---
title: "Singleton"
type: "design-pattern"
slug: "singleton"
url: "http://localhost:3000/en/design-patterns/singleton.md"
category: "Creational Patterns"
description: "Singleton is a creational design pattern that lets you ensure that a class has only one instance, while providing a global access point to this instance."
languages: ["java", "csharp", "cpp", "go", "php", "python", "ruby", "rust", "swift", "typescript"]
---
# Singleton

> Singleton is a creational design pattern that lets you ensure that a class has only one instance, while providing a global access point to this instance.

## Intent

**Singleton** is a creational design pattern that lets you ensure that a class has only one instance, while providing a global access point to this instance.

## Problem

The Singleton pattern solves two problems at the same time, violating the _Single Responsibility Principle_:

1. **Ensure that a class has just a single instance**. Why would anyone want to control how many instances a class has? The most common reason for this is to control access to some shared resource—for example, a database or a file.
Here’s how it works: imagine that you created an object, but after a while decided to create a new one. Instead of receiving a fresh object, you’ll get the one you already created.
Note that this behavior is impossible to implement with a regular constructor since a constructor call **must** always return a new object by design.

Clients may not even realize that they’re working with the same object all the time.

1. **Provide a global access point to that instance**. Remember those global variables that you (all right, me) used to store some essential objects? While they’re very handy, they’re also very unsafe since any code can potentially overwrite the contents of those variables and crash the app.
Just like a global variable, the Singleton pattern lets you access some object from anywhere in the program. However, it also protects that instance from being overwritten by other code.
There’s another side to this problem: you don’t want the code that solves problem #1 to be scattered all over your program. It’s much better to have it within one class, especially if the rest of your code already depends on it.

Nowadays, the Singleton pattern has become so popular that people may call something a _singleton_ even if it solves just one of the listed problems.

## Solution

All implementations of the Singleton have these two steps in common:

* Make the default constructor private, to prevent other objects from using the `new` operator with the Singleton class.
* Create a static creation method that acts as a constructor. Under the hood, this method calls the private constructor to create an object and saves it in a static field. All following calls to this method return the cached object.

If your code has access to the Singleton class, then it’s able to call the Singleton’s static method. So whenever that method is called, the same object is always returned.

## Structure

1. The **Singleton** class declares the static method `getInstance` that returns the same instance of its own class.
The Singleton’s constructor should be hidden from the client code. Calling the `getInstance` method should be the only way of getting the Singleton object.

## Pseudocode

In this example, the database connection class acts as a **Singleton**. This class doesn’t have a public constructor, so the only way to get its object is to call the `getInstance` method. This method caches the first created object and returns it in all subsequent calls.

// The Database class defines the `getInstance` method that lets
// clients access the same instance of a database connection
// throughout the program.
class Database is
    // The field for storing the singleton instance should be
    // declared static.
    private static field instance: Database

    // The singleton's constructor should always be private to
    // prevent direct construction calls with the `new`
    // operator.
    private constructor Database() is
        // Some initialization code, such as the actual
        // connection to a database server.
        // ...

    // The static method that controls access to the singleton
    // instance.
    public static method getInstance() is
        if (Database.instance == null) then
            acquireThreadLock() and then
                // Ensure that the instance hasn't yet been
                // initialized by another thread while this one
                // has been waiting for the lock's release.
                if (Database.instance == null) then
                    Database.instance = new Database()
        return Database.instance

    // Finally, any singleton should define some business logic
    // which can be executed on its instance.
    public method query(sql) is
        // For instance, all database queries of an app go
        // through this method. Therefore, you can place
        // throttling or caching logic here.
        // ...

class Application is
    method main() is
        Database foo = Database.getInstance()
        foo.query("SELECT ...")
        // ...
        Database bar = Database.getInstance()
        bar.query("SELECT ...")
        // The variable `bar` will contain the same object as
        // the variable `foo`.

## Applicability

Use the Singleton pattern when a class in your program should have just a single instance available to all clients; for example, a single database object shared by different parts of the program.

 The Singleton pattern disables all other means of creating objects of a class except for the special creation method. This method either creates a new object or returns an existing one if it has already been created.

 Use the Singleton pattern when you need stricter control over global variables.

 Unlike global variables, the Singleton pattern guarantees that there’s just one instance of a class. Nothing, except for the Singleton class itself, can replace the cached instance.

Note that you can always adjust this limitation and allow creating any number of Singleton instances. The only piece of code that needs changing is the body of the `getInstance` method.

## How to Implement

1. Add a private static field to the class for storing the singleton instance.
2. Declare a public static creation method for getting the singleton instance.
3. Implement “lazy initialization” inside the static method. It should create a new object on its first call and put it into the static field. The method should always return that instance on all subsequent calls.
4. Make the constructor of the class private. The static method of the class will still be able to call the constructor, but not the other objects.
5. Go over the client code and replace all direct calls to the singleton’s constructor with calls to its static creation method.

## Pros

* You can be sure that a class has only a single instance.
* You gain a global access point to that instance.
* The singleton object is initialized only when it’s requested for the first time.

## Cons

* Violates the _Single Responsibility Principle_. The pattern solves two problems at the time.
* The Singleton pattern can mask bad design, for instance, when the components of the program know too much about each other.
* The pattern requires special treatment in a multithreaded environment so that multiple threads won’t create a singleton object several times.
* It may be difficult to unit test the client code of the Singleton because many test frameworks rely on inheritance when producing mock objects. Since the constructor of the singleton class is private and overriding static methods is impossible in most languages, you will need to think of a creative way to mock the singleton. Or just don’t write the tests. Or don’t use the Singleton pattern.

## Relations with Other Patterns

* A [Facade](/design-patterns/facade) class can often be transformed into a [Singleton](/design-patterns/singleton) since a single facade object is sufficient in most cases.
* [Flyweight](/design-patterns/flyweight) would resemble [Singleton](/design-patterns/singleton) if you somehow managed to reduce all shared states of the objects to just one flyweight object. But there are two fundamental differences between these patterns:

  1. There should be only one Singleton instance, whereas a _Flyweight_ class can have multiple instances with different intrinsic states.
  2. The _Singleton_ object can be mutable. Flyweight objects are immutable.
* [Abstract Factories](/design-patterns/abstract-factory), [Builders](/design-patterns/builder) and [Prototypes](/design-patterns/prototype) can all be implemented as [Singletons](/design-patterns/singleton).
## Relations

**Related patterns**

- [Facade](/en/design-patterns/facade.md)
- [Flyweight](/en/design-patterns/flyweight.md)
- [Abstract Factory](/en/design-patterns/abstract-factory.md)
- [Builder](/en/design-patterns/builder.md)
- [Prototype](/en/design-patterns/prototype.md)

## Code Examples

### java

```java
package refactoring_guru.singleton.example.non_thread_safe;

public final class Singleton {
    private static Singleton instance;
    public String value;

    private Singleton(String value) {
        // The following code emulates slow initialization.
        try {
            Thread.sleep(1000);
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
        this.value = value;
    }

    public static Singleton getInstance(String value) {
        if (instance == null) {
            instance = new Singleton(value);
        }
        return instance;
    }
}

package refactoring_guru.singleton.example.non_thread_safe;

public class DemoSingleThread {
    public static void main(String[] args) {
        System.out.println("If you see the same value, then singleton was reused (yay!)" + "\n" +
                "If you see different values, then 2 singletons were created (booo!!)" + "\n\n" +
                "RESULT:" + "\n");
        Singleton singleton = Singleton.getInstance("FOO");
        Singleton anotherSingleton = Singleton.getInstance("BAR");
        System.out.println(singleton.value);
        System.out.println(anotherSingleton.value);
    }
}

If you see the same value, then singleton was reused (yay!)
If you see different values, then 2 singletons were created (booo!!)

RESULT:

FOO
FOO

package refactoring_guru.singleton.example.non_thread_safe;

public final class Singleton {
    private static Singleton instance;
    public String value;

    private Singleton(String value) {
        // The following code emulates slow initialization.
        try {
            Thread.sleep(1000);
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
        this.value = value;
    }

    public static Singleton getInstance(String value) {
        if (instance == null) {
            instance = new Singleton(value);
        }
        return instance;
    }
}

package refactoring_guru.singleton.example.non_thread_safe;

public class DemoMultiThread {
    public static void main(String[] args) {
        System.out.println("If you see the same value, then singleton was reused (yay!)" + "\n" +
                "If you see different values, then 2 singletons were created (booo!!)" + "\n\n" +
                "RESULT:" + "\n");
        Thread threadFoo = new Thread(new ThreadFoo());
        Thread threadBar = new Thread(new ThreadBar());
        threadFoo.start();
        threadBar.start();
    }

    static class ThreadFoo implements Runnable {
        @Override
        public void run() {
            Singleton singleton = Singleton.getInstance("FOO");
            System.out.println(singleton.value);
        }
    }

    static class ThreadBar implements Runnable {
        @Override
        public void run() {
            Singleton singleton = Singleton.getInstance("BAR");
            System.out.println(singleton.value);
        }
    }
}

If you see the same value, then singleton was reused (yay!)
If you see different values, then 2 singletons were created (booo!!)

RESULT:

FOO
BAR

package refactoring_guru.singleton.example.thread_safe;

public final class Singleton {
    // The field must be declared volatile so that double check lock would work
    // correctly.
    private static volatile Singleton instance;

    public String value;

    private Singleton(String value) {
        this.value = value;
    }

    public static Singleton getInstance(String value) {
        // The approach taken here is called double-checked locking (DCL). It
        // exists to prevent race condition between multiple threads that may
        // attempt to get singleton instance at the same time, creating separate
        // instances as a result.
        //
        // It may seem that having the `result` variable here is completely
        // pointless. There is, however, a very important caveat when
        // implementing double-checked locking in Java, which is solved by
        // introducing this local variable.
        //
        // You can read more info DCL issues in Java here:
        // https://refactoring.guru/java-dcl-issue
        Singleton result = instance;
        if (result != null) {
            return result;
        }
        synchronized(Singleton.class) {
            if (instance == null) {
                instance = new Singleton(value);
            }
            return instance;
        }
    }
}

package refactoring_guru.singleton.example.thread_safe;

public class DemoMultiThread {
    public static void main(String[] args) {
        System.out.println("If you see the same value, then singleton was reused (yay!)" + "\n" +
                "If you see different values, then 2 singletons were created (booo!!)" + "\n\n" +
                "RESULT:" + "\n");
        Thread threadFoo = new Thread(new ThreadFoo());
        Thread threadBar = new Thread(new ThreadBar());
        threadFoo.start();
        threadBar.start();
    }

    static class ThreadFoo implements Runnable {
        @Override
        public void run() {
            Singleton singleton = Singleton.getInstance("FOO");
            System.out.println(singleton.value);
        }
    }

    static class ThreadBar implements Runnable {
        @Override
        public void run() {
            Singleton singleton = Singleton.getInstance("BAR");
            System.out.println(singleton.value);
        }
    }
}

If you see the same value, then singleton was reused (yay!)
If you see different values, then 2 singletons were created (booo!!)

RESULT:

BAR
BAR
```

### csharp

```csharp
using System;

namespace RefactoringGuru.DesignPatterns.Singleton.Conceptual.NonThreadSafe
{
    // The Singleton class defines the `GetInstance` method that serves as an
    // alternative to constructor and lets clients access the same instance of
    // this class over and over.

    // EN : The Singleton should always be a 'sealed' class to prevent class
    // inheritance through external classes and also through nested classes.
    public sealed class Singleton
    {
        // The Singleton's constructor should always be private to prevent
        // direct construction calls with the `new` operator.
        private Singleton() { }

        // The Singleton's instance is stored in a static field. There there are
        // multiple ways to initialize this field, all of them have various pros
        // and cons. In this example we'll show the simplest of these ways,
        // which, however, doesn't work really well in multithreaded program.
        private static Singleton _instance;

        // This is the static method that controls the access to the singleton
        // instance. On the first run, it creates a singleton object and places
        // it into the static field. On subsequent runs, it returns the client
        // existing object stored in the static field.
        public static Singleton GetInstance()
        {
            if (_instance == null)
            {
                _instance = new Singleton();
            }
            return _instance;
        }

        // Finally, any singleton should define some business logic, which can
        // be executed on its instance.
        public void someBusinessLogic()
        {
            // ...
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            // The client code.
            Singleton s1 = Singleton.GetInstance();
            Singleton s2 = Singleton.GetInstance();

            if (s1 == s2)
            {
                Console.WriteLine("Singleton works, both variables contain the same instance.");
            }
            else
            {
                Console.WriteLine("Singleton failed, variables contain different instances.");
            }
        }
    }
}

Singleton works, both variables contain the same instance.

using System;
using System.Threading;

namespace Singleton
{
    // This Singleton implementation is called "double check lock". It is safe
    // in multithreaded environment and provides lazy initialization for the
    // Singleton object.
    class Singleton
    {
        private Singleton() { }

        private static Singleton _instance;

        // We now have a lock object that will be used to synchronize threads
        // during first access to the Singleton.
        private static readonly object _lock = new object();

        public static Singleton GetInstance(string value)
        {
            // This conditional is needed to prevent threads stumbling over the
            // lock once the instance is ready.
            if (_instance == null)
            {
                // Now, imagine that the program has just been launched. Since
                // there's no Singleton instance yet, multiple threads can
                // simultaneously pass the previous conditional and reach this
                // point almost at the same time. The first of them will acquire
                // lock and will proceed further, while the rest will wait here.
                lock (_lock)
                {
                    // The first thread to acquire the lock, reaches this
                    // conditional, goes inside and creates the Singleton
                    // instance. Once it leaves the lock block, a thread that
                    // might have been waiting for the lock release may then
                    // enter this section. But since the Singleton field is
                    // already initialized, the thread won't create a new
                    // object.
                    if (_instance == null)
                    {
                        _instance = new Singleton();
                        _instance.Value = value;
                    }
                }
            }
            return _instance;
        }

        // We'll use this property to prove that our Singleton really works.
        public string Value { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            // The client code.
            
            Console.WriteLine(
                "{0}\n{1}\n\n{2}\n",
                "If you see the same value, then singleton was reused (yay!)",
                "If you see different values, then 2 singletons were created (booo!!)",
                "RESULT:"
            );
            
            Thread process1 = new Thread(() =>
            {
                TestSingleton("FOO");
            });
            Thread process2 = new Thread(() =>
            {
                TestSingleton("BAR");
            });
            
            process1.Start();
            process2.Start();
            
            process1.Join();
            process2.Join();
        }
        
        public static void TestSingleton(string value)
        {
            Singleton singleton = Singleton.GetInstance(value);
            Console.WriteLine(singleton.Value);
        } 
    }
}

FOO
FOO
```

### cpp

```cpp
/**
 * The Singleton class defines the `GetInstance` method that serves as an
 * alternative to constructor and lets clients access the same instance of this
 * class over and over.
 */
class Singleton
{

    /**
     * The Singleton's constructor should always be private to prevent direct
     * construction calls with the `new` operator.
     */

protected:
    Singleton(const std::string value): value_(value)
    {
    }

    static Singleton* singleton_;

    std::string value_;

public:

    /**
     * Singletons should not be cloneable.
     */
    Singleton(Singleton &other) = delete;
    /**
     * Singletons should not be assignable.
     */
    void operator=(const Singleton &) = delete;
    /**
     * This is the static method that controls the access to the singleton
     * instance. On the first run, it creates a singleton object and places it
     * into the static field. On subsequent runs, it returns the client existing
     * object stored in the static field.
     */

    static Singleton *GetInstance(const std::string& value);
    /**
     * Finally, any singleton should define some business logic, which can be
     * executed on its instance.
     */
    void SomeBusinessLogic()
    {
        // ...
    }

    std::string value() const{
        return value_;
    } 
};

Singleton* Singleton::singleton_= nullptr;;

/**
 * Static methods should be defined outside the class.
 */
Singleton *Singleton::GetInstance(const std::string& value)
{
    /**
     * This is a safer way to create an instance. instance = new Singleton is
     * dangeruous in case two instance threads wants to access at the same time
     */
    if(singleton_==nullptr){
        singleton_ = new Singleton(value);
    }
    return singleton_;
}

void ThreadFoo(){
    // Following code emulates slow initialization.
    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    Singleton* singleton = Singleton::GetInstance("FOO");
    std::cout << singleton->value() << "\n";
}

void ThreadBar(){
    // Following code emulates slow initialization.
    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    Singleton* singleton = Singleton::GetInstance("BAR");
    std::cout << singleton->value() << "\n";
}


int main()
{
    std::cout <<"If you see the same value, then singleton was reused (yay!\n" <<
                "If you see different values, then 2 singletons were created (booo!!)\n\n" <<
                "RESULT:\n";   
    std::thread t1(ThreadFoo);
    std::thread t2(ThreadBar);
    t1.join();
    t2.join();

    return 0;
}

If you see the same value, then singleton was reused (yay!
If you see different values, then 2 singletons were created (booo!!)

RESULT:
BAR
FOO

/**
 * The Singleton class defines the `GetInstance` method that serves as an
 * alternative to constructor and lets clients access the same instance of this
 * class over and over.
 */
class Singleton
{

    /**
     * The Singleton's constructor/destructor should always be private to
     * prevent direct construction/desctruction calls with the `new`/`delete`
     * operator.
     */
private:
    static Singleton * pinstance_;
    static std::mutex mutex_;

protected:
    Singleton(const std::string value): value_(value)
    {
    }
    ~Singleton() {}
    std::string value_;

public:
    /**
     * Singletons should not be cloneable.
     */
    Singleton(Singleton &other) = delete;
    /**
     * Singletons should not be assignable.
     */
    void operator=(const Singleton &) = delete;
    /**
     * This is the static method that controls the access to the singleton
     * instance. On the first run, it creates a singleton object and places it
     * into the static field. On subsequent runs, it returns the client existing
     * object stored in the static field.
     */

    static Singleton *GetInstance(const std::string& value);
    /**
     * Finally, any singleton should define some business logic, which can be
     * executed on its instance.
     */
    void SomeBusinessLogic()
    {
        // ...
    }
    
    std::string value() const{
        return value_;
    } 
};

/**
 * Static methods should be defined outside the class.
 */

Singleton* Singleton::pinstance_{nullptr};
std::mutex Singleton::mutex_;

/**
 * The first time we call GetInstance we will lock the storage location
 *      and then we make sure again that the variable is null and then we
 *      set the value. RU:
 */
Singleton *Singleton::GetInstance(const std::string& value)
{
    std::lock_guard<std::mutex> lock(mutex_);
    if (pinstance_ == nullptr)
    {
        pinstance_ = new Singleton(value);
    }
    return pinstance_;
}

void ThreadFoo(){
    // Following code emulates slow initialization.
    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    Singleton* singleton = Singleton::GetInstance("FOO");
    std::cout << singleton->value() << "\n";
}

void ThreadBar(){
    // Following code emulates slow initialization.
    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    Singleton* singleton = Singleton::GetInstance("BAR");
    std::cout << singleton->value() << "\n";
}

int main()
{   
    std::cout <<"If you see the same value, then singleton was reused (yay!\n" <<
                "If you see different values, then 2 singletons were created (booo!!)\n\n" <<
                "RESULT:\n";   
    std::thread t1(ThreadFoo);
    std::thread t2(ThreadBar);
    t1.join();
    t2.join();
    
    return 0;
}

If you see the same value, then singleton was reused (yay!
If you see different values, then 2 singletons were created (booo!!)

RESULT:
FOO
FOO
```

### go

```go
package main

import (
	"fmt"
	"sync"
)

var lock = &sync.Mutex{}

type single struct {
}

var singleInstance *single

func getInstance() *single {
	if singleInstance == nil {
		lock.Lock()
		defer lock.Unlock()
		if singleInstance == nil {
			fmt.Println("Creating single instance now.")
			singleInstance = &single{}
		} else {
			fmt.Println("Single instance already created.")
		}
	} else {
		fmt.Println("Single instance already created.")
	}

	return singleInstance
}

package main

import (
	"fmt"
)

func main() {

	for i := 0; i < 30; i++ {
		go getInstance()
	}

	// Scanln is similar to Scan, but stops scanning at a newline and
	// after the final item there must be a newline or EOF.
	fmt.Scanln()
}

Creating single instance now.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.

package main

import (
	"fmt"
	"sync"
)

var once sync.Once

type single struct {
}

var singleInstance *single

func getInstance() *single {
	if singleInstance == nil {
		once.Do(
			func() {
				fmt.Println("Creating single instance now.")
				singleInstance = &single{}
			})
	} else {
		fmt.Println("Single instance already created.")
	}

	return singleInstance
}

package main

import (
	"fmt"
)

func main() {

	for i := 0; i < 30; i++ {
		go getInstance()
	}

	// Scanln is similar to Scan, but stops scanning at a newline and
	// after the final item there must be a newline or EOF.
	fmt.Scanln()
}

Creating single instance now.
Single instance already created.
Single instance already created.
```

### php

```php
<?php

namespace RefactoringGuru\Singleton\Conceptual;

/**
 * The Singleton class defines the `GetInstance` method that serves as an
 * alternative to constructor and lets clients access the same instance of this
 * class over and over.
 */
class Singleton
{
    /**
     * The Singleton's instance is stored in a static field. This field is an
     * array, because we'll allow our Singleton to have subclasses. Each item in
     * this array will be an instance of a specific Singleton's subclass. You'll
     * see how this works in a moment.
     */
    private static $instances = [];

    /**
     * The Singleton's constructor should always be private to prevent direct
     * construction calls with the `new` operator.
     */
    protected function __construct()
    {
    }

    /**
     * Singletons should not be cloneable.
     */
    protected function __clone()
    {
    }

    /**
     * Singletons should not be restorable from strings.
     */
    public function __wakeup()
    {
        throw new \Exception("Cannot unserialize a singleton.");
    }

    /**
     * This is the static method that controls the access to the singleton
     * instance. On the first run, it creates a singleton object and places it
     * into the static field. On subsequent runs, it returns the client existing
     * object stored in the static field.
     *
     * This implementation lets you subclass the Singleton class while keeping
     * just one instance of each subclass around.
     */
    public static function getInstance(): Singleton
    {
        $cls = static::class;
        if (!isset(self::$instances[$cls])) {
            self::$instances[$cls] = new static();
        }

        return self::$instances[$cls];
    }

    /**
     * Finally, any singleton should define some business logic, which can be
     * executed on its instance.
     */
    public function someBusinessLogic()
    {
        // ...
    }
}

/**
 * The client code.
 */
function clientCode()
{
    $s1 = Singleton::getInstance();
    $s2 = Singleton::getInstance();
    if ($s1 === $s2) {
        echo "Singleton works, both variables contain the same instance.";
    } else {
        echo "Singleton failed, variables contain different instances.";
    }
}

clientCode();

Singleton works, both variables contain the same instance.

<?php

namespace RefactoringGuru\Singleton\RealWorld;

/**
 * If you need to support several types of Singletons in your app, you can
 * define the basic features of the Singleton in a base class, while moving the
 * actual business logic (like logging) to subclasses.
 */
class Singleton
{
    /**
     * The actual singleton's instance almost always resides inside a static
     * field. In this case, the static field is an array, where each subclass of
     * the Singleton stores its own instance.
     */
    private static $instances = [];

    /**
     * Singleton's constructor should not be public. However, it can't be
     * private either if we want to allow subclassing.
     */
    protected function __construct()
    {
    }

    /**
     * Cloning and unserialization are not permitted for singletons.
     */
    protected function __clone()
    {
    }

    public function __wakeup()
    {
        throw new \Exception("Cannot unserialize singleton");
    }

    /**
     * The method you use to get the Singleton's instance.
     */
    public static function getInstance()
    {
        $subclass = static::class;
        if (!isset(self::$instances[$subclass])) {
            // Note that here we use the "static" keyword instead of the actual
            // class name. In this context, the "static" keyword means "the name
            // of the current class". That detail is important because when the
            // method is called on the subclass, we want an instance of that
            // subclass to be created here.

            self::$instances[$subclass] = new static();
        }
        return self::$instances[$subclass];
    }
}

/**
 * The logging class is the most known and praised use of the Singleton pattern.
 * In most cases, you need a single logging object that writes to a single log
 * file (control over shared resource). You also need a convenient way to access
 * that instance from any context of your app (global access point).
 */
class Logger extends Singleton
{
    /**
     * A file pointer resource of the log file.
     */
    private $fileHandle;

    /**
     * Since the Singleton's constructor is called only once, just a single file
     * resource is opened at all times.
     *
     * Note, for the sake of simplicity, we open the console stream instead of
     * the actual file here.
     */
    protected function __construct()
    {
        $this->fileHandle = fopen('php://stdout', 'w');
    }

    /**
     * Write a log entry to the opened file resource.
     */
    public function writeLog(string $message): void
    {
        $date = date('Y-m-d');
        fwrite($this->fileHandle, "$date: $message\n");
    }

    /**
     * Just a handy shortcut to reduce the amount of code needed to log messages
     * from the client code.
     */
    public static function log(string $message): void
    {
        $logger = static::getInstance();
        $logger->writeLog($message);
    }
}

/**
 * Applying the Singleton pattern to the configuration storage is also a common
 * practice. Often you need to access application configurations from a lot of
 * different places of the program. Singleton gives you that comfort.
 */
class Config extends Singleton
{
    private $hashmap = [];

    public function getValue(string $key): string
    {
        return $this->hashmap[$key];
    }

    public function setValue(string $key, string $value): void
    {
        $this->hashmap[$key] = $value;
    }
}

/**
 * The client code.
 */
Logger::log("Started!");

// Compare values of Logger singleton.
$l1 = Logger::getInstance();
$l2 = Logger::getInstance();
if ($l1 === $l2) {
    Logger::log("Logger has a single instance.");
} else {
    Logger::log("Loggers are different.");
}

// Check how Config singleton saves data...
$config1 = Config::getInstance();
$login = "test_login";
$password = "test_password";
$config1->setValue("login", $login);
$config1->setValue("password", $password);
// ...and restores it.
$config2 = Config::getInstance();
if (
    $login == $config2->getValue("login") &&
    $password == $config2->getValue("password")
) {
    Logger::log("Config singleton also works fine.");
}

Logger::log("Finished!");

2018-06-04: Started!
2018-06-04: Logger has a single instance.
2018-06-04: Config singleton also works fine.
2018-06-04: Finished!
```

### python

```python
class SingletonMeta(type):
    """
    The Singleton class can be implemented in different ways in Python. Some
    possible methods include: base class, decorator, metaclass. We will use the
    metaclass because it is best suited for this purpose.
    """

    _instances = {}

    def __call__(cls, *args, **kwargs):
        """
        Possible changes to the value of the `__init__` argument do not affect
        the returned instance.
        """
        if cls not in cls._instances:
            instance = super().__call__(*args, **kwargs)
            cls._instances[cls] = instance
        return cls._instances[cls]


class Singleton(metaclass=SingletonMeta):
    def some_business_logic(self):
        """
        Finally, any singleton should define some business logic, which can be
        executed on its instance.
        """

        # ...


if __name__ == "__main__":
    # The client code.

    s1 = Singleton()
    s2 = Singleton()

    if id(s1) == id(s2):
        print("Singleton works, both variables contain the same instance.")
    else:
        print("Singleton failed, variables contain different instances.")

Singleton works, both variables contain the same instance.

from threading import Lock, Thread


class SingletonMeta(type):
    """
    This is a thread-safe implementation of Singleton.
    """

    _instances = {}

    _lock: Lock = Lock()
    """
    We now have a lock object that will be used to synchronize threads during
    first access to the Singleton.
    """

    def __call__(cls, *args, **kwargs):
        """
        Possible changes to the value of the `__init__` argument do not affect
        the returned instance.
        """
        # Now, imagine that the program has just been launched. Since there's no
        # Singleton instance yet, multiple threads can simultaneously pass the
        # previous conditional and reach this point almost at the same time. The
        # first of them will acquire lock and will proceed further, while the
        # rest will wait here.
        with cls._lock:
            # The first thread to acquire the lock, reaches this conditional,
            # goes inside and creates the Singleton instance. Once it leaves the
            # lock block, a thread that might have been waiting for the lock
            # release may then enter this section. But since the Singleton field
            # is already initialized, the thread won't create a new object.
            if cls not in cls._instances:
                instance = super().__call__(*args, **kwargs)
                cls._instances[cls] = instance
        return cls._instances[cls]


class Singleton(metaclass=SingletonMeta):
    value: str = None
    """
    We'll use this property to prove that our Singleton really works.
    """

    def __init__(self, value: str) -> None:
        self.value = value

    def some_business_logic(self):
        """
        Finally, any singleton should define some business logic, which can be
        executed on its instance.
        """


def test_singleton(value: str) -> None:
    singleton = Singleton(value)
    print(singleton.value)


if __name__ == "__main__":
    # The client code.

    print("If you see the same value, then singleton was reused (yay!)\n"
          "If you see different values, "
          "then 2 singletons were created (booo!!)\n\n"
          "RESULT:\n")

    process1 = Thread(target=test_singleton, args=("FOO",))
    process2 = Thread(target=test_singleton, args=("BAR",))
    process1.start()
    process2.start()

If you see the same value, then singleton was reused (yay!)
If you see different values, then 2 singletons were created (booo!!)

RESULT:

FOO
FOO
```

### ruby

```ruby
# The Singleton class defines the `instance` method that lets clients access the
# unique singleton instance.
class Singleton
  @instance = new

  private_class_method :new

  # The static method that controls the access to the singleton instance.
  #
  # This implementation let you subclass the Singleton class while keeping just
  # one instance of each subclass around.
  def self.instance
    @instance
  end

  # Finally, any singleton should define some business logic, which can be
  # executed on its instance.
  def some_business_logic
    # ...
  end
end

# The client code.

s1 = Singleton.instance
s2 = Singleton.instance

if s1.equal?(s2)
  print 'Singleton works, both variables contain the same instance.'
else
  print 'Singleton failed, variables contain different instances.'
end

Singleton works, both variables contain the same instance.

# The Singleton class defines the `instance` method that lets clients access the
# unique singleton instance.
class Singleton
  attr_reader :value

  @instance_mutex = Mutex.new

  private_class_method :new

  def initialize(value)
    @value = value
  end

  # The static method that controls the access to the singleton instance.
  #
  # This implementation let you subclass the Singleton class while keeping just
  # one instance of each subclass around.
  def self.instance(value)
    return @instance if @instance

    @instance_mutex.synchronize do
      @instance ||= new(value)
    end

    @instance
  end

  # Finally, any singleton should define some business logic, which can be
  # executed on its instance.
  def some_business_logic
    # ...
  end
end

# @param [String] value
def test_singleton(value)
  singleton = Singleton.instance(value)
  puts singleton.value
end

# The client code.

puts "If you see the same value, then singleton was reused (yay!)\n"\
     "If you see different values, then 2 singletons were created (booo!!)\n\n"\
     "RESULT:\n\n"

process1 = Thread.new { test_singleton('FOO') }
process2 = Thread.new { test_singleton('BAR') }
process1.join
process2.join

If you see the same value, then singleton was reused (yay!)
If you see different values, then 2 singletons were created (booo!!)

RESULT:

FOO
FOO
```

### rust

```rust
//! A pure safe way to implement Singleton in Rust is using no static variables
//! and passing everything around through function arguments.
//! The oldest living variable is an object created at the start of the `main()`.

fn change(global_state: &mut u32) {
    *global_state += 1;
}

fn main() {
    let mut global_state = 0u32;

    change(&mut global_state);

    println!("Final state: {}", global_state);
}

Final state: 1

//! Taken from: https://stackoverflow.com/questions/27791532/how-do-i-create-a-global-mutable-singleton
//!
//! Rust doesn't really allow a singleton pattern without `unsafe` because it
//! doesn't have a safe mutable global state.
//!
//! `lazy-static` allows declaring a static variable with lazy initialization
//! at first access. It is actually implemented via `unsafe` with `static mut`
//! manipulation, however, it keeps your code clear of `unsafe` blocks.
//!
//! `Mutex` provides safe access to a single object.

use lazy_static::lazy_static;
use std::sync::Mutex;

lazy_static! {
    static ref ARRAY: Mutex<Vec<u8>> = Mutex::new(vec![]);
}

fn do_a_call() {
    ARRAY.lock().unwrap().push(1);
}

fn main() {
    do_a_call();
    do_a_call();
    do_a_call();

    println!("Called {}", ARRAY.lock().unwrap().len());
}

Called 3

//! ructc 1.63
//! https://stackoverflow.com/questions/27791532/how-do-i-create-a-global-mutable-singleton
//!
//! Starting with Rust 1.63, it can be easier to work with global mutable
//! singletons, although it's still preferable to avoid global variables in most
//! cases.
//!
//! Now that `Mutex::new` is `const`, you can use global static `Mutex` locks
//! without needing lazy initialization.

use std::sync::Mutex;

static ARRAY: Mutex<Vec<i32>> = Mutex::new(Vec::new());

fn do_a_call() {
    ARRAY.lock().unwrap().push(1);
}

fn main() {
    do_a_call();
    do_a_call();
    do_a_call();

    let array = ARRAY.lock().unwrap();
    println!("Called {} times: {:?}", array.len(), array);
    drop(array);

    *ARRAY.lock().unwrap() = vec![3, 4, 5];

    println!("New singleton object: {:?}", ARRAY.lock().unwrap());
}

Called 3 times
```

### swift

```swift
import XCTest

/// The Singleton class defines the `shared` field that lets clients access the
/// unique singleton instance.
class Singleton {

    /// The static field that controls the access to the singleton instance.
    ///
    /// This implementation let you extend the Singleton class while keeping
    /// just one instance of each subclass around.
    static var shared: Singleton = {
        let instance = Singleton()
        // ... configure the instance
        // ...
        return instance
    }()

    /// The Singleton's initializer should always be private to prevent direct
    /// construction calls with the `new` operator.
    private init() {}

    /// Finally, any singleton should define some business logic, which can be
    /// executed on its instance.
    func someBusinessLogic() -> String {
        // ...
        return "Result of the 'someBusinessLogic' call"
    }
}

/// Singletons should not be cloneable.
extension Singleton: NSCopying {

    func copy(with zone: NSZone? = nil) -> Any {
        return self
    }
}

/// The client code.
class Client {
    // ...
    static func someClientCode() {
        let instance1 = Singleton.shared
        let instance2 = Singleton.shared

        if (instance1 === instance2) {
            print("Singleton works, both variables contain the same instance.")
        } else {
            print("Singleton failed, variables contain different instances.")
        }
    }
    // ...
}

/// Let's see how it all works together.
class SingletonConceptual: XCTestCase {

    func testSingletonConceptual() {
        Client.someClientCode()
    }
}

Singleton works, both variables contain the same instance.

import XCTest

/// Singleton Design Pattern
///
/// Intent: Ensure that class has a single instance, and provide a global point
/// of access to it.

class SingletonRealWorld: XCTestCase {

    func testSingletonRealWorld() {

        /// There are two view controllers.
        ///
        /// MessagesListVC displays a list of last messages from a user's chats.
        /// ChatVC displays a chat with a friend.
        ///
        /// FriendsChatService fetches messages from a server and provides all
        /// subscribers (view controllers in our example) with new and removed
        /// messages.
        ///
        /// FriendsChatService is used by both view controllers. It can be
        /// implemented as an instance of a class as well as a global variable.
        ///
        /// In this example, it is important to have only one instance that
        /// performs resource-intensive work.

        let listVC = MessagesListVC()
        let chatVC = ChatVC()

        listVC.startReceiveMessages()
        chatVC.startReceiveMessages()

        /// ... add view controllers to the navigation stack ...
    }
}


class BaseVC: UIViewController, MessageSubscriber {

    func accept(new messages: [Message]) {
        /// Handles new messages in the base class
    }

    func accept(removed messages: [Message]) {
        /// Hanldes removed messages in the base class
    }

    func startReceiveMessages() {

        /// The singleton can be injected as a dependency. However, from an
        /// informational perspective, this example calls FriendsChatService
        /// directly to illustrate the intent of the pattern, which is: "...to
        /// provide the global point of access to the instance..."

        FriendsChatService.shared.add(subscriber: self)
    }
}

class MessagesListVC: BaseVC {

    override func accept(new messages: [Message]) {
        print("MessagesListVC accepted 'new messages'")
        /// Handles new messages in the child class
    }

    override func accept(removed messages: [Message]) {
        print("MessagesListVC accepted 'removed messages'")
        /// Handles removed messages in the child class
    }

    override func startReceiveMessages() {
        print("MessagesListVC starts receive messages")
        super.startReceiveMessages()
    }
}

class ChatVC: BaseVC {

    override func accept(new messages: [Message]) {
        print("ChatVC accepted 'new messages'")
        /// Handles new messages in the child class
    }

    override func accept(removed messages: [Message]) {
        print("ChatVC accepted 'removed messages'")
        /// Handles removed messages in the child class
    }

    override func startReceiveMessages() {
        print("ChatVC starts receive messages")
        super.startReceiveMessages()
    }
}

/// Protocol for call-back events

protocol MessageSubscriber {

    func accept(new messages: [Message])
    func accept(removed messages: [Message])
}

/// Protocol for communication with a message service

protocol MessageService {

    func add(subscriber: MessageSubscriber)
}

/// Message domain model

struct Message {

    let id: Int
    let text: String
}


class FriendsChatService: MessageService {

    static let shared = FriendsChatService()

    private var subscribers = [MessageSubscriber]()

    func add(subscriber: MessageSubscriber) {

        /// In this example, fetching starts again by adding a new subscriber
        subscribers.append(subscriber)

        /// Please note, the first subscriber will receive messages again when
        /// the second subscriber is added
        startFetching()
    }

    func startFetching() {

        /// Set up the network stack, establish a connection...
        /// ...and retrieve data from a server

        let newMessages = [Message(id: 0, text: "Text0"),
                           Message(id: 5, text: "Text5"),
                           Message(id: 10, text: "Text10")]

        let removedMessages = [Message(id: 1, text: "Text0")]

        /// Send updated data to subscribers
        receivedNew(messages: newMessages)
        receivedRemoved(messages: removedMessages)
    }
}

private extension FriendsChatService {

    func receivedNew(messages: [Message]) {

        subscribers.forEach { item in
            item.accept(new: messages)
        }
    }

    func receivedRemoved(messages: [Message]) {

        subscribers.forEach { item in
            item.accept(removed: messages)
        }
    }
}

MessagesListVC starts receive messages
MessagesListVC accepted 'new messages'
MessagesListVC accepted 'removed messages'
======== At this point, the second subscriber is added ======
ChatVC starts receive messages
MessagesListVC accepted 'new messages'
ChatVC accepted 'new messages'
MessagesListVC accepted 'removed messages'
ChatVC accepted 'removed messages'
```

### typescript

```typescript
/**
 * The Singleton class defines an `instance` getter, that lets clients access
 * the unique singleton instance.
 */
class Singleton {
    static #instance: Singleton;

    /**
     * The Singleton's constructor should always be private to prevent direct
     * construction calls with the `new` operator.
     */
    private constructor() { }

    /**
     * The static getter that controls access to the singleton instance.
     *
     * This implementation allows you to extend the Singleton class while
     * keeping just one instance of each subclass around.
     */
    public static get instance(): Singleton {
        if (!Singleton.#instance) {
            Singleton.#instance = new Singleton();
        }

        return Singleton.#instance;
    }

    /**
     * Finally, any singleton can define some business logic, which can be
     * executed on its instance.
     */
    public someBusinessLogic() {
        // ...
    }
}

/**
 * The client code.
 */
function clientCode() {
    const s1 = Singleton.instance;
    const s2 = Singleton.instance;

    if (s1 === s2) {
        console.log(
            'Singleton works, both variables contain the same instance.'
        );
    } else {
        console.log('Singleton failed, variables contain different instances.');
    }
}

clientCode();

Singleton works, both variables contain the same instance.
```