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.

<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>

C#

A thread safe singleton implementation. To make it non-thread safe remove lockObject and the lock() statement.

<lang csharp> // Usage: Singleton.Instance.SomeMethod() class Singleton {

       private static Singleton _singleton;
       private static lockObject = new object();
       private Singleton() {}
       public static Singleton Instance 
       {
           get
           {
               lock(lockObject)
               {
                   if (_singleton == null)
                       _singleton = new Singleton();
               }
               return _singleton;
           }
       }
       // The rest of the methods

} </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>

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> Sample 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

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>

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!

Icon and Unicon

Icon

Icon is not object oriented.

Unicon

<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 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.)

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

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>

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>

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>

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 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;

}

- (void)release {

   // prevent release

}

- (id)autorelease {

   return self;

}

@end</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".

PureBasic

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>

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>

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>

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>

Singleton

Contents

ActionScript

Ada

Non Thread Safe

Thread Safe

AutoHotkey

C

C++

Thread-safe

Non-Thread-Safe

C#

Erlang

Common Lisp

D

E

Factor

Icon and Unicon

Icon

Unicon

Io

J

Java

Thread-safe

Non-Thread-Safe

Perl

Perl 6

PHP

PicoLisp

Python

Objeck

Objective-C

Non-Thread-Safe

Oz

PureBasic

Ruby

Scala

Slate

Tcl