Singleton: Difference between revisions

=={{header|ActionScript}}==

{

public class Singleton

}

=={{header|Ada}}==

===Non Thread Safe===

procedure Set_Data (Value : Integer);

function Get_Data return Integer;

end record;

Instance : Instance_Type;

--------------

end Get_Data;

===Thread Safe===

procedure Set_Data (Value : Integer);

function Get_Data return Integer;

Data : Integer := 0;

end Instance_Type;

--------------

end Instance;

=={{header|AutoHotkey}}==

{{works with | AutoHotkey_L}}

Translation of python borg pattern

b2 := borg()

msgbox % "b1 is b2? " . (b1 == b2)

brg[1, name] := val

Return val

=={{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.

#define SILLY_H

extern int PlayFetchWithDog( float weightOfStick);

Then in a separate C source (.c) file, define your structures, variables and functions.

#include "silly.h"

{ ...

if(weightOfStick < lazyDog.max_stick_weight){...

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.

#include "silly.h"

...

JumpOverTheDog( 4);

retrieved = PlayFetchWithDog( 3.1);

=={{header|C sharp|C#}}==

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).

{

private static Singleton1 instance;

}

}

===Fixes excessive locking by double-checking for null.===

Still uses locking and implementation is ugly and verbose.

{

private static Singleton2 instance;

}

}

===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.

{

private static Singleton3 Instance { get; } = new Singleton3();

static Singleton3() { }

===Truly lazy by using an inner class.===

This version is completely lazy but the code looks more complicated than it needs to be.

{

public static Singleton4 Instance => SingletonHolder.instance;

internal static readonly Singleton4 instance = new Singleton4();

}

===Using Lazy<T>===

C# has a dedicated type for lazy initialization: Lazy<T>.<br />

It makes implementing a Singleton really easy. Recommended.

{

private static readonly Lazy<Singleton5> lazy = new Lazy<Singleton5>(() => new Singleton5());

public static Singleton5 Instance => lazy.Value;

=={{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.

#include <stdexcept>

controller::instance().work();

}

=={{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.

/// 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

}

{{out|Examples}}

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.

(:documentation "Concatenate two phrases."))

(dolist (q (list 'the-empty-phrase

(make-instance 'nonempty-phrase :text "up the hill")))

Thread safety is irrelevant since the singleton is created at load time, not first access.

=={{header|D}}==

import std.stdio ;

import std.thread ;

x.wait ; y.wait ; z.wait ;

{{out}}

<pre>>>Mary come in.

=={{header|Delphi}} and {{header|Pascal}}==

Detailed explanation [http://www.yanniel.info/2010/10/singleton-pattern-delphi.html here]. (Delphi started out as an object-oriented version of Pascal.)

interface

end;

=={{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).

# ...

=={{header|Eiffel}}==

'''Implementation:'''

SINGLETON

create {SINGLETON_ACCESS}

feature

-- singleton features go here

SINGLETON_ACCESS

feature

Result /= Void

end

'''Usage:'''

=={{header|Elena}}==

Stateless singleton

singleton Singleton

{

// ...

}

Normal singleton

{

object theField;

}

=={{header|Epoxy}}==

if this.self then return this.self cls

var new: {}

NewSingleton>>setName("Test")

{{out}}

<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.

-export([get/0, set/1, start/0]).

From ! ok,

loop(NewValue)

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).

not_set

2> singleton:set(apple).

ok

7> singleton:get().

=={{header|Factor}}==

IN: singleton-demo

SINGLETON: bar

GENERIC: foo ( obj -- )

( scratchpad ) bar foo

Hello!

Needs the FMS-SI (single inheritance) library code located here:

http://soton.mpeforth.com/flag/fms/index.html

\ A singleton is created by using normal Forth data

s1 printb \ => 9

s2 printa \ => 4

=={{header|FreeBASIC}}==

REM Sacado del forum de FreeBASIC (https://www.freebasic.net/forum/viewtopic.php?t=20432)

Delete ps3

Sleep

{{out}}

<pre>

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.

import (

w.Wait()

log.Println("after trying both, instance =", instance)

{{out}}

<pre>

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.

// package level data declarations serve as singleton instance variables

func F() int {

return Y - X

Example program using the package:

import (

fmt.Println(singlep.X, singlep.Y)

fmt.Println(singlep.F())

{{out}}

<pre>

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.

import (

once.Do(func() { color = c })

log.Println("color initialized to", color)

import (

log.Println("trying to set red")

single.SetColor("red")

import (

log.Println("trying to set blue")

single.SetColor("blue")

import (

}

log.Println(single.Color())

{{out}}

<pre>

=={{header|Groovy}}==

class SingletonClass {

SingletonClass.instance.invokeMe()

}

{{out}}

<pre>invoking method of a singleton class</pre>

==Icon and {{header|Unicon}}==

Icon is not object oriented, but Unicon supports O-O programming.

method print()

write("Hi there.")

Singleton().print()

Singleton().print()

This Unicon example uses a number of Icon features.

=={{header|Io}}==

Io does not have globals. But it is easy to make singleton objects:

=={{header|J}}==

===Thread-safe===

[[wp:Double-checked locking]]; only use with Java 1.5+

{

private static Singleton myInstance;

// Any other methods

===Thread-Safe Lazy-Loaded===

This is the [[wp:Initialization-on-demand holder idiom]].

private Singleton() {

// Constructor code goes here.

return LazyHolder.INSTANCE;

}

===Non-Thread-Safe===

{

private static Singleton myInstance;

// Any other methods

=={{header|JavaScript}}==

if(Singleton._instance) return Singleton._instance;

this.set("");

c.append("!!!");

=={{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:

struct IAmaSingleton end

println("x == y is $(x == y) and x === y is $(x === y).")

=={{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:

object Singleton {

fun main(args: Array<String>) {

Singleton.speak()

{{out}}

===Server wide singleton===

// New definition supersede any current threads.

#b->switch = 'b'

===Thread level singleton===

define singleton => type {

#b->switch = 'b'

=={{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.

self id := 0.

self newID := {

println: Singleton newID. ; 1

println: Singleton newID. ; 2

=={{header|Lingo}}==

In Lingo a Singleton class can be implemented like this:

property _instance

me._someProperty = me._someProperty + x

return me._someProperty

=={{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.

:- public(value/1).

state(0).

A simple usage example after compiling and loading the code above:

Value = 0

yes

| ?- singleton::(set_value(1), value(Value)).

Value = 1

=={{header|NetRexx}}==

Uses a static field to avoid synchronization problems and the ''flawed'' &quot;double-checked locking&quot; 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.

options replace format comments java crossref symbols binary

return info

{{out}}

=={{header|Nim}}==

In the file <code>singleton.nim</code> we don't export the type, so new objects can't be created:

foo*: int

Then in another file we can use the singleton object:

single.foo = 12

=={{header|Objeck}}==

@singleton : static : Singleton;

...

}

=={{header|Objective-C}}==

===Non-Thread-Safe===

(Using Cocoa/OpenStep's NSObject as a base class)

@interface SomeSingleton : NSObject

{

+ (SomeSingleton *)sharedInstance;

@implementation SomeSingleton

}

===Thread-Safe===

Same as above except:

{

static SomeSingleton *sharedInstance = nil;

}

return sharedInstance;

===With GCD===

Same as above except:

{

static SomeSingleton *sharedInstance = nil;

});

return sharedInstance;

===With class methods===

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.

Sequence method: initialize(initialValue)

Channel newSize(1) := channel

@channel send(initialValue) drop ;

Usage :

: testSequence

| s i |

Sequence new(0) ->s

=={{header|ooRexx}}==

a = .singleton~new

b = .singleton~new

a singleton.

::attribute foo

=={{header|OxygenBasic}}==

Class Singleton

static sys inst 'private

new Singleton MySingleton

'endif

=={{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.

local

class Singleton

end

end

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}}==

use strict;

use warnings;

my $s2 = Singleton->new;

=={{header|Phix}}==

or keep check() private and invoke it internally from a few critical routines.

Needs 0.8.1+

object chk = NULL

class singleton

s.check()

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).

<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:

if chk==NULL then

chk = new("singleton")

end if

return chk

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

=={{header|PHP}}==

protected static $instance = null;

public $test_var;

echo $bar->test_var; //Prints 'One'

=={{header|PicoLisp}}==

As there is no physical difference between classes and objects, we can use the

class symbol itself.

(dm message1> ()

(dm message2> ()

{{out}}

<pre>: (message1> '+Singleton)

===Native version===

Thread safe version.

Interface OO_Interface ; Interface for any value of this type

Data.i @OO_Set()

Data.i @OO_Destroy()

===Simple OOP extension===

Using the open-source precompiler [http://www.development-lounge.de/viewtopic.php?t=5915 SimpleOOP].

BeginPrivate

Name$

EndMethod

=={{header|Python}}==

Every instance of the Borg class will share the same state:

__state = {}

def __init__(self):

>>> b1.datum is b2.datum

True

===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.

import abc

Singleton.printSelf()

print Singleton.state

When executed this code should print out the following:<br>

<br>

class Singleton(type):

_instances = {}

class Logger(object):

__metaclass__ = Singleton

or in Python3

class Logger(metaclass=Singleton):

pass

=={{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:

#lang racket

(provide instance)

(super-new)))

(define instance (new singleton%))

Or better, not name the class at all:

#lang racket

(provide instance)

(define/public (foo) 123)

(super-new))))

=={{header|Raku}}==

(formerly Perl 6)

# 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; }

=={{header|Ruby}}==

class MySingleton

include Singleton

a = MySingleton.instance # instance is only created the first time it is requested

b = MySingleton.instance

=={{header|Scala}}==

The '''object''' construct in Scala is a singleton.

// any code here gets executed as if in a constructor

=={{header|Sidef}}==

static instance;

s2.name = 'bar'; # change name in s2

=={{header|Slate}}==

Clones of Oddball themselves may not be cloned.

Methods and slots may still be defined on them:

=={{header|Smalltalk}}==

SomeClass class>>sharedInstance

SharedInstance ifNil: [SharedInstance := self basicNew initialize].

^ SharedInstance

=={{header|Swift}}==

class SingletonClass {

// Usage

let sharedObject = SingletonClass.sharedInstance

=={{header|Tcl}}==

ref http://wiki.tcl.tk/21595

# This is a metaclass, a class that defines the behavior of other classes

return [incr count]

}

Demonstrating in an interactive shell:

::oo::Obj20

% set b [example new] ;# note how this returns the same object name

3

% $b counter

=={{header|Tern}}==

Tern has built-in support for singletons via module declarations.

speak() {

println("I am a singleton");

}

{{out}}

=={{header|Vala}}==

static Singleton? instance;

print("Equal.\n");

}

=={{header|Wren}}==

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.

// Returns the singleton. If it hasn't been created, creates it first.

static instance { __instance == null ? __instance = Singleton.new_() : __instance }

var s2 = Singleton.instance

System.print("s1 and s2 are same object = %(Object.same(s1, s2))")

{{out}}

A class declared static only has one instance, ever.

However, a class with the same name & structure could be created in another scope.

b1 := Borg; b2 := Borg();

b1 == b2 //--> True

{{omit from|6502 Assembly}}

{{omit from|68000 Assembly}}