singleton pattern

Singletons are often preferred to global variables because they do not pollute the global namespace (or their containing namespace). Additionally, they permit lazy allocation and initialization, whereas global variables in many languages will always consume resources.{{cite news |last1=Soni |first1=Devin |date=31 July 2019 |title=What Is a Singleton? |work=BetterProgramming |url=https://betterprogramming.pub/what-is-a-singleton-2dc38ca08e92 |access-date=28 August 2021}}

The singleton pattern can also be used as a basis for other design patterns, such as the abstract factory, factory method, builder and prototype patterns. Facade objects are also often singletons because only one facade object is required.

Logging is a common real-world use case for singletons, because all objects that wish to log messages require a uniform point of access and conceptually write to a single source.{{cite news |last1=Rainsberger |first1=J.B. |title=Use your singletons wisely |url=https://www.ibm.com/developerworks/library/co-single/ |access-date=28 August 2021 |publisher=IBM |date=1 July 2001 |archive-url=https://web.archive.org/web/20210224180356/https://www.ibm.com/developerworks/library/co-single/ |archive-date=24 February 2021}}

Implementations of the singleton pattern ensure that only one instance of the singleton class ever exists and typically provide global access to that instance.

Typically, this is accomplished by:

• Declaring all constructors of the class to be private, which prevents it from being instantiated by other objects
• Providing a static method that returns a reference to the instance

The instance is usually stored as a private static variable; the instance is created when the variable is initialized, at some point before when the static method is first called.

This C++23 implementation is based on the pre-C++98 implementation in the book {{Citation needed|reason=the book is ambiguous here.|date=March 2024}}.

import std;

class Singleton {

public:

// defines an class operation that lets clients access its unique instance.

static Singleton& get() {

// may be responsible for creating its own unique instance.

if (nullptr == instance) instance = new Singleton;

return *instance;

}

Singleton(const Singleton&) = delete; // rule of three

Singleton& operator=(const Singleton&) = delete;

static void destruct() {

delete instance;

instance = nullptr;

}

// existing interface goes here

int getValue() {

return value;

}

void setValue(int value_) {

value = value_;

}

private:

Singleton() = default; // no public constructor

~Singleton() = default; // no public destructor

static Singleton* instance; // declaration class variable

int value;

};

Singleton* Singleton::instance = nullptr; // definition class variable

int main() {

Singleton::get().setValue(42);

std::println("value={}", Singleton::get().getValue());

Singleton::destruct();

}

The program output is

value=42

This is an implementation of the Meyers singleton{{cite book |author=Scott Meyers |title=More Effective C++ |publisher=Addison Wesley |year=1997 |isbn=0-201-63371-X |pages= 146 ff}} in C++11. The Meyers singleton has no destruct method. The program output is the same as above.

import std;

class Singleton {

public:

static Singleton& get() {

static Singleton instance;

return instance;

}

int getValue() {

return value;

}

void setValue(int value_) {

value = value_;

}

private:

Singleton() = default;

~Singleton() = default;

int value;

};

int main() {

Singleton::get().setValue(42);

std::println("value={}", Singleton::get().getValue());

}

A singleton implementation may use lazy initialization in which the instance is created when the static method is first invoked. In multithreaded programs, this can cause race conditions that result in the creation of multiple instances. The following Java 5+ example{{Cite book|author=Eric Freeman, Elisabeth Freeman, Kathy Sierra, and Bert Bates|title=Head First Design Patterns|publisher=O'Reilly Media, Inc|date=October 2004|edition=First|chapter=5: One of a Kind Objects: The Singleton Pattern|page=182|isbn=978-0-596-00712-6|chapter-url=https://books.google.com/books?id=GGpXN9SMELMC&pg=PA182}} is a thread-safe implementation, using lazy initialization with double-checked locking.

public class Singleton {

private static volatile Singleton instance = null;

private Singleton() {}

public static Singleton getInstance() {

if (instance == null) {

synchronized(Singleton.class) {

if (instance == null) {

instance = new Singleton();

}

}

}

return instance;

}

}

Some consider the singleton to be an anti-pattern that introduces global state into an application, often unnecessarily. This introduces a potential dependency on the singleton by other objects, requiring analysis of implementation details to determine whether a dependency actually exists.{{cite web |title=Why Singletons Are Controversial |url=https://code.google.com/archive/p/google-singleton-detector/wikis/WhySingletonsAreControversial.wiki |archive-url=https://web.archive.org/web/20210506162753/https://code.google.com/archive/p/google-singleton-detector/wikis/WhySingletonsAreControversial.wiki |archive-date=6 May 2021 |access-date=28 August 2021 |website=Google Code Archive}} This increased coupling can introduce difficulties with unit testing.{{cite news |last1=Button |first1=Brian |date=25 May 2004 |title=Why Singletons are Evil |work=Being Scott Densmore |publisher=Microsoft |url=https://docs.microsoft.com/en-us/archive/blogs/scottdensmore/why-singletons-are-evil |access-date=28 August 2021 |archive-url=https://web.archive.org/web/20210715184717/https://docs.microsoft.com/en-us/archive/blogs/scottdensmore/why-singletons-are-evil |archive-date=15 July 2021}} In turn, this places restrictions on any abstraction that uses the singleton, such as preventing concurrent use of multiple instances.Steve Yegge. [http://steve.yegge.googlepages.com/singleton-considered-stupid Singletons considered stupid], September 2004Hevery, Miško, "[http://googletesting.blogspot.com/2008/11/clean-code-talks-global-state-and.html Global State and Singletons]", Clean Code Talks, 21 November 2008.

Singletons also violate the single-responsibility principle because they are responsible for enforcing their own uniqueness along with performing their normal functions.

• Initialization-on-demand holder idiom
• Multiton pattern
• Software design pattern

{{reflist}}

{{external links|date=November 2016}}

{{wikibooks|Computer Science/Design Patterns|Singleton|Singleton implementations in various languages}}

{{commons category}}

• Complete article "[https://howtodoinjava.com/design-patterns/creational/singleton-design-pattern-in-java/ Java Singleton Pattern Explained]"
• Four different ways to implement singleton in Java "[https://web.archive.org/web/20150709155148/http://www.javaexperience.com/design-patterns-singleton-design-pattern/ Ways to implement singleton in Java]"
• Book extract: [https://csharpindepth.com/Articles/Singleton Implementing the Singleton Pattern in C#] by Jon Skeet
• [https://docs.microsoft.com/en-us/previous-versions/msp-n-p/ff650849(v=pandp.10) Singleton at Microsoft patterns & practices Developer Center]
• IBM article "[https://www.ibm.com/developerworks/library/j-dcl/ Double-checked locking and the Singleton pattern]" by Peter Haggar
• {{cite web |last1=Geary |first1=David |date=2003-04-25 |df=mdy |url=https://www.infoworld.com/article/2073352/core-java-simply-singleton.html |title=How to navigate the deceptively simple Singleton pattern |department=Java Design Patterns |work=JavaWorld |access-date=2020-07-21}}
• [https://code.google.com/archive/p/google-singleton-detector/ Google Singleton Detector] (analyzes Java bytecode to detect singletons)

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