Singleton pattern

In software engineering, the singleton pattern is a design pattern that is used to restrict instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. Sometimes it is generalized to systems that operate more efficiently when only one or a few objects exist. It is also considered an anti-pattern since it is often used as a euphemism for global variable.

Implementation

The singleton pattern is implemented by creating a class with a method that creates a new instance of the class if one does not exist. If an instance already exists, it simply returns a reference to that object. To make sure that the object cannot be instantiated any other way, the constructor is made either private or protected. Note the distinction between a simple static instance of a class and a singleton: although a singleton can be implemented as a static instance, it can also be lazily constructed, requiring no memory or resources until needed. Another notable difference is that all static member class cannot implement an interface, unless that interface is simply a marker. So if the class has to realize a contract expressed by an interface, you really have to make it a singleton.

The singleton pattern must be carefully constructed in multi-threaded applications. If two threads are to execute the creation method at the same time when a singleton does not yet exist, they both must check for an instance of the singleton and then only one should create the new one. If the programming language has concurrent processing capabilities the method should be constructed to execute as a mutually exclusive operation.

The classic solution to this problem is to use mutual exclusion on the class that indicates that the object is being instantiated.

Common uses

• The Abstract Factory, Builder, and Prototype patterns can use Singletons in their implementation.
• Façade objects are often Singletons because only one Façade object is required.
• State objects are often Singletons.
• Singletons are often preferred to global variables because:
  • They don't pollute the global namespace (or, in languages with namespaces, their containing namespace) with unnecessary variables.
  • They permit lazy allocation and initialization, where global variables in many languages will always consume resources.
• Singletons behave differently depending on the lifetime of the virtual machine. While a software development kit may start a new virtual machine for every run which results in a new instance of the singleton being created, calls to a singleton e.g. within the virtual machine of an application server behave differently. There the virtual machine remains alive, therefore the instance of the singleton remains as well. Running the code again therefore can retrieve the "old" instance of the singleton which then may be contaminated with values in local fields which are the result of the first run.

Class diagram

Example implementations

Java

A thread-safe Java programming language lazy-loaded solution suggested by Bill Pugh, known as the initialization on demand holder idiom, follows:

public class Singleton
{ 
  // Private constructor suppresses generation of a (public) default constructor
  private Singleton() {}

  /**
   * SingletonHolder is loaded on the first execution of Singleton.getInstance() 
   * or the first access to SingletonHolder.INSTANCE, not before.
   */
  private static class SingletonHolder
  { 
    private final static Singleton INSTANCE = new Singleton();
  }
  
  public static Singleton getInstance()
  {
    return SingletonHolder.INSTANCE;
  }
}

Traditional simple way:

 public class Singleton {
   private final static Singleton INSTANCE = new Singleton();
 
   // Private constructor suppresses generation of a (public) default constructor
   private Singleton() {}
 
   public static Singleton getInstance() {
     return INSTANCE;
   }
 }

PHP 5

Singleton pattern in PHP 5:

<?php
class Singleton {
  // object instance
  private static $instance;
  
  private function __construct() {}
  
  private function __clone() {}
  
  public static function getInstance() {
    if (self::$instance === null) {
      self::$instance = new Singleton();
    }
    return self::$instance;
  }

  public function doAction() {
    ...
  }
}

//usage
Singleton::getInstance()->doAction();

?>

Objective-C

A common way to implement a singleton in Objective-C is the following:

@interface MySingleton : NSObject
{
}

+ (MySingleton *)sharedSingleton;
@end

@implementation MySingleton

+ (MySingleton *)sharedSingleton
{
  static MySingleton *sharedSingleton;
  
  @synchronized(self)
  {
    if (!sharedSingleton)
      sharedSingleton = [[MySingleton alloc] init];
    
    return sharedSingleton;
  }
}

@end

If thread-safety is not required, the synchronization can be left out, leaving the +sharedSingleton method like this:

+ (MySingleton *)sharedSingleton
{
  static MySingleton *sharedSingleton;

  if (!sharedSingleton)
    sharedSingleton = [[MySingleton alloc] init];

  return sharedSingleton;
}

This pattern is widely used in the Cocoa frameworks (see for instance, NSApplication, NSColorPanel, NSFontPanel or NSWorkspace, to name but a few).

Some may argue that this is not, strictly speaking, a Singleton, because it is possible to allocate more than one instance of the object. A common way around this is to use assertions or exceptions to prevent this double allocation.

@interface MySingleton : NSObject
{
}

+ (MySingleton *)sharedSingleton;
@end

@implementation MySingleton

static MySingleton *sharedSingleton;

+ (MySingleton *)sharedSingleton
{
  @synchronized(self)
  {
    if (!sharedSingleton)
      [[MySingleton alloc] init];
    
    return sharedSingleton;
  }
}

+(id)alloc
{
  @synchronized(self)
  {
    NSAssert(sharedSingleton == nil, @"Attempted to allocate a second instance of a singleton.");
    sharedSingleton = [super alloc];
    return sharedSingleton;
  }
}

@end

There are alternative ways to express the Singleton pattern in Objective-C, but they are not always as simple or as easily understood, not least because they may rely on the -init method returning an object other than self. Some of the Cocoa "Class Clusters" (e.g. NSString, NSNumber) are known to exhibit this type of behaviour.

Note that @synchronized is not available in some Objective-C configurations, as it relies on the NeXT/Apple runtime. It is also comparatively slow, because it has to look up the lock based on the object in parentheses. Check the history of this page for a different implementation using an NSConditionLock.

C++ (using pthreads)

A common design pattern for thread safety with the singleton class is to use double-checked locking. However, due to the ability of modern processors to re-order instructions (as long as the result is consistent with their architecturally-specified memory model), and the absence of any consideration being given to multiple threads of execution in the language standard, double-checked locking is intrinsically prone to failure in C++. There is no model — other than runtime libraries (e.g. POSIX threads, designed to provide concurrency primitives) — that can provide the necessary execution order.[1]

By adding a mutex to the singleton class, a thread-safe implementation may be obtained.

#include <pthread.h>
#include <memory>
#include <iostream>

class Mutex
{
  public:
    Mutex()
    { pthread_mutex_init(&m, 0); }

    void lock()
    { pthread_mutex_lock(&m); }

    void unlock()
    { pthread_mutex_unlock(&m); }

  private:
    pthread_mutex_t m;
};

class MutexLocker
{
  public:
    MutexLocker(Mutex& pm): m(pm) { m.lock(); }
    ~MutexLocker() { m.unlock(); }
  private:
    Mutex& m;
};

class Singleton
{
  public:
    static Singleton& Instance();
    int example_data;
    ~Singleton() { }

  protected:
    Singleton(): example_data(42) { }

  private:
    static std::auto_ptr<Singleton> theSingleInstance;
    static Mutex m;
};
 
Singleton& Singleton::Instance()
{
    MutexLocker obtain_lock(m);
    if (theSingleInstance.get() == 0)
      theSingleInstance.reset(new Singleton);
    return *theSingleInstance;
}

std::auto_ptr<Singleton> Singleton::theSingleInstance;
Mutex Singleton::m;
 
int main()
{
    std::cout << Singleton::Instance().example_data << std::endl;
    return 0;
}

Note the use of the MutexLocker class in the Singleton::Instance() function. The MutexLocker is being used as an RAII object, also known as scoped lock, guaranteeing that the mutex lock will be relinquished even if an exception is thrown during the execution of Singleton::Instance(), since the language specification pledges that the destructors of automatically allocated objects are invoked during stack unwind.

This implementation invokes the mutex-locking primitives for each call to Singleton::Instance(), even though the mutex is only needed once, the first time the method is called. To get rid of the extra mutex operations, the programmer can explicitly construct Singleton::theSingleInstance early in the program (say, in main); or, the class can be optimized (in this case, using pthread_once) to cache the value of the initialized pointer in thread-local storage.

More code must be written if the singleton code is located in a static library, but the program is divided into DLLs. Each DLL that uses the singleton will create a new and distinct instance of the singleton. To avoid that, the singleton code must be linked in a DLL. Alternatively, the singleton can be rewritten to use a memory-mapped file to store theSingleInstance.

C# (using generics)

This example is thread-safe with lazy initialization.

/// <summary>
/// Generic class implements singleton pattern.
/// </summary>
/// <typeparam name="T">
/// Reference type.
/// </typeparam>
public class Singleton<T> where T : class, new()
{
        /// <summary>
        /// Get the Singleton Instance.
        /// </summary>
        public static T Instance
        {
            get { return SingletonCreator._instance ; }
        }

        class SingletonCreator
        {
            static SingletonCreator() { }

            internal static readonly T _instance = new T();
        }
}

C# (using generics and reflection)

This example is thread-safe with lazy initialization. In addition, this example is suit for classes which have private constructor.

/// <summary>
/// Generic class implements singleton pattern.
/// </summary>
/// <typeparam name="T"></typeparam>
[CLSCompliant(true)]
public static class Singleton<T> where T : class
{
    private static T _instance;

    static Singleton() {
        Type type = typeof(T);
        BindingFlags searchPattern = BindingFlags.NonPublic | 
                                     BindingFlags.Public | 
                                     BindingFlags.Instance;
        ConstructorInfo constructor = type.GetConstructor(searchPattern, 
                                                          null, 
                                                          Type.EmptyTypes, 
                                                          null);

        if (constructor == null) {
            string errorMsg = String.Format(
                "The type {0} does not have a default constructor.", type.FullName
            );
            throw new ArgumentNullException(errorMsg);
        }

        _instance = (T)constructor.Invoke(new object[0]);
    }

    /// <summary>
    /// Get the singleton instance of the type.
    /// </summary>
    public static T Instance {
        get { return _instance; }
    }
}

public class Session
{
    private Session() {
    }
}

public class Program
{
    private static void Main() {
        Session instanceA = Singleton<Session>.Instance;
        Session instanceB = Singleton<Session>.Instance;

        // Are they the same?
        Console.WriteLine(instanceA == instanceB); // true
    }
}

Python

According to influential Python programmer Alex Martelli, The Singleton design pattern (DP) has a catchy name, but the wrong focus—on identity rather than on state. The Borg design pattern has all instances share state instead. A rough consensus in the Python community is that sharing state among instances is more elegant, at least in Python, than is caching creation of identical instances on class initialization. Coding shared state is nearly transparent:

class Borg:
   __shared_state = {}
   def __init__(self):
       self.__dict__ = self.__shared_state
   # and whatever else you want in your class -- that's all!

But with the new style class, this is a better solution, because only one instance is created:

class  Singleton (object):
   instance = None       
    def __new__(cls, *args, **kargs): 
        if cls.instance is None:
            cls.instance = object.__new__(cls, *args, **kargs)
        return cls.instance

#Usage
mySingleton1 =  Singleton()
mySingleton2 =  Singleton()
 
#mySingleton1 and  mySingleton2 are the same instance.
assert mySingleton1 is mySingleton2

Two caveats:

• The __init__-method is called every time you call Singleton().
• If you want to inherit from the Singleton-class, instance should probably be a dictionary belonging explicitly to the Singleton-class.

class  InheritableSingleton (object):
    instances = {}
    def __new__(cls, *args, **kargs): 
        if InheritableSingleton.instances.get(cls) is None:
            InheritableSingleton.instances[cls] = object.__new__(cls, *args, **kargs)
        return InheritableSingleton.instances[cls]

Prototype-based singleton

In a prototype-based programming language, where objects but not classes are used, a "singleton" simply refers to an object without copies or that is not used as the prototype for any other object. Example in Io:

Foo := Object clone
Foo clone := Foo

Example of use with the factory method pattern

The singleton pattern is often used in conjunction with the factory method pattern to create a system-wide resource whose specific type is not known to the code that uses it. An example of using these two patterns together is the Java Abstract Windowing Toolkit (AWT).

java.awt.Toolkit is an abstract class that binds the various AWT components to particular native toolkit implementations. The Toolkit class has a Toolkit.getDefaultToolkit() factory method that returns the platform-specific subclass of Toolkit. The Toolkit object is a singleton because the AWT needs only a single object to perform the binding and the object is relatively expensive to create. The toolkit methods must be implemented in an object and not as static methods of a class because the specific implementation is not known by the platform-independent components. The name of the specific Toolkit subclass used is specified by the "awt.toolkit" environment property accessed through System.getProperties().

The binding performed by the toolkit allows, for example, the backing implementation of a java.awt.Window to bound to the platform-specific java.awt.peer.WindowPeer implementation. Neither the Window class nor the application using the window needs to be aware of which platform-specific subclass of the peer is used.

References

• "C++ and the Perils of Double-Checked Locking" Meyers, Scott and Alexandrescu, Andrei, September 2004.
• "The Boost.Threads Library" Kempf, B., Dr. Dobb's Portal, April 2003.

External links

• Java Singleton Pattern by Hemanth Balaji
• A Pattern Enforcing Compiler™ that enforces the Singleton pattern amongst other patterns
• Description from the Portland Pattern Repository
• Singleton pattern discussion with 1-page examples in C++ and Java by Vince Huston
• Implementing the Singleton Pattern in C# by Jon Skeet
• A Threadsafe C++ Template Singleton Pattern for Windows Platforms by O. Patrick Barnes
• Implementing the Inheritable Singleton Pattern in PHP5
• Singleton Pattern and Thread Safety
• PHP patterns
• Javascript implementation of a Singleton Pattern by Christian Schaefer
• Singletons Cause Cancer
• Singleton examples
• Article "Double-checked locking and the Singleton pattern" by Peter Haggar
• Article "Use your singletons wisely" by J. B. Rainsberger
• Article "Simply Singleton" by David Geary
• Article "Description of Singleton" by Aruna
• Article "Why Singletons Are Controversial"
• The Google Singleton Detector analyzes Java bytecode to detect singletons, so that their usefulness can be evaluated.