OOPSLA (Object-Oriented Programming Systems, Languages and Applications). San Diego, California, United States. pp. 46–57. doi:10.1145/62083.62089. ISBN 0-89791-284-5.
  6. ^ "The Flyweight design pattern - Structure and Collaboration". Retrieved 2017-08-12.
  7. ^ "Date Model §". The (online) Python Language Reference. Python Software Foundation. Retrieved 7 March 2017.