Singleton
A Global Singleton is a class of which only one instance exists within a program.
You are encouraged to solve this task according to the task description, using any language you may know.
Any attempt to use non-static members of the class involves performing operations on this one instance.
ActionScript
<lang actionscript>package {
public class Singleton
{
private static var instance:Singleton;
// ActionScript does not allow private or protected constructors.
public function Singleton(enforcer:SingletonEnforcer) {
}
public static function getInstance():Singleton {
if (instance == null) instance = new Singleton(new SingletonEnforcer());
return instance;
}
}
}
internal class SingletonEnforcer {}</lang>
Ada
Non Thread Safe
<lang ada>package Global_Singleton is
procedure Set_Data (Value : Integer); function Get_Data return Integer;
private
type Instance_Type is record
-- Define instance data elements
Data : Integer := 0;
end record;
Instance : Instance_Type;
end Global_Singleton;</lang>
<lang ada>package body Global_Singleton is
-------------- -- Set_Data -- --------------
procedure Set_Data (Value : Integer) is
begin
Instance.Data := Value;
end Set_Data;
-------------- -- Get_Data -- --------------
function Get_Data return Integer is
begin
return Instance.Data;
end Get_Data;
end Global_Singleton;</lang>
Thread Safe
<lang ada>package Protected_Singleton is
procedure Set_Data (Value : Integer); function Get_Data return Integer;
private
protected Instance is
procedure Set(Value : Integer);
function Get return Integer;
private
Data : Integer := 0;
end Instance_Type;
end Protected_Singleton;</lang>
<lang ada>package body Protected_Singleton is
-------------- -- Set_Data -- --------------
procedure Set_Data (Value : Integer) is
begin
Instance.Set(Value);
end Set_Data;
-------------- -- Get_Data -- --------------
function Get_Data return Integer is
begin
return Instance.Get;
end Get_Data;
-------------- -- Instance -- --------------
protected body Instance is
---------
-- Set --
---------
procedure Set (Value : Integer) is
begin
Data := Value;
end Set;
---------
-- Get --
---------
function Get return Integer is
begin
return Data;
end Get;
end Instance;
end Protected_Singleton;</lang>
AutoHotkey
Translation of python borg pattern <lang AutoHotkey>b1 := borg() b2 := borg() msgbox % "b1 is b2? " . (b1 == b2) b1.datum := 3 msgbox % "b1.datum := 3`n...`nb1 datum: " b1.datum "`nb2 datum: " b2.datum ; is 3 also msgbox % "b1.datum is b2.datum ? " (b1.datum == b2.datum) return
borg(){
static borg
If !borg
borg := Object("__Set", "Borg_Set"
, "__Get", "Borg_Get")
return object(1, borg, "base", borg)
}
Borg_Get(brg, name)
{
Return brg[1, name]
}
Borg_Set(brg, name, val) {
brg[1, name] := val Return val
}</lang>
C
Since C doesn't really support classes anyhow, there's not much to do. If you want somethin akin to a singleton, what you do is first declare the interface functions in a header (.h) file. <lang c>#ifndef SILLY_H
- define SILLY_H
extern void JumpOverTheDog( int numberOfTimes); extern int PlayFetchWithDog( float weightOfStick);
- endif</lang>
Then in a separate C source (.c) file, define your structures, variables and functions. <lang c>...
- include "silly.h"
struct sDog {
float max_stick_weight; int isTired; int isAnnoyed;
};
static struct sDog lazyDog = { 4.0, 0,0 };
/* define functions used by the functions in header as static */ static int RunToStick( ) {... } /* define functions declared in the header file. */
void JumpOverTheDog(int numberOfTimes) { ...
lazyDog.isAnnoyed = TRUE;
} int PlayFetchWithDog( float weightOfStick ) { ...
if(weightOfStick < lazyDog.max_stick_weight){...
}</lang> Code using the singleton includes the header and cannot create a struct sDog as the definition is only in the C source (or other header privately included by the silly.c source). Only the functions declared in the header may be used externally. <lang c>...
- include "silly.h"
... /* code using the dog methods */
JumpOverTheDog( 4); retrieved = PlayFetchWithDog( 3.1);
...</lang>
C#
First attempt at thread-safety using locking.
Performance suffers because the lock is acquired every time Instance is accessed.
This implementation is extremely slow and should not be used (but is seen often).
<lang csharp>public sealed class Singleton1 //Lazy: Yes ||| Thread-safe: Yes ||| Uses locking: Yes
{
private static Singleton1 instance;
private static readonly object lockObj = new object();
public static Singleton1 Instance {
get {
lock(lockObj) {
if (instance == null) {
instance = new Singleton1();
}
}
return instance;
}
}
}</lang>
Fixes excessive locking by double-checking for null.
Still uses locking and implementation is ugly and verbose. <lang csharp>public sealed class Singleton2 //Lazy: Yes ||| Thread-safe: Yes ||| Uses locking: Yes, but only once {
private static Singleton2 instance; private static readonly object lockObj = new object();
public static Singleton2 Instance {
get {
if (instance == null) {
lock(lockObj) {
if (instance == null) {
instance = new Singleton2();
}
}
}
return instance;
}
}
}</lang>
Really simple implementation without locking.
It still is not completely lazy. If there are other static members, accessing any of those will still cause initialization. <lang csharp>public sealed class Singleton3 //Lazy: Yes, but not completely ||| Thread-safe: Yes ||| Uses locking: No {
private static Singleton3 Instance { get; } = new Singleton3();
static Singleton3() { }
}</lang>
Truly lazy by using an inner class.
This version is completely lazy but the code looks more complicated than it needs to be. <lang csharp>public sealed class Singleton4 //Lazy: Yes ||| Thread-safe: Yes ||| Uses locking: No {
public static Singleton4 Instance => SingletonHolder.instance;
private class SingletonHolder
{
static SingletonHolder() { }
internal static readonly Singleton4 instance = new Singleton4();
}
}</lang>
Using Lazy<T>
C# has a dedicated type for lazy initialization: Lazy<T>.
It makes implementing a Singleton really easy. Recommended.
<lang csharp>public sealed class Singleton5 //Lazy: Yes ||| Thread-safe: Yes ||| Uses locking: No
{
private static readonly Lazy<Singleton5> lazy = new Lazy<Singleton5>(() => new Singleton5()); public static Singleton5 Instance => lazy.Value;
}</lang>
C++
A generic singleton template class (implemented via the "Curiously Recurring Template Pattern"[1]). Warning: if using a version of C++ prior to C++11, a mutex (or similar) is required to access static variables within a multi-threaded program.
<lang cpp>
- include <stdexcept>
template <typename Self> class singleton { protected: static Self* sentry; public: static Self& instance() { return *sentry; } singleton() { if(sentry) throw std::logic_error("Error: attempt to instantiate a singleton over a pre-existing one!"); sentry = (Self*)this; } virtual ~singleton() { if(sentry == this) sentry = 0; } }; template <typename Self> Self* singleton<Self>::sentry = 0;
/* Example usage:
- /
- include <iostream>
- include <string>
using namespace std;
class controller : public singleton<controller> { public: controller(string const& name) : name(name) { trace("begin"); } ~controller() { trace("end"); } void work() { trace("doing stuff"); } void trace(string const& message) { cout << name << ": " << message << endl; } string name; }; int main() { controller* first = new controller("first"); controller::instance().work(); delete first; /* No problem, our first controller no longer exists...
- /
controller second("second"); controller::instance().work(); try { /* Never happens... */ controller goner("goner"); controller::instance().work(); } catch(exception const& error) { cout << error.what() << endl; } controller::instance().work(); /* Never happens (and depending on your system this may or may not print a helpful message!)
- /
controller goner("goner"); controller::instance().work(); } </lang>
Caché ObjectScript
In Caché, each job runs in a self-contained execution environment (i.e. a separate process instead of a thread). However, it is possible for each process to share data through multidimensional storage (global variables). This is because when the Caché virtual machine starts, it allocates a single, large chunk of shared memory to allow all Caché processes to access this data simultaneously. However, it is the responsibility of the application developer to ensure read and write access to objects is properly co-ordinated (or 'synchronized') between processes to prevent concurrency problems. Also, Caché defines any global variable whose name starts with 'CacheTemp' as being temporary, which means changes are not usually written to disk and are instead maintained within the in-memory buffer pool.
<lang cos> /// The <CLASS>Singleton</CLASS> class represents a global singleton object that can /// be instantiated by multiple processes. The 'Get' class method is used to obtain /// an in-memory object reference and the 'Set' method is used to save any changes to /// state. See below for an example. /// /// <EXAMPLE> /// Set one=##class(Singleton).Get(,.sc) /// Set one.GlobalProperty="Some Value" /// Set sc=one.Set() /// </EXAMPLE> /// /// This class can also be extended. Class User.Singleton Extends %SerialObject {
Property GlobalProperty As %String;
/// Refer to <LINK href=/AboutConcurrency.html>About Concurrency</LINK> for more details /// on the optional pConcurrency argument. ClassMethod Get(pConcurrency As %Integer = -1, Output pStatus As %Status = {$$$OK}) As Singleton [ Final ] { // check if singleton object already instantiated Set oRef = "" For { Set oRef = $ZObjNext(oRef) If oRef = "" Quit If oRef.%ClassName(1) = ..%ClassName(1) Quit } If $IsObject(oRef) Quit oRef
// determine what lock needs to be applied If '$IsValidNum(pConcurrency, 0, -1, 4) { Set pStatus = $$$ERROR($$$LockTypeInvalid, pConcurrency) Quit $$$NULLOREF } If pConcurrency = -1 Set pConcurrency = $Xecute("Quit "_..#DEFAULTCONCURRENCY)
// acquire lock for global singleton object Set lockTO = $ZUtil(115,4), lockOK = 1 If pConcurrency<4, pConcurrency { Lock +^CacheTempUser("Singleton", ..%ClassName(1))#"S":lockTO Set lockOK = $Test } ElseIf pConcurrency = 4 { Lock +^CacheTempUser("Singleton", ..%ClassName(1)):lockTO Set lockOK = $Test } If 'lockOK { If pConcurrency = 4 { Set pStatus = $$$ERROR($$$LockFailedToAcquireExclusive, ..%ClassName(1)) } Else { Set pStatus = $$$ERROR($$$LockFailedToAcquireRead, ..%ClassName(1)) } Quit $$$NULLOREF }
// retrieve global singleton object and deserialise Set oId = $Get(^CacheTempUser("Singleton", ..%ClassName(1))) Set oRef = ..%Open(oId) //,, .pStatus) If '$IsObject(oRef) Set pStatus = $$$ERROR($$$GeneralError, "Failed to load singleton object.")
// release temporary lock If (pConcurrency = 1) || (pConcurrency = 2) { Lock -^CacheTempUser("Singleton", ..%ClassName(1))#"S" }
// singleton object failed to load If $$$ISERR(pStatus) { // release retained lock If pConcurrency = 3 { Lock -^CacheTempUser("Singleton", ..%ClassName(1))#"S" } If pConcurrency = 4 { Lock -^CacheTempUser("Singleton", ..%ClassName(1)) } Quit $$$NULLOREF }
// store concurrency state and return in-memory object reference Set oRef.Concurrency = pConcurrency Quit oRef }
Method Set() As %Status [ Final ] { // check for version change Set oId0 = $Get(^CacheTempUser("Singleton", ..%ClassName(1))) Set oRef0 = ..%Open(oId0) //,, .sc) If '$IsObject(oRef0) Quit $$$ERROR($$$GeneralError, "Failed to load singleton object.") If oRef0.Version = ..Version { Set ..Version = ..Version + 1 } Else { Quit $$$ERROR($$$ConcurrencyVersionMismatch, ..%ClassName(1)) }
// serialise local singleton object and check status code Set sc = ..%GetSwizzleObject(,.oId) If $$$ISERR(sc) Quit sc
// acquire exclusive lock on global singleton object Set lockTO = $ZUtil(115,4) Lock +^CacheTempUser("Singleton", ..%ClassName(1)):lockTO If '$Test Quit $$$ERROR($$$LockFailedToAcquireExclusive, ..%ClassName(1))
// update global singleton object and release lock Set ^CacheTempUser("Singleton", ..%ClassName(1)) = oId Lock -^CacheTempUser("Singleton", ..%ClassName(1)) Quit $$$OK }
Method %OnNew() As %Status [ Final, Internal ] { // do not allow constructor method to be called Quit $$$ERROR($$$GeneralError, "Can't instantiate directly.") }
Method %OnConstructClone() As %Status [ Final, Internal ] { // do not allow singleton object to be cloned Quit $$$ERROR($$$GeneralError, "Can't clone instance.") }
Method %OnClose() As %Status [ Final, Internal ] { // reference count for singleton object is now zero, so // release lock on global singleton object, if applicable If ..Concurrency = 3 Lock -^CacheTempUser("Singleton", ..%ClassName(1))#"S" If ..Concurrency = 4 Lock -^CacheTempUser("Singleton", ..%ClassName(1)) Quit $$$OK }
Property Concurrency As %Integer [ Final, Private, Transient ];
Property Version As %Integer [ Final, Private ];
} </lang>
- Examples:
USER>Set one=##class(Singleton).Get() USER>Set one.GlobalProperty="Some Value" USER>Set sc=one.Set()
Common Lisp
Since Common Lisp uses generic functions for dispatch, creating a class is not necessary. If the superclasses of the singleton are not important, the simplest thing to do is to use a particular symbol; methods use eql specializers to be applicable to only that object.
For a simple example, the following program constructs English sentences without worrying about extra space occurring at points where no text (the-empty-phrase, our singleton) is inserted.
<lang lisp>(defgeneric concat (a b)
(:documentation "Concatenate two phrases."))
(defclass nonempty-phrase ()
((text :initarg :text :reader text)))
(defmethod concat ((a nonempty-phrase) (b nonempty-phrase))
(make-instance 'nonempty-phrase :text (concatenate 'string (text a) " " (text b))))
(defmethod concat ((a (eql 'the-empty-phrase)) b)
b)
(defmethod concat (a (b (eql 'the-empty-phrase)))
a)
(defun example ()
(let ((before (make-instance 'nonempty-phrase :text "Jack"))
(mid (make-instance 'nonempty-phrase :text "went"))
(after (make-instance 'nonempty-phrase :text "to fetch a pail of water")))
(dolist (p (list 'the-empty-phrase
(make-instance 'nonempty-phrase :text "and Jill")))
(dolist (q (list 'the-empty-phrase
(make-instance 'nonempty-phrase :text "up the hill")))
(write-line (text (reduce #'concat (list before p mid q after))))))))</lang>
Thread safety is irrelevant since the singleton is created at load time, not first access.
D
<lang d>module singleton ; import std.stdio ; import std.thread ; import std.random ; import std.c.time ;
class Dealer {
private static Dealer me ;
static Dealer Instance() {
writefln(" Calling Dealer... ") ;
if(me is null) // Double Checked Lock
synchronized // this part of code can only be executed by one thread a time
if(me is null)
me = new Dealer ;
return me ;
}
private static string[] str = ["(1)Enjoy", "(2)Rosetta", "(3)Code"] ;
private int state ;
private this() {
for(int i = 0 ; i < 3 ; i++) {
writefln("...calling Dealer... ") ;
msleep(rand() & 2047) ;
}
writefln(">>Dealer is called to come in!") ;
state = str.length - 1 ;
}
Dealer nextState() {
synchronized(this) // accessed to Object _this_ is locked ... is it necessary ???
state = (state + 1) % str.length ;
return this ;
}
string toString() { return str[state] ; }
}
class Coder : Thread {
private string name_ ;
Coder hasName(string name) { name_ = name ; return this ; }
override int run() {
msleep(rand() & 1023) ;
writefln(">>%s come in.", name_) ;
Dealer single = Dealer.Instance ;
msleep(rand() & 1023) ;
for(int i = 0 ; i < 3 ; i++) {
writefln("%9s got %-s", name_, single.nextState) ;
msleep(rand() & 1023) ;
}
return 0 ;
}
}
void main() {
Coder x = new Coder ;
Coder y = new Coder ;
Coder z = new Coder ;
x.hasName("Peter").start() ;
y.hasName("Paul").start() ;
z.hasName("Mary").start() ;
x.wait ; y.wait ; z.wait ;
}</lang>
- Output:
>>Mary come in.
Calling Dealer...
...calling Dealer...
>>Peter come in.
Calling Dealer...
>>Paul come in.
Calling Dealer...
...calling Dealer...
...calling Dealer...
>>Dealer is called to come in!
Mary got (1)Enjoy
Peter got (2)Rosetta
Mary got (3)Code
Paul got (1)Enjoy
Peter got (2)Rosetta
Paul got (3)Code
Paul got (1)Enjoy
Mary got (2)Rosetta
Peter got (3)Code
Delphi and Pascal
Detailed explanation here. (Delphi started out as an object-oriented version of Pascal.) <lang Delphi>unit Singleton;
interface
type
TSingleton = class private //Private fields and methods here...
class var _instance: TSingleton; protected //Other protected methods here... public //Global point of access to the unique instance class function Create: TSingleton;
destructor Destroy; override;
//Other public methods and properties here... end;
implementation
{ TSingleton }
class function TSingleton.Create: TSingleton; begin
if (_instance = nil) then _instance:= inherited Create as Self;
result:= _instance;
end;
destructor TSingleton.Destroy; begin
_instance:= nil; inherited;
end;
end.</lang>
E
Since E uses closure-style objects rather than classes, a singleton is simply an object which is defined at the top level of the program, not inside any method. There are no thread-safety issues since the singleton, like every other object, belongs to some particular vat (but can be remotely invoked from other vats). <lang e>def aSingleton {
# ...
}</lang>
Eiffel
Non-Thread Safe
Taken from this dated site
Implementation: <lang Eiffel>class SINGLETON create {SINGLETON_ACCESS} default_create feature -- singleton features go here end</lang> <lang Eiffel>frozen class SINGLETON_ACCESS feature singleton: SINGLETON once ("PROCESS") create Result ensure Result /= Void end end</lang> Usage: <lang Eiffel>s: SINGLETON -- declaration somewhere
s := (create{SINGLETON_ACCESS}).singleton -- in some routine</lang>
Elena
Stateless singleton <lang elena> singleton Singleton {
// ...
} </lang> Normal singleton <lang elena>class Singleton {
object theField;
// ...
}
static singleton = new Singleton();</lang>
Erlang
Erlang is not object-oriented, so there is no such thing as a singleton class. The singleton is something of an anti-pattern in Erlang, so if you are tempted to do this, there is probably a better architecture. If you do want something akin to a singleton, you start and register a process that maintains its state in a message loop and provides its state to anyone that wants it or needs to change it. Since this is done with message passing, it's safe for concurrent use. <lang Erlang>-module(singleton).
-export([get/0, set/1, start/0]).
-export([loop/1]).
% spec singleton:get() -> {ok, Value::any()} | not_set get() ->
?MODULE ! {get, self()},
receive
{ok, not_set} -> not_set;
Answer -> Answer
end.
% spec singleton:set(Value::any()) -> ok set(Value) ->
?MODULE ! {set, self(), Value},
receive
ok -> ok
end.
start() ->
register(?MODULE, spawn(?MODULE, loop, [not_set])).
loop(Value) ->
receive
{get, From} ->
From ! {ok, Value},
loop(Value);
{set, From, NewValue} ->
From ! ok,
loop(NewValue)
end.</lang>
Here is an example of how to use it (from the shell). It assumes singleton:start/0 was already called from the supervisor tree (as would be typical if you were using something like this).
<lang Erlang>1> singleton:get(). not_set 2> singleton:set(apple). ok 3> singleton:get(). {ok,apple} 4> singleton:set("Pear"). ok 5> singleton:get(). {ok,"Pear"} 6> singleton:set(42). ok 7> singleton:get(). {ok,42}</lang>
Factor
<lang factor>USING: classes.singleton kernel io prettyprint ; IN: singleton-demo
SINGLETON: bar GENERIC: foo ( obj -- ) M: bar foo drop "Hello!" print ;</lang>
( scratchpad ) bar foo Hello!
Forth
Works with any ANS Forth
Needs the FMS-SI (single inheritance) library code located here: http://soton.mpeforth.com/flag/fms/index.html <lang forth>include FMS-SI.f
\ A singleton is created by using normal Forth data \ allocation words such as value or variable as instance variables. \ Any number of instances of a singleton class may be \ instantiated but messages will all operate on the same shared data \ so it is the same as if only one object has been created. \ The data name space will remain private to the class.
- class singleton
0 value a 0 value b :m printa a . ;m :m printb b . ;m :m add-a ( n -- ) a + to a ;m :m add-b ( n -- ) b + to b ;m
- class
singleton s1 singleton s2 singleton s3
4 s1 add-a 9 s2 add-b s3 printa \ => 4 s3 printb \ => 9 s1 printb \ => 9 s2 printa \ => 4 </lang>
Go
sync.Once
From the Go standard library, sync.Once provides a way to ensure that some "step," effectively an initialization step, is performed no more than once even if it might be attempted from multiple concurrent goroutines. This capability might be considered similar to some mechanism ensuring that singleton constructor code is only run once. <lang go>package main
import (
"log" "math/rand" "sync" "time"
)
var (
instance string once sync.Once // initialize instance with once.Do
)
func claim(color string, w *sync.WaitGroup) {
time.Sleep(time.Duration(rand.Intn(1e8))) // hesitate up to .1 sec
log.Println("trying to claim", color)
once.Do(func() { instance = color })
log.Printf("tried %s. instance: %s", color, instance)
w.Done()
}
func main() {
rand.Seed(time.Now().Unix())
var w sync.WaitGroup
w.Add(2)
go claim("red", &w) // these two attempts run concurrently
go claim("blue", &w)
w.Wait()
log.Println("after trying both, instance =", instance)
}</lang>
- Output:
2016/07/01 20:36:02 trying to claim red 2016/07/01 20:36:02 tried red. instance: red 2016/07/01 20:36:02 trying to claim blue 2016/07/01 20:36:02 tried blue. instance: red 2016/07/01 20:36:02 after trying both, instance = red
Packages as singletons
Go packages are singletons, in a way. Go does not use the word "class," and while Go structs might seem most like classes of other languages, Go packages are also like classes in that they represent an organization of declarations, including data and functions. All declarations in a package form a single package block. This block is delimited syntactically, has an associated identifier, and its members are accessed by this package identifier. This is much like classes in other languages.
Because packages cannot be imported multiple times, data declared at package level will only ever have a single instance, and the package as a whole serves as a singleton. <lang go>package singlep
// package level data declarations serve as singleton instance variables var X, Y int
// package level initialization can serve as constructor code func init() {
X, Y = 2, 3
}
// package level functions serve as methods for a package-as-a-singleton func F() int {
return Y - X
}</lang> Example program using the package: <lang go>package main
import (
"fmt" "singlep"
)
func main() {
// dot selector syntax references package variables and functions fmt.Println(singlep.X, singlep.Y) fmt.Println(singlep.F())
}</lang>
- Output:
2 3 1
Package data initialization with sync.Once
This example combines the two previous concepts and also shows some additional concepts. It has packages imported with a "diamond" dependency. While both red and blue import single, only a single variable color will exist in memory. The init() mechanism shown above actually runs before main(). In contrast, the sync.Once mechanism can serve as constructor code after main() begins.
<lang go>package single
import (
"log" "sync"
)
var (
color string once sync.Once
)
func Color() string {
if color == "" {
panic("color not initialized")
}
return color
}
func SetColor(c string) {
log.Println("color initialization")
once.Do(func() { color = c })
log.Println("color initialized to", color)
}</lang> <lang go>package red
import (
"log"
"single"
)
func SetColor() {
log.Println("trying to set red")
single.SetColor("red")
}</lang> <lang go>package blue
import (
"log"
"single"
)
func SetColor() {
log.Println("trying to set blue")
single.SetColor("blue")
}</lang> <lang go>package main
import (
"log" "math/rand" "time"
"blue" "red" "single"
)
func main() {
rand.Seed(time.Now().Unix())
switch rand.Intn(3) {
case 1:
red.SetColor()
blue.SetColor()
case 2:
blue.SetColor()
red.SetColor()
}
log.Println(single.Color())
}</lang>
- Output:
2016/07/01 20:52:18 trying to set red 2016/07/01 20:52:18 color initialization 2016/07/01 20:52:18 color initialized to red 2016/07/01 20:52:18 trying to set blue 2016/07/01 20:52:18 color initialization 2016/07/01 20:52:18 color initialized to red 2016/07/01 20:52:18 red
Groovy
<lang groovy>@Singleton class SingletonClass {
def invokeMe() {
println 'invoking method of a singleton class'
}
static void main(def args) {
SingletonClass.instance.invokeMe()
}
}</lang>
- Output:
invoking method of a singleton class
Icon and Unicon
Icon is not object oriented, but Unicon supports O-O programming. <lang unicon>class Singleton
method print()
write("Hi there.")
end
initially
write("In constructor!")
Singleton := create |self
end
procedure main()
Singleton().print() Singleton().print()
end</lang>
This Unicon example uses a number of Icon features.
- The class descriptor Singleton is a first-class global object.
- The create keyword yields a co-routine which can be activated like a function call.
- The monadic operator | repeatedly yields the iteration of it's argument - in this case, it yields the object created (self).
- The initializer of each object actually replaces the global object Singleton with a coroutine that returns ... the first object created. Therefore there is no further access to the true Singleton constructor; future attempts to create the object instead just activates the co-routine.
NOTE: this could be subverted by capturing a reference to Singleton prior to the first object construction.
Io
Io does not have globals. But it is easy to make singleton objects: <lang io>Singleton := Object clone Singleton clone = Singleton</lang>
J
In J, all classes are singletons though their objects are not. (Class names may be used in any context where object references may be used, and object references can be used in almost every context where a class name may be used.)
Singletons should not have a constructor so any attempt to construct an instance of a singleton (dyadic conew) would fail. Other than that, singletons are defined like any other class in J.
Java
Thread-safe
wp:Double-checked locking; only use with Java 1.5+ <lang java>class Singleton {
private static Singleton myInstance;
public static Singleton getInstance()
{
if (myInstance == null)
{
synchronized(Singleton.class)
{
if (myInstance == null)
{
myInstance = new Singleton();
}
}
}
return myInstance; }
protected Singleton()
{
// Constructor code goes here.
}
// Any other methods
}</lang>
Thread-Safe Lazy-Loaded
This is the wp:Initialization-on-demand holder idiom. <lang java>public class Singleton {
private Singleton() {
// Constructor code goes here.
}
private static class LazyHolder {
private static final Singleton INSTANCE = new Singleton();
}
public static Singleton getInstance() {
return LazyHolder.INSTANCE;
}
}</lang>
Non-Thread-Safe
<lang java>class Singleton {
private static Singleton myInstance;
public static Singleton getInstance()
{
if (myInstance == null)
{
myInstance = new Singleton();
}
return myInstance; }
protected Singleton()
{
// Constructor code goes here.
}
// Any other methods
}</lang>
JavaScript
<lang JavaScript>function Singleton() { if(Singleton._instance) return Singleton._instance; this.set(""); Singleton._instance = this; }
Singleton.prototype.set = function(msg) { this.msg = msg; } Singleton.prototype.append = function(msg) { this.msg += msg; } Singleton.prototype.get = function() { return this.msg; }
var a = new Singleton();
var b = new Singleton();
var c = new Singleton();
a.set("Hello"); b.append(" World"); c.append("!!!");
document.write( (new Singleton()).get() );</lang>
Julia
Julia allows singletons as type declarations without further specifiers. There can be only one instance of such a type, and if more than one variable is bound to such a type they are actually all bound to the same instance in memory: <lang julia> struct IAmaSingleton end
x = IAmaSingleton() y = IAmaSingleton()
println("x == y is $(x == y) and x === y is $(x === y).") </lang>
Kotlin
Kotlin has built-in support for singletons via object declarations. To refer to the singleton, we simply use its name which can be any valid identifier other than a keyword: <lang scala>// version 1.1.2
object Singleton {
fun speak() = println("I am a singleton")
}
fun main(args: Array<String>) {
Singleton.speak()
}</lang>
- Output:
I am a singleton
Lasso
Lasso supports singletons on two levels.
Server wide singleton
<lang Lasso>// Define the thread if it doesn't exist // New definition supersede any current threads.
not ::serverwide_singleton->istype ? define serverwide_singleton => thread {
data public switch = 'x'
}
local(
a = serverwide_singleton, b = serverwide_singleton,
)
- a->switch = 'a'
- b->switch = 'b'
- a->switch // b</lang>
Thread level singleton
<lang Lasso>// Define thread level singleton
define singleton => type {
data public switch = 'x' public oncreate => var(.type)->isa(.type) ? var(.type) | var(.type) := self
}
local(
a = singleton, b = singleton,
)
- a->switch = 'a'
- b->switch = 'b'
- a->switch // b</lang>
Latitude
Latitude objects are prototypes, so any new object can be treated as a singleton by simply not cloning it. For added security, one can always override clone to make it clear that the object should not be cloned, but this is generally overkill.
<lang latitude>Singleton ::= Object clone tap {
self id := 0.
self newID := {
self id := self id + 1.
}.
self clone := {
err ArgError clone tap { self message := "Singleton object!". } throw.
}.
}.
println: Singleton newID. ; 1 println: Singleton newID. ; 2 println: Singleton newID. ; 3</lang>
Lingo
In Lingo a Singleton class can be implemented like this: <lang lingo>-- parent script "SingletonDemo"
property _instance property _someProperty
-- @constructor
on new (me)
if not voidP(me.script._instance) then return me.script._instance me.script._instance = me me._someProperty = 0 return me
end
-- sample method
on someMethod (me, x)
me._someProperty = me._someProperty + x return me._someProperty
end</lang>
Logtalk
Logtalk supports both classes and prototypes. A prototype is a much simpler solution for defining a singleton object than defining a class with only an instance. <lang logtalk>:- object(singleton).
:- public(value/1).
value(Value) :-
state(Value).
:- public(set_value/1).
set_value(Value) :-
retract(state(_)),
assertz(state(Value)).
:- private(state/1). :- dynamic(state/1). state(0).
- - end_object.</lang>
A simple usage example after compiling and loading the code above: <lang logtalk>| ?- singleton::value(Value). Value = 0 yes
| ?- singleton::(set_value(1), value(Value)). Value = 1 yes</lang>
NetRexx
Uses a static field to avoid synchronization problems and the flawed "double-checked locking" idiom in JVMs. See www.ibm.com/developerworks/java/library/j-dcl/index.html for a detailed explanation. <lang NetRexx>/* NetRexx */ options replace format comments java crossref symbols binary
import java.util.random
class RCSingleton public
method main(args = String[]) public static RCSingleton.Testcase.main(args) return
-- --------------------------------------------------------------------------- class RCSingleton.Instance public
properties private static
_instance = Instance()
properties private
_refCount = int
_random = Random
method Instance() private
this._refCount = 0
this._random = Random()
return
method getInstance public static returns RCSingleton.Instance
return _instance
method getRandom public returns Random return _random
method addRef public protect
_refCount = _refCount + 1
return
method release public protect
if _refCount > 0 then
_refCount = _refCount - 1
return
method getRefCount public protect returns int
return _refCount
-- --------------------------------------------------------------------------- class RCSingleton.Testcase public implements Runnable
properties private _instance = RCSingleton.Instance
method run public
say threadInfo'|-'
thud = Thread.currentThread
_instance = RCSingleton.Instance.getInstance
thud.yield
_instance.addRef
say threadInfo'|'_instance.getRefCount
thud.yield
do
thud.sleep(_instance.getRandom.nextInt(1000))
catch ex = InterruptedException
ex.printStackTrace
end
_instance.release
say threadInfo'|'_instance.getRefCount
return
method main(args = String[]) public static
threads = [ Thread -
Thread(Testcase()), Thread(Testcase()), Thread(Testcase()), -
Thread(Testcase()), Thread(Testcase()), Thread(Testcase()) ]
say threadInfo'|-'
mn = Testcase()
mn._instance = RCSingleton.Instance.getInstance
say mn.threadInfo'|'mn._instance.getRefCount
mn._instance.addRef
say mn.threadInfo'|'mn._instance.getRefCount
do
loop tr over threads
(Thread tr).start
end tr
Thread.sleep(400)
catch ex = InterruptedException
ex.printStackTrace
end
mn._instance.release
say mn.threadInfo'|'mn._instance.getRefCount
return
method threadInfo public static returns String
trd = Thread.currentThread
tid = trd.getId
hc = trd.hashCode
info = Rexx(trd.getName).left(16, '_')':' -
|| Rexx(Long.toString(tid)).right(10, 0)':' -
|| '@'Rexx(Integer.toHexString(hc)).right(8, 0)
return info
</lang>
- Output:
main____________:0000000001:@035a8767|- main____________:0000000001:@035a8767|0 main____________:0000000001:@035a8767|1 Thread-1________:0000000010:@22998b08|- Thread-1________:0000000010:@22998b08|2 Thread-2________:0000000011:@7a6d084b|- Thread-2________:0000000011:@7a6d084b|3 Thread-3________:0000000012:@2352544e|- Thread-4________:0000000013:@457471e0|- Thread-5________:0000000014:@7ecec0c5|- Thread-6________:0000000015:@3dac2f9c|- Thread-3________:0000000012:@2352544e|4 Thread-4________:0000000013:@457471e0|5 Thread-5________:0000000014:@7ecec0c5|6 Thread-6________:0000000015:@3dac2f9c|7 Thread-5________:0000000014:@7ecec0c5|6 main____________:0000000001:@035a8767|5 Thread-3________:0000000012:@2352544e|4 Thread-1________:0000000010:@22998b08|3 Thread-6________:0000000015:@3dac2f9c|2 Thread-2________:0000000011:@7a6d084b|1 Thread-4________:0000000013:@457471e0|0
Nim
In the file singleton.nim we don't export the type, so new objects can't be created:
<lang nim>type Singleton = object # Singleton* would export
foo*: int
var single* = Singleton(foo: 0)</lang> Then in another file we can use the singleton object: <lang nim>import singleton
single.foo = 12 echo single.foo</lang>
Objeck
<lang objeck>class Singleton {
@singleton : static : Singleton;
New : private () {
}
function : GetInstance() ~ Singleton {
if(@singleton <> Nil) {
@singleton := Singleton->New();
};
return @singleton; }
method : public : DoStuff() ~ Nil {
...
}
}</lang>
Objective-C
Non-Thread-Safe
(Using Cocoa/OpenStep's NSObject as a base class) <lang objc>// SomeSingleton.h @interface SomeSingleton : NSObject {
// any instance variables
}
+ (SomeSingleton *)sharedInstance;
@end</lang>
<lang objc>// SomeSingleton.m @implementation SomeSingleton
+ (SomeSingleton *) sharedInstance {
static SomeSingleton *sharedInstance = nil;
if (!sharedInstance) {
sharedInstance = [[SomeSingleton alloc] init];
}
return sharedInstance;
}
- (id)copyWithZone:(NSZone *)zone {
return self;
}
- (id)retain {
return self;
}
- (unsigned)retainCount {
return UINT_MAX;
}
- (oneway void)release {
// prevent release
}
- (id)autorelease {
return self;
}
@end</lang>
Thread-Safe
Same as above except: <lang objc>+ (SomeSingleton *) sharedInstance {
static SomeSingleton *sharedInstance = nil;
@synchronized(self) {
if (!sharedInstance) {
sharedInstance = [[SomeSingleton alloc] init];
}
}
return sharedInstance;
}</lang>
With GCD
Same as above except: <lang objc>+ (SomeSingleton *) sharedInstance {
static SomeSingleton *sharedInstance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedInstance = [[SomeSingleton alloc] init];
});
return sharedInstance;
}</lang>
With class methods
It's possible to accomplish the same thing with class methods of some class, rather than instance methods on the instance of a singleton class. Data that needs to be kept as "instance variables" would instead be kept as static (file-local) global variables. "Initialization" of the singleton object would be done in the +initialize method, which is guaranteed to be called at most once for every class, the first time the class is messaged. This way, the singleton is also "lazy loaded" as needed.
In other words, here the class object serves as the singleton object. The "singleton class" is the metaclass of the class. The downside of this approach is that the "singleton class" (the metaclass of the class) cannot be made to explicitly inherit from a class of the user's choice, or implement a protocol of the user's choice. Also, there is no way to prevent subclasses of the class from being made, thus effectively creating "multiple instances" of the singleton class. Also, one cannot declare properties on the singleton (the class object).
Oforth
Oforth does not have global variables, class attributes or some kind of shared mutable memory that can be updated by different tasks.
In Oforth, singleton is an anti-pattern because it needs synchronisation in order to be safe between parallel tasks.
If the goal is to keep and update a value in a safe way, a channel can be used.
For instance, this Sequence class creates instances that increment an integer and send it. If a task tries to get the next value before it is incremented, it will wait until the channel is no more empty and holds the new value. This won't work if the value is a mutable value (you will get an exception if you try to send a mutable object into channel). A mutable object can't be shared between tasks. Here we send a new integer each time.
<lang Oforth>Object Class new: Sequence(channel) Sequence method: initialize(initialValue)
Channel newSize(1) := channel @channel send(initialValue) drop ;
Sequence method: nextValue @channel receive dup 1 + @channel send drop ;</lang>
Usage : <lang Oforth>import: parallel
- testSequence
| s i |
Sequence new(0) ->s 100 loop: i [ #[ s nextValue println ] & ] ;</lang>
ooRexx
<lang ooRexx> a = .singleton~new b = .singleton~new
a~foo = "Rick" if a~foo \== b~foo then say "A and B are not the same object"
- class singleton
-- initialization method for the class
- method init class
expose singleton -- mark this as unallocated. We could also just allocate -- the singleton now, but better practice is probably wait -- until it is requested singleton = .nil
-- override the new method. Since this is a guarded -- method by default, this is thread safe
- method new class
expose singleton
-- first request? Do the real creation now
if singleton == .nil then do
-- forward to the super class. We use this form of
-- FORWARD rather than explicit call ~new:super because
-- this takes care of any arguments passed to NEW as well.
forward class(super) continue
singleton = result
end
return singleton
-- an attribute that can be used to demonstrate this really is a singleton.
- attribute foo
</lang>
OxygenBasic
The singleton contains static members only. It may be instantiated any number of times, but the members will all be shared. <lang oxygenbasic> class singleton static sys a,b,c static string s,t,u static double x,y,z end class
'TEST '====
singleton A singleton B A.c=3 print B.c 'result 3 print sizeof B 'result 0 </lang>
Oz
Singleton is not a common pattern in Oz programs. It can be implemented by limiting the scope of the class definition such that only the GetInstance function has access to it.
<lang oz>declare
local
class Singleton
meth init skip end
end
L = {NewLock}
Instance
in
fun {GetInstance}
lock L then if {IsFree Instance} then Instance = {New Singleton init} end Instance end
end end</lang>
This will work as long as all functors are linked with import statements. If you use multiple calls to Module.link instead, you will get multiple instances of the "Singleton".
Perl
<lang Perl>package Singleton; use strict; use warnings;
my $Instance;
sub new {
my $class = shift;
$Instance ||= bless {}, $class; # initialised once only
}
sub name {
my $self = shift;
$self->{name};
}
sub set_name {
my ($self, $name) = @_;
$self->{name} = $name;
}
package main;
my $s1 = Singleton->new; $s1->set_name('Bob'); printf "name: %s, ref: %s\n", $s1->name, $s1;
my $s2 = Singleton->new; printf "name: %s, ref: %s\n", $s2->name, $s2;</lang>
Phix
Not really any special handling for singletons in Phix, but you can do something like this, or keep check() private and invoke it internally from a few critical routines. Needs 0.8.1+ <lang Phix>-- <separate include file> object chk = NULL class singleton
public procedure check()
if chk==NULL then
chk = this
elsif this!=chk then
?9/0
end if
?"ok"
end procedure
end class
global singleton s = new() --global singleton s2 = new() -- </separate include file>
s.check()
--s2.check() -- dies</lang>
While all classes are technically global in the sense that builtins\structs.e knows all about them, the
implicit associated user defined type is by default private (ie w/o "global" in front of the class def).
The above separate file prohibits the use of (other) singleton s = new() everywhere, however
class s = new("singleton") could still be used anywhere, and that way get duplicates.
One obvious alternative (of no special merit imo) might be to replace that global singleton s with:
<lang Phix>global function get_singleton()
if chk==NULL then
chk = new("singleton")
end if
return chk
end function</lang> Technically, builtins/struct.e looks like it could easily be modified to support something very similar to the Python Borg pattern, by appropriately sharing cdx/tid in new(). However it would be even better to share the whole delete_routine()'d res, and that would probably be best achieved by setting a new flag, say S_SINGLETON or S_STATIC, alongside and triggered via similar syntax to S_ABSTRACT/S_NULLABLE. Maybe.
PHP
<lang PHP>class Singleton {
protected static $instance = null;
public $test_var;
private function __construct(){
//Any constructor code
}
public static function getInstance(){
if (is_null(self::$instance)){
self::$instance = new self();
}
return self::$instance;
}
}
$foo = Singleton::getInstance(); $foo->test_var = 'One';
$bar = Singleton::getInstance(); echo $bar->test_var; //Prints 'One'
$fail = new Singleton(); //Fatal error</lang>
PicoLisp
As there is no physical difference between classes and objects, we can use the class symbol itself. <lang PicoLisp>(class +Singleton)
(dm message1> ()
(prinl "This is method 1 on " This) )
(dm message2> ()
(prinl "This is method 2 on " This) )</lang>
- Output:
: (message1> '+Singleton) This is method 1 on +Singleton -> +Singleton : (message2> '+Singleton) This is method 2 on +Singleton -> +Singleton
PureBasic
Native version
Thread safe version. <lang PureBasic>Global SingletonSemaphore=CreateSemaphore(1)
Interface OO_Interface ; Interface for any value of this type
Get.i() Set(Value.i) Destroy()
EndInterface
Structure OO_Structure ; The *VTable structure
Get.i Set.i Destroy.i
EndStructure
Structure OO_Var
*VirtualTable.OO_Structure Value.i
EndStructure
Procedure OO_Get(*Self.OO_Var)
ProcedureReturn *Self\Value
EndProcedure
Procedure OO_Set(*Self.OO_Var, n)
*Self\Value = n
EndProcedure
Procedure CreateSingleton()
If TrySemaphore(SingletonSemaphore)
*p.OO_Var = AllocateMemory(SizeOf(OO_Var))
If *p
*p\VirtualTable = ?VTable
EndIf
EndIf
ProcedureReturn *p
EndProcedure
Procedure OO_Destroy(*Self.OO_Var)
FreeMemory(*Self) SignalSemaphore(SingletonSemaphore)
EndProcedure
DataSection
VTable: Data.i @OO_Get() Data.i @OO_Set() Data.i @OO_Destroy()
EndDataSection</lang>
Simple OOP extension
Using the open-source precompiler SimpleOOP. <lang PureBasic>Singleton Class Demo
BeginPrivate
Name$
X.i
EndPrivate
Public Method Init(Name$)
This\Name$ = Name$
EndMethod
Public Method GetX()
MethodReturn This\X
EndMethod
Public Method SetX(n)
This\X = n
EndMethod
Public Method Hello()
MessageRequester("Hello!", "I'm "+This\Name$)
EndMethod
EndClass</lang>
Python
per Borg Design
In Python we use the Borg pattern to share state between instances rather than concentrate on identity.
Every instance of the Borg class will share the same state: <lang python>>>> class Borg(object): __state = {} def __init__(self): self.__dict__ = self.__state # Any other class names/methods
>>> b1 = Borg()
>>> b2 = Borg()
>>> b1 is b2
False
>>> b1.datum = range(5)
>>> b1.datum
[0, 1, 2, 3, 4]
>>> b2.datum
[0, 1, 2, 3, 4]
>>> b1.datum is b2.datum
True
>>> # For any datum!</lang>
per MetaClass/AbstractBaseClass
An approximation of the singleton can be made using only class attributes to store data instead of the instance attributes, providing at least one abstract instance method (class can not be instantiated then) and making the rest of the methods being class methods. E.g.
<lang python> import abc
class Singleton(object):
"""
Singleton class implementation
"""
__metaclass__ = abc.ABCMeta
state = 1 #class attribute to be used as the singleton's attribute
@abc.abstractmethod
def __init__(self):
pass #this prevents instantiation!
@classmethod
def printSelf(cls):
print cls.state #prints out the value of the singleton's state
- demonstration
if __name__ == "__main__":
try:
a = Singleton() #instantiation will fail!
except TypeError as err:
print err
Singleton.printSelf()
print Singleton.state
Singleton.state = 2
Singleton.printSelf()
print Singleton.state
</lang>
When executed this code should print out the following:
Can't instantiate abstract class Singleton with abstract methods __init__
1
1
2
2
So, instantiation is not possible. Only a single object is available, and it behaves as a singleton.
per MetaClass
<lang python> class Singleton(type):
_instances = {}
def __call__(cls, *args, **kwargs):
if cls not in cls._instances:
cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
return cls._instances[cls]
class Logger(object):
__metaclass__ = Singleton
</lang>
or in Python3
<lang python>
class Logger(metaclass=Singleton):
pass
</lang>
Racket
Singletons are not very useful in Racket, because functions that use module state are more straightforward. However, classes are first class values, and therefore they follow the same rules as all other bindings. For example, a class can be made and instantiated but not provided to client files: <lang racket>
- lang racket
(provide instance) (define singleton%
(class object% (super-new)))
(define instance (new singleton%)) </lang>
Or better, not name the class at all: <lang racket>
- lang racket
(provide instance) (define instance
(new (class object%
(define/public (foo) 123)
(super-new))))
</lang>
Raku
(formerly Perl 6) <lang perl6>class Singleton {
# We create a lexical variable in the class block that holds our single instance.
my Singleton $instance = Singleton.bless; # You can add initialization arguments here.
method new {!!!} # Singleton.new dies.
method instance { $instance; }
}</lang>
Ruby
<lang ruby>require 'singleton' class MySingleton
include Singleton # constructor and/or methods go here
end
a = MySingleton.instance # instance is only created the first time it is requested b = MySingleton.instance puts a.equal?(b) # outputs "true"</lang>
Scala
The object construct in Scala is a singleton. <lang scala>object Singleton {
// any code here gets executed as if in a constructor
}</lang>
Sidef
<lang ruby>class Singleton(name) {
static instance;
method new(name) {
instance := Singleton.bless(Hash(:name => name));
}
method new {
Singleton.new(nil);
}
}
var s1 = Singleton('foo'); say s1.name; #=> 'foo' say s1.object_id; #=> '30424504'
var s2 = Singleton(); say s2.name; #=> 'foo' say s2.object_id; #=> '30424504'
s2.name = 'bar'; # change name in s2 say s1.name; #=> 'bar'</lang>
Slate
Clones of Oddball themselves may not be cloned. Methods and slots may still be defined on them: <lang slate>define: #Singleton &builder: [Oddball clone]</lang>
Smalltalk
<lang smalltalk> SomeClass class>>sharedInstance
SharedInstance ifNil: [SharedInstance := self basicNew initialize].
^ SharedInstance
</lang>
Swift
<lang swift>
class SingletonClass {
static let sharedInstance = SingletonClass()
///Override the init method and make it private
private override init(){
// User can do additional manipulations here.
}
} // Usage let sharedObject = SingletonClass.sharedInstance </lang>
Tcl
or
ref http://wiki.tcl.tk/21595 <lang tcl>package require TclOO
- This is a metaclass, a class that defines the behavior of other classes
oo::class create singleton {
superclass oo::class
variable object
unexport create ;# Doesn't make sense to have named singletons
method new args {
if {![info exists object]} {
set object [next {*}$args]
}
return $object
}
}
singleton create example {
method counter {} {
my variable count
return [incr count]
}
}</lang> Demonstrating in an interactive shell: <lang tcl>% set a [example new]
- oo::Obj20
% set b [example new] ;# note how this returns the same object name
- oo::Obj20
% expr {$a == $b} 1 % $a counter 1 % $b counter 2 % $a counter 3 % $b counter 4</lang>
Tern
Tern has built-in support for singletons via module declarations. <lang tern>module Singleton {
speak() {
println("I am a singleton");
}
}
Singleton.speak();</lang>
- Output:
I am a singleton
Vala
<lang Vala>public class Singleton : Object {
static Singleton? instance;
// Private constructor
Singleton() {
}
// Public constructor
public static Singleton get_instance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
void main() {
Singleton a = Singleton.get_instance();
Singleton b = Singleton.get_instance();
if (a == b) {
print("Equal.\n");
}
}</lang>
Wren
Although it's possible to create a singleton in Wren, you have to rely on no one calling the 'private' constructor directly. This is because there is currently no way to create a private method in Wren - all you can do is to suffix the name with an underscore to indicate by convention it's for internal use only.
In practice, it's unlikely anyone would bother; they'd just create a class with static methods and/or fields only which is effectively a singleton as there's only ever a single instance of a static field. <lang ecmascript>class Singleton {
// Returns the singleton. If it hasn't been created, creates it first.
static instance { __instance == null ? __instance = Singleton.new_() : __instance }
// Private constructor.
construct new_() {}
// instance method
speak() { System.print("I'm a singleton.") }
}
var s1 = Singleton.instance var s2 = Singleton.instance System.print("s1 and s2 are same object = %(Object.same(s1, s2))") s1.speak() // call instance method</lang>
- Output:
s1 and s2 are same object = true I'm a singleton.
zkl
A class declared static only has one instance, ever. However, a class with the same name & structure could be created in another scope. <lang zkl>class [static] Borg{ var v } b1 := Borg; b2 := Borg(); b1 == b2 //--> True b1.v=123; b2.v.println(); //--> 123</lang>