Singleton: Difference between revisions
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{{task|Object oriented}}
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.
<br><br>
=={{header|ActionScript}}==
<
{
public class Singleton
Line 21 ⟶ 25:
}
internal class SingletonEnforcer {}</
=={{header|Ada}}==
===Non Thread Safe===
<
procedure Set_Data (Value : Integer);
function Get_Data return Integer;
Line 34 ⟶ 38:
end record;
Instance : Instance_Type;
end Global_Singleton;</
<
--------------
Line 56 ⟶ 60:
end Get_Data;
end Global_Singleton;</
===Thread Safe===
<
procedure Set_Data (Value : Integer);
function Get_Data return Integer;
Line 69 ⟶ 73:
Data : Integer := 0;
end Instance_Type;
end Protected_Singleton;</
<
--------------
Line 117 ⟶ 121:
end Instance;
end Protected_Singleton;</
=={{header|AutoHotkey}}==
{{works with | AutoHotkey_L}}
Translation of python borg pattern
<syntaxhighlight 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
}</syntaxhighlight>
=={{header|BASIC}}==
==={{header|FreeBASIC}}===
<syntaxhighlight lang="freebasic">
REM Sacado del forum de FreeBASIC (https://www.freebasic.net/forum/viewtopic.php?t=20432)
Type singleton
Public :
Declare Static Function crearInstancia() As singleton Ptr
Declare Destructor ()
Dim i As Integer
Private :
Declare Constructor()
Declare Constructor(Byref rhs As singleton)
Declare Static Function instancia(Byval crear As Integer) As singleton Ptr
End Type
Static Function singleton.crearInstancia() As singleton Ptr
Return singleton.instancia(1)
End Function
Static Function singleton.instancia(Byval crear As Integer) As singleton Ptr
Static ref As singleton Ptr = 0
Function = 0
If crear = 0 Then
ref = 0
Elseif ref = 0 Then
ref = New singleton
Function = ref
End If
End Function
Constructor singleton ()
End Constructor
Destructor singleton()
singleton.instancia(0)
End Destructor
'-----------------------------------------------------------------------------
Dim As singleton Ptr ps1 = singleton.crearinstancia()
ps1->i = 1234
Print ps1, ps1->i
Dim As singleton Ptr ps2 = singleton.crearinstancia()
Print ps2
Delete ps1
Dim As singleton Ptr ps3 = singleton.crearinstancia()
Print ps3, ps3->i
Delete ps3
Sleep
</syntaxhighlight>
{{out}}
<pre>
2038352 1234
0
2038352 0
</pre>
==={{header|OxygenBasic}}===
<syntaxhighlight lang="oxygenbasic">
Class Singleton
static sys inst 'private
int instantiated() { return inst }
void constructor(){ if not inst then inst=@this }
'all other methods start with @this=inst
end class
'if not singleton.instantiated
new Singleton MySingleton
'endif
</syntaxhighlight>
==={{header|PureBasic}}===
====Native version====
Thread safe version.
<syntaxhighlight 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</syntaxhighlight>
====Simple OOP extension====
Using the open-source precompiler [http://www.development-lounge.de/viewtopic.php?t=5915 SimpleOOP].
<syntaxhighlight 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</syntaxhighlight>
=={{header|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.
<syntaxhighlight lang="c">#ifndef SILLY_H
#define SILLY_H
extern void JumpOverTheDog( int numberOfTimes);
extern int PlayFetchWithDog( float weightOfStick);
#endif</syntaxhighlight>
Then in a separate C source (.c) file, define your structures, variables and functions.
<syntaxhighlight 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){...
}</syntaxhighlight>
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.
<syntaxhighlight lang="c">...
#include "silly.h"
...
/* code using the dog methods */
JumpOverTheDog( 4);
retrieved = PlayFetchWithDog( 3.1);
...</syntaxhighlight>
=={{header|C sharp|C#}}==
===First attempt at thread-safety using locking.===
Performance suffers because the lock is acquired every time Instance is accessed.<br />
This implementation is extremely slow and should not be used (but is seen often).
<syntaxhighlight 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;
}
}
}</syntaxhighlight>
===Fixes excessive locking by double-checking for null.===
Still uses locking and implementation is ugly and verbose.
<syntaxhighlight 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;
}
}
}</syntaxhighlight>
===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.
<syntaxhighlight 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() { }
}</syntaxhighlight>
===Truly lazy by using an inner class.===
This version is completely lazy but the code looks more complicated than it needs to be.
<syntaxhighlight 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();
}
}</syntaxhighlight>
===Using Lazy<T>===
C# has a dedicated type for lazy initialization: Lazy<T>.<br />
It makes implementing a Singleton really easy. Recommended.
<syntaxhighlight 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;
}</syntaxhighlight>
=={{header|C++}}==
A generic singleton template class (implemented via the "Curiously Recurring Template Pattern"[https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern]). Warning: if using a version of C++ prior to C++11, a [[Mutex#C|mutex]] (or similar) is required to access static variables within a multi-threaded program.
<syntaxhighlight 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();
}
</syntaxhighlight>
=={{header|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.
<syntaxhighlight 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 <var>pConcurrency</var> 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 ];
}
</syntaxhighlight>
{{out|Examples}}
<pre>
USER>Set one=##class(Singleton).Get()
USER>Set one.GlobalProperty="Some Value"
USER>Set sc=one.Set()
</pre>
=={{header|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 (<code>the-empty-phrase</code>, our singleton) is inserted.
<syntaxhighlight lang="lisp">(defgeneric concat (a b)
(:documentation "Concatenate two phrases."))
Line 223 ⟶ 705:
(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))))))))</
Thread safety is irrelevant since the singleton is created at load time, not first access.
=={{header|D}}==
<
import std.stdio ;
import std.thread ;
Line 288 ⟶ 769:
x.wait ; y.wait ; z.wait ;
}</
{{out}}
<pre>>>Mary come in.
Calling Dealer...
Line 310 ⟶ 791:
Peter got (3)Code</pre>
=={{header|
Detailed explanation [http://www.yanniel.info/2010/10/singleton-pattern-delphi.html here]. (Delphi started out as an object-oriented version of Pascal.)
<syntaxhighlight 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.</syntaxhighlight>
=={{header|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 [http://www.erights.org/elib/concurrency/vat.html vat] (but can be remotely invoked from other vats).
<syntaxhighlight lang="e">def aSingleton {
# ...
}</
=={{header|
The <code>static</code> keyword in a class declaration will compile that class as a singleton. It is legal to define <code>const</code> (i.e. immutable) and <code>service</code> classes as singletons. Modules, packages, and enumeration values are always singleton classes. It is <b>not</b> legal to define normal <code>class</code> classes as singletons, because normal classes are mutable, and Ecstasy does not allow shared mutable state.
<syntaxhighlight lang="ecstasy">
module test {
static service Singleton {
private Int counter;
String fooHasBeenCalled() {
return $"{++counter} times";
}
}
void run() {
@Inject Console console;
for (Int i : 1..5) {
console.print($"{Singleton.fooHasBeenCalled()=}");
}
}
}
</syntaxhighlight>
{{out}}
<pre>
x$ xec test
Singleton.fooHasBeenCalled()=1 times
Singleton.fooHasBeenCalled()=2 times
Singleton.fooHasBeenCalled()=3 times
Singleton.fooHasBeenCalled()=4 times
Singleton.fooHasBeenCalled()=5 times
</pre>
=={{header|Eiffel}}==
===Non-Thread Safe===
Taken from [http://www.jot.fm/issues/issue_2004_04/article5/ this dated site]
'''Implementation:'''
<syntaxhighlight lang="eiffel">class
SINGLETON
create {SINGLETON_ACCESS}
default_create
feature
-- singleton features go here
end</syntaxhighlight>
<syntaxhighlight lang="eiffel">frozen class
SINGLETON_ACCESS
feature
singleton: SINGLETON
once ("PROCESS")
create Result
ensure
Result /= Void
end
end</syntaxhighlight>
'''Usage:'''
<syntaxhighlight lang="eiffel">s: SINGLETON -- declaration somewhere
s := (create{SINGLETON_ACCESS}).singleton -- in some routine</syntaxhighlight>
=={{header|Elena}}==
Stateless singleton
<syntaxhighlight lang="elena">
singleton Singleton
{
// ...
}
</syntaxhighlight>
Normal singleton
<syntaxhighlight lang="elena">class Singleton
{
object theField;
// ...
}
static singleton = new Singleton();</syntaxhighlight>
=={{header|EMal}}==
<syntaxhighlight lang="emal">
type Singleton
model
text greeting
fun speak = void by block do writeLine(me.greeting + " I'm a singleton") end
end
Singleton instance
fun getInstance = Singleton by block
if instance == null do instance = Singleton() end
return instance
end
type SomeOtherType
Singleton s1 = Singleton.getInstance()
s1.greeting = "Hello"
Singleton s2 = Singleton.getInstance()
s2.greeting.append(", World!")
writeLine(s1 + " and " + s2 + " are the same object: " + (s1 == s2) + ", s2: " + s2.greeting)
s1.speak() # call instance method
</syntaxhighlight>
{{out}}
<pre>
§(0x02bf8098) and §(0x02bf8098) are the same object: ⊤, s2: Hello, World!
Hello, World! I'm a singleton
</pre>
=={{header|Epoxy}}==
<syntaxhighlight lang="epoxy">fn Singleton()
if this.self then return this.self cls
var new: {}
iter k,v as this._props do
new[k]:v
cls
this.self:new
return new
cls
Singleton._props: {
name: "Singleton",
fn setName(self,new)
self.name:new
cls,
}
var MySingleton: Singleton()
log(MySingleton == Singleton()) --true
log(MySingleton.name) --Singleton
var NewSingleton: Singleton()
NewSingleton>>setName("Test")
log(MySingleton.name) --Test</syntaxhighlight>
{{out}}
<pre>
true
Singleton
Test
</pre>
=={{header|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.
<syntaxhighlight 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.</syntaxhighlight>
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).
<syntaxhighlight 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}</syntaxhighlight>
=={{header|Factor}}==
<syntaxhighlight lang="factor">USING: classes.singleton kernel io prettyprint ;
IN: singleton-demo
SINGLETON: bar
GENERIC: foo ( obj -- )
M: bar foo drop "Hello!" print ;</syntaxhighlight>
( scratchpad ) bar foo
Hello!
=={{header|Forth}}==
{{works with|Forth}}
Works with any ANS Forth
Needs the FMS2VT Forth extension located here:
https://github.com/DouglasBHoffman/FMS2/tree/master/FMS2VT
<syntaxhighlight lang="forth">include FMS2VT.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
</syntaxhighlight>
=={{header|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.
<syntaxhighlight 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)
}</syntaxhighlight>
{{out}}
<pre>
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
</pre>
'''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.
<syntaxhighlight 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
}</syntaxhighlight>
Example program using the package:
<syntaxhighlight 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())
}</syntaxhighlight>
{{out}}
<pre>
2 3
1
</pre>
'''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 <code>red</code> and <code>blue</code> import <code>single</code>, only a single variable <code>color</code> will exist in memory. The <code>init()</code> mechanism shown above actually runs before <code>main()</code>. In contrast, the <code>sync.Once</code> mechanism can serve as constructor code after <code>main()</code> begins.
<syntaxhighlight 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)
}</syntaxhighlight>
<syntaxhighlight lang="go">package red
import (
"log"
"single"
)
func SetColor() {
log.Println("trying to set red")
single.SetColor("red")
}</syntaxhighlight>
<syntaxhighlight lang="go">package blue
import (
"log"
"single"
)
func SetColor() {
log.Println("trying to set blue")
single.SetColor("blue")
}</syntaxhighlight>
<syntaxhighlight 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())
}</syntaxhighlight>
{{out}}
<pre>
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
</pre>
=={{header|Groovy}}==
<syntaxhighlight lang="groovy">@Singleton
class SingletonClass {
def invokeMe() {
println 'invoking method of a singleton class'
}
static void main(def args) {
SingletonClass.instance.invokeMe()
}
}</syntaxhighlight>
{{out}}
<pre>invoking method of a singleton class</pre>
==Icon and {{header|Unicon}}==
Icon is not object oriented, but Unicon supports O-O programming.
<syntaxhighlight 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</syntaxhighlight>
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.
=={{header|Io}}==
Io does not have globals. But it is easy to make singleton objects:
<syntaxhighlight lang="io">Singleton := Object clone
Singleton clone = Singleton</syntaxhighlight>
=={{header|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 <code>conew</code>) would fail. Other than that, singletons are defined like any other class in J.
=={{header|Java}}==
===Thread-safe===
[[wp:Double-checked locking]]; only use with Java 1.5+
<syntaxhighlight lang="java">class Singleton
{
private static Singleton myInstance;
Line 363 ⟶ 1,302:
{
if (myInstance == null)
{
synchronized(Singleton.class)
{
if (myInstance == null)
{
myInstance = new Singleton();
}
}
}
return myInstance;
Line 376 ⟶ 1,321:
// Any other methods
}</
===
This is the [[wp:Initialization-on-demand holder idiom]].
<syntaxhighlight 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;
}
}</syntaxhighlight>
===Thread-Safe Using Enum ===
Enums in Java are fully-fledged classes with specific instances, and are an idiomatic way to create singletons.
<syntaxhighlight lang="java">public enum Singleton {
INSTANCE;
// Fields, constructors and methods...
private int value;
Singleton() {
value = 0;
}
public int getValue() {
return value;
}
public void setValue(int value) {
this.value = value;
}
}</syntaxhighlight>
===Non-Thread-Safe===
<syntaxhighlight lang="java">class Singleton
{
private static Singleton myInstance;
Line 386 ⟶ 1,364:
{
if (myInstance == null)
{
myInstance = new Singleton();
}
return myInstance;
Line 397 ⟶ 1,377:
// Any other methods
}</
=={{header|JavaScript}}==
<syntaxhighlight 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() );</syntaxhighlight>
=={{header|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:
<syntaxhighlight lang="julia">
struct IAmaSingleton end
x = IAmaSingleton()
y = IAmaSingleton()
println("x == y is $(x == y) and x === y is $(x === y).")
</syntaxhighlight>
=={{header|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:
<syntaxhighlight lang="scala">// version 1.1.2
object Singleton {
fun speak() = println("I am a singleton")
}
fun main(args: Array<String>) {
Singleton.speak()
}</syntaxhighlight>
{{out}}
<pre>
I am a singleton
</pre>
=={{header|M2000 Interpreter}}==
<syntaxhighlight lang="m2000 interpreter">
Module CheckSingleton {
\\ singleton
\\ pointers and static groups are the same object because
\\ each one has a pointer to same state (a tuple)
\\ but from outside we do the miracle to have a static group to act as a pointer
\\ We need a lambda function to hold the pointer to Singleton as closure
Global One=lambda M=pointer() (aValue=0)-> {
If M is type null then
\\ one time happen
Group Singleton {
Type:One
Private:
state=(aValue,)
Public:
module Add (x) {
.state+=x
}
Set {Drop}
Value {
=.state#val(0)
}
}
M->group(Singleton)
end if
\\ return M which is a pointer
=M
}
K=One(100)
Print Eval(K)=100
M=One()
Print Eval(M)=100
Print K is M = true
Print K is type One = true
K=>add 500
Print eval(K)=600
\\ copy K to Z (no pointer to Z, Z is named group)
Z=Group(K)
Print eval(z)=600, z=600
Z.add 1000
Print Z=1600, Eval(M)=1600, Eval(K)=1600
\\ push a copy of Z, but state is pointer so we get a copy of a pointer
Push Group(Z)
Read beta
Beta.add 1000
Print Z=2600, Eval(M)=2600, Eval(K)=2600
\\ convert pointer to group (a copy of group)
group delta=One()
delta.add 1000
Print Z=3600, beta=3600, delta=3600, Eval(M)=3600, Eval(K)=3600
\\ M and K are pointers to groups
M=>add 400
Print Z=4000, beta=4000, delta=4000, Eval(M)=4000, Eval(K)=4000
}
CheckSingleton
</syntaxhighlight>
=={{header|Lasso}}==
Lasso supports singletons on two levels.
===Server wide singleton===
<syntaxhighlight 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</syntaxhighlight>
===Thread level singleton===
<syntaxhighlight 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</syntaxhighlight>
=={{header|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 <code>clone</code> to make it clear that the object should not be cloned, but this is generally overkill.
<syntaxhighlight 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</syntaxhighlight>
=={{header|Lingo}}==
In Lingo a Singleton class can be implemented like this:
<syntaxhighlight 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</syntaxhighlight>
=={{header|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.
<syntaxhighlight 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.</syntaxhighlight>
A simple usage example after compiling and loading the code above:
<syntaxhighlight lang="logtalk">| ?- singleton::value(Value).
Value = 0
yes
| ?- singleton::(set_value(1), value(Value)).
Value = 1
yes</syntaxhighlight>
=={{header|NetRexx}}==
Uses a static field to avoid synchronization problems and the ''flawed'' "double-checked locking" idiom in JVMs. See [http://www.ibm.com/developerworks/java/library/j-dcl/index.html www.ibm.com/developerworks/java/library/j-dcl/index.html] for a detailed explanation.
<syntaxhighlight 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
</syntaxhighlight>
{{out}}
<pre style="height:30ex;overflow:scroll">
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
</pre>
=={{header|Nim}}==
In the file <code>singleton.nim</code> we don't export the type, so new objects can't be created:
<syntaxhighlight lang="nim">type Singleton = object # Singleton* would export
foo*: int
var single* = Singleton(foo: 0)</syntaxhighlight>
Then in another file we can use the singleton object:
<syntaxhighlight lang="nim">import singleton
single.foo = 12
echo single.foo</syntaxhighlight>
=={{header|Objeck}}==
<syntaxhighlight lang="objeck">class Singleton {
@singleton : static : Singleton;
New : private () {
}
function : GetInstance() ~ Singleton {
if(@singleton <> Nil) {
@singleton := Singleton->New();
};
return @singleton;
}
method : public : DoStuff() ~ Nil {
...
}
}</syntaxhighlight>
=={{header|Objective-C}}==
===Non-Thread-Safe===
(Using Cocoa/OpenStep's NSObject as a base class)
<syntaxhighlight lang="objc">// SomeSingleton.h
@interface SomeSingleton : NSObject
{
// any instance variables
}
+ (SomeSingleton *)sharedInstance;
@end</syntaxhighlight>
<syntaxhighlight 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</syntaxhighlight>
===Thread-Safe===
Same as above except:
<syntaxhighlight lang="objc">+ (SomeSingleton *) sharedInstance
{
static SomeSingleton *sharedInstance = nil;
@synchronized(self) {
if (!sharedInstance) {
sharedInstance = [[SomeSingleton alloc] init];
}
}
return sharedInstance;
}</syntaxhighlight>
===With GCD===
Same as above except:
<syntaxhighlight lang="objc">+ (SomeSingleton *) sharedInstance
{
static SomeSingleton *sharedInstance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedInstance = [[SomeSingleton alloc] init];
});
return sharedInstance;
}</syntaxhighlight>
===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 <code>static</code> (file-local) global variables. "Initialization" of the singleton object would be done in the <code>+initialize</code> 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).
=={{header|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.
<syntaxhighlight 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 ;</syntaxhighlight>
Usage :
<syntaxhighlight lang="oforth">import: parallel
: testSequence
| s i |
Sequence new(0) ->s
100 loop: i [ #[ s nextValue println ] & ] ;</syntaxhighlight>
=={{header|ooRexx}}==
<syntaxhighlight 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
</syntaxhighlight>
=={{header|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 <code>GetInstance</code> function has access to it.
<syntaxhighlight 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</syntaxhighlight>
This will work as long as all functors are linked with <code>import</code> statements. If you use multiple calls to <code>Module.link</code> instead, you will get multiple instances of the "Singleton".
=={{header|Perl}}==
<syntaxhighlight 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;</syntaxhighlight>
=={{header|Phix}}==
{{libheader|Phix/Class}}
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+
<syntaxhighlight 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</syntaxhighlight>
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) <code>singleton s = new()</code> everywhere, however
<code>class s = new("singleton")</code> 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:
<syntaxhighlight lang="phix">global function get_singleton()
if chk==NULL then
chk = new("singleton")
end if
return chk
end function</syntaxhighlight>
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.
=={{header|PHP}}==
<syntaxhighlight 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</syntaxhighlight>
=={{header|PicoLisp}}==
As there is no physical difference between classes and objects, we can use the
class symbol itself.
<syntaxhighlight lang="picolisp">(class +Singleton)
(dm message1> ()
(prinl "This is method 1 on " This) )
(dm message2> ()
(prinl "This is method 2 on " This) )</syntaxhighlight>
{{out}}
<pre>: (message1> '+Singleton)
This is method 1 on +Singleton
-> +Singleton
: (message2> '+Singleton)
This is method 2 on +Singleton
-> +Singleton</pre>
=={{header|Python}}==
===per Borg Design===
In Python we use the [http://code.activestate.com/recipes/66531/ Borg pattern] to share state between instances rather than concentrate on identity.
Every instance of the Borg class will share the same state:
<
__state = {}
def __init__(self):
Line 421 ⟶ 2,062:
>>> b1.datum is b2.datum
True
>>> # For any datum!</
===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.
<syntaxhighlight 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
</syntaxhighlight>
When executed this code should print out the following:<br>
<br>
Can't instantiate abstract class Singleton with abstract methods __init__<br>
1<br>
1<br>
2<br>
2<br>
<br>
So, instantiation is not possible. Only a single object is available, and it behaves as a singleton.
===per MetaClass===
<syntaxhighlight 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
</syntaxhighlight>
or in Python3
<syntaxhighlight lang="python">
class Logger(metaclass=Singleton):
pass
</syntaxhighlight>
=={{header|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:
<syntaxhighlight lang="racket">
#lang racket
(provide instance)
(define singleton%
(class object%
(super-new)))
(define instance (new singleton%))
</syntaxhighlight>
Or better, not name the class at all:
<syntaxhighlight lang="racket">
#lang racket
(provide instance)
(define instance
(new (class object%
(define/public (foo) 123)
(super-new))))
</syntaxhighlight>
=={{header|Raku}}==
(formerly Perl 6)
<syntaxhighlight lang="raku" line>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; }
}</syntaxhighlight>
=={{header|Ruby}}==
<syntaxhighlight lang="ruby">require 'singleton'
class MySingleton
include Singleton
Line 433 ⟶ 2,174:
a = MySingleton.instance # instance is only created the first time it is requested
b = MySingleton.instance
puts a.equal?(b) # outputs "true"</
=={{header|Scala}}==
The '''object''' construct in Scala is a singleton.
<syntaxhighlight lang="scala">object Singleton {
// any code here gets executed as if in a constructor
}</
=={{header|Sidef}}==
<syntaxhighlight 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'</syntaxhighlight>
=={{header|Slate}}==
Clones of Oddball themselves may not be cloned.
Methods and slots may still be defined on them:
<syntaxhighlight lang="slate">define: #Singleton &builder: [Oddball clone]</syntaxhighlight>
=={{header|Smalltalk}}==
<syntaxhighlight lang="smalltalk">
SomeClass class>>sharedInstance
SharedInstance ifNil: [SharedInstance := self basicNew initialize].
^ SharedInstance
</syntaxhighlight>
=={{header|Swift}}==
<syntaxhighlight 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
</syntaxhighlight>
=={{header|Tcl}}==
{{works with|Tcl|8.6}}
ref http://wiki.tcl.tk/21595
<syntaxhighlight lang
# This is a metaclass, a class that defines the behavior of other classes
Line 472 ⟶ 2,258:
return [incr count]
}
}</
Demonstrating in an interactive shell:
<
::oo::Obj20
% set b [example new] ;# note how this returns the same object name
Line 487 ⟶ 2,273:
3
% $b counter
4</
=={{
Tern has built-in support for singletons via module declarations.
<syntaxhighlight lang="tern">module Singleton {
speak() {
println("I am a singleton");
}
}
Singleton.speak();</syntaxhighlight>
{{out}}
<pre>
I am a singleton
</pre>
=={{header|TXR}}==
<syntaxhighlight lang="txrlisp">;; Custom (:singleton) clause which adds behavior to a class
;; asserting against multiple instantiation.
(define-struct-clause :singleton ()
^((:static inst-count 0)
(:postinit (me)
(assert (<= (inc me.inst-count) 1)))))
(defstruct singleton-one ()
(:singleton)
(:method speak (me)
(put-line "I am singleton-one")))
(defstruct singleton-two ()
(:singleton)
(:method speak (me)
(put-line "I am singleton-two")))
;; Test
;; Global singleton
(defvarl s1 (new singleton-one))
;; Local singleton in function (like static in C)
;; load-time evaluates once.
(defun fn ()
(let ((s2 (load-time (new singleton-two))))
s2.(speak)))
s1.(speak)
(fn) ;; multiple calls to fn don't re-instantiate singleton-two
(fn)
(put-line "so far, so good")
(new singleton-two) ;; assertion gooes off</syntaxhighlight>
{{out}}
<pre>I am singleton-one
I am singleton-two
I am singleton-two
so far, so good
txr: unhandled exception of type assert:
txr: assertion (<= (inc me.inst-count)
1) failed in singleton.tl:6
txr: during evaluation at singleton.tl:6 of form (sys:rt-assert-fail "singleton.tl"
6 '(<= (inc me.inst-count)
1))</pre>
=={{header|Vala}}==
<syntaxhighlight 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");
}
}</syntaxhighlight>
=={{header|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.
<syntaxhighlight lang="wren">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</syntaxhighlight>
{{out}}
<pre>
s1 and s2 are same object = true
I'm a singleton.
</pre>
=={{header|zkl}}==
A class declared static only has one instance, ever.
However, a class with the same name & structure could be created in another scope.
<syntaxhighlight lang="zkl">class [static] Borg{ var v }
b1 := Borg; b2 := Borg();
b1 == b2 //--> True
b1.v=123; b2.v.println(); //--> 123</syntaxhighlight>
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