In computer science, the singleton design pattern is designed to restrict instantiation of a class to one (or a few) objects. 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.

The singleton pattern is implemented by creating a class with a method that creates a new instance of the object if one does not exist. If one does exist it returns a reference to the object that already exists. To make sure that the object cannot be instantiated any other way the constructor is made either private or protected.

The singleton pattern must be carefully constructed in multi-threaded applications. If both threads execute the creation method at the same time when a singleton does not yet exist, they both do a check for an instance of the singleton and then only one should create a new one.

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

A Java programming language solution is as follows (based on the Q&A link, modified for multi-threading):

public class Singleton {
    private static Singleton INSTANCE = null;

    // Private constructor suppresses 
    // default public constructor
    private Singleton() {}

    //synchronized creator to defend against multi-threading issues
    //another if check here to avoid multiple instantiation
    private synchronized static void createInstance() {
        if (INSTANCE == null){ 
            INSTANCE = new Singleton();
        }
    }

    public static Singleton getInstance() {
        if (INSTANCE == null) createInstance();
        return INSTANCE;
    }
}

A possible C++ solution (also known as Meyers singleton) where the singleton is a static local object (note: this solution is not thread-safe and is designed to give an idea of how singletons work rather than a solution usable in large-scale software project).

template<typename T> class Singleton
{

  public:
    static T& Instance()
    {
        static T theSingleInstance; //assumes T has a default constructor
        return theSingleInstance;
    }
};

class OnlyOne : public Singleton<OnlyOne>
{
    //..rest of interface defined here
};

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.

See also

• Double checked locking pattern
• Multiton pattern

External links

• A Q&A page (java.sun.com)
• A Pattern Enforcing Compiler™ that enfoces the Singleton pattern amongst other patterns
• Description from the Pattern Digest
• Description from the Portland Pattern Repository
• Description by Vince Huston
• Article "Use your singletons wisely" by J. B. Rainsberger
• Implementing the Singleton Pattern in C#