Singleton

A Global Singleton is a class of which only one instance exists within a program.

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>

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++

Thread-safe

Operating System: Microsoft Windows NT/XP/Vista

Uses a Win32 flavor Mutex - a POSIX flavor Mutex could be used.

<lang cpp>class Singleton { public:

    static Singleton* Instance()
    {
         // We need to ensure that we don't accidentally create two Singletons
         HANDLE hMutex = CreateMutex(NULL, FALSE, "MySingletonMutex");
         WaitForSingleObject(hMutex, INFINITE);

         // Create the instance of the class.
         // Since it's a static variable, if the class has already been created,
         // It won't be created again.
         static Singleton myInstance;

         // Release our mutex so that other application threads can use this function
         ReleaseMutex( hMutex );

         // Free the handle
         CloseHandle( hMutex );

         // Return a pointer to our mutex instance.
         return &myInstance;
    }

    // Any other public methods

protected:

    Singleton()
    {
         // Constructor code goes here.
    }
    ~Singleton()
    {
         // Destructor code goes here.
    }

    // And any other protected methods.

}</lang>

Non-Thread-Safe

This version doesn't require Mutex, but it is not safe in a multi-threaded environment (before C++11). <lang cpp>class Singleton { public:

    static Singleton* Instance()
    {
         // Since it's a static variable, if the class has already been created,
         // It won't be created again.
         static Singleton myInstance;

         // Return a pointer to our mutex instance.
         return &myInstance;
    }

    // Any other public methods

protected:

    Singleton()
    {
         // Constructor code goes here.
    }
    ~Singleton()
    {
         // Destructor code goes here.
    }

    // And any other protected methods.

}</lang>

Thread safe (since C++11)

This will be thread safe since C++11, where static variables is always thread-safe (according to section 6.7 of The Standard).

<lang cpp>class Singleton { public:

   static Singleton & Instance()
   {
       // Since it's a static variable, if the class has already been created,
       // It won't be created again.
       // And it **is** thread-safe in C++11.

       static Singleton myInstance;

       // Return a reference to our instance.
       return myInstance;
   }
   
   // delete copy and move constructors and assign operators
   Singleton(Singleton const&) = delete;             // Copy construct
   Singleton(Singleton&&) = delete;                  // Move construct
   Singleton& operator=(Singleton const&) = delete;  // Copy assign
   Singleton& operator=(Singleton &&) = delete;      // Move assign

   // Any other public methods

protected:

   Singleton()
   {
        // Constructor code goes here.
   }

   ~Singleton()
   {
        // Destructor code goes here.
   }

    // And any other protected methods.

}</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>

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> class singleton = {

   // ...

}. </lang> Normal singleton <lang elena>class $Singleton {

   object theField.

   // ...

}

static singleton = $Singleton new.</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: 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>

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>

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>

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>

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

Python

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>

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.

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>

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>

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

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

Works with: Tcl version 8.6

or

Library: TclOO

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>

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>

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>

Singleton

Non Thread Safe

Thread Safe

Thread-safe

Non-Thread-Safe

Thread safe (since C++11)

First attempt at thread-safety using locking.

Fixes excessive locking by double-checking for null.

Really simple implementation without locking.

Truly lazy by using an inner class.

Using Lazy<T>

Delphi and Pascal

Non-Thread Safe

Icon and Unicon

Thread-safe

Thread-Safe Lazy-Loaded

Non-Thread-Safe

Server wide singleton

Thread level singleton

Non-Thread-Safe

Thread-Safe

With GCD

With class methods

Native version

Simple OOP extension