object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called procedures or methods and impl ...

, the singleton pattern is a

software design pattern In software engineering, a software design pattern or design pattern is a general, reusable solution to a commonly occurring problem in many contexts in software design. A design pattern is not a rigid structure to be transplanted directly into s ...

that restricts the instantiation of a

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

to a singular instance. It is one of the well-known "Gang of Four" design patterns, which describe how to solve recurring problems in object-oriented software. The pattern is useful when exactly one object is needed to coordinate actions across a system. More specifically, the singleton pattern allows classes to: * Ensure they only have one instance * Provide easy access to that instance * Control their instantiation (for example, hiding the constructors of a

) The term comes from the mathematical concept of a singleton.

Common uses

Singletons are often preferred to

global variables In computer programming, a global variable is a variable with global scope, meaning that it is visible (hence accessible) throughout the program, unless shadowed. The set of all global variables is known as the ''global environment'' or ''global ...

because they do not pollute the global

namespace In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified. Namespaces ...

(or their containing namespace). Additionally, they permit lazy allocation and initialization, whereas global variables in many languages will always consume resources. The singleton pattern can also be used as a basis for other design patterns, such as the

abstract factory The abstract factory pattern in software engineering is a design pattern that provides a way to create families of related objects without imposing their concrete classes, by encapsulating a group of individual factories that have a common theme w ...

, factory method,

builder Builder may refer to: Construction * Construction worker, who specializes in building work * Carpenter, a skilled craftsman who works with wood * General contractor, that specializes in building work ** Subcontractor * Real estate developer, who ...

and

prototype A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototype ...

patterns. Facade objects are also often singletons because only one facade object is required.

Logging Logging is the process of cutting, processing, and moving trees to a location for transport. It may include skidder, skidding, on-site processing, and loading of trees or trunk (botany), logs onto logging truck, trucks

Implementations

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 Private or privates may refer to: Music * "In Private", by Dusty Springfield from the 1990 album ''Reputation'' * Private (band), a Denmark-based band * "Private" (Ryōko Hirosue song), from the 1999 album ''Private'', written and also recorded ...

, which prevents it from being instantiated by other objects * Providing a

static method A method in object-oriented programming (OOP) is a procedure associated with an object, and generally also a message. An object consists of ''state data'' and ''behavior''; these compose an ''interface'', which specifies how the object may be us ...

that returns a

reference A reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''nam ...

to the instance The instance is usually stored as a private

static variable In computer programming, a static variable is a variable that has been allocated "statically", meaning that its lifetime (or "extent") is the entire run of the program. This is in contrast to shorter-lived automatic variables, whose storage is ...

; the instance is created when the variable is initialized, at some point before when the static method is first called. This

C++23 C++23, formally ISO/IEC 14882:2024, is the current open standard for the C++ programming language that follows C++20. The final draft of this version is N4950. In February 2020, at the final meeting for C++20 in Prague, an overall plan for C++ ...

implementation is based on the pre-C++98 implementation in the book . import std; class Singleton ; Singleton* Singleton::instance = nullptr; // definition class variable int main() The program output is value=42 This is an implementation of the Meyers singleton in C++11. The Meyers singleton has no destruct method. The program output is the same as above. import std; class Singleton ; int main()

Lazy initialization

A singleton implementation may use

lazy initialization In computer programming, lazy initialization is the tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed. It is a kind of lazy evaluation that refers specificall ...

in which the instance is created when the static method is first invoked. In multithreaded programs, this can cause

race conditions A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of other uncontrollable events, leading to unexpected or inconsistent ...

that result in the creation of multiple instances. The following Java 5+ example is a

thread-safe In multi-threaded computer programming, a function is thread-safe when it can be invoked or accessed concurrently by multiple threads without causing unexpected behavior, race conditions, or data corruption. As in the multi-threaded context where ...

implementation, using lazy initialization with

double-checked locking In software engineering, double-checked locking (also known as "double-checked locking optimization") is a software design pattern used to reduce the overhead of acquiring a lock by testing the locking criterion (the "lock hint") before acquiring t ...

. public class Singleton

Criticism

Some consider the singleton to be an

anti-pattern An anti-pattern in software engineering, project management, and business processes is a common response to a recurring problem that is usually ineffective and risks being highly counterproductive. The term, coined in 1995 by computer programmer An ...

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. This increased

coupling A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mo ...

can introduce difficulties with

unit testing Unit testing, component or module testing, is a form of software testing by which isolated source code is tested to validate expected behavior. Unit testing describes tests that are run at the unit-level to contrast testing at the Integration ...

. In turn, this places restrictions on any abstraction that uses the singleton, such as preventing

concurrent Concurrent means happening at the same time. Concurrency, concurrent, or concurrence may refer to: Law * Concurrence, in jurisprudence, the need to prove both ''actus reus'' and ''mens rea'' * Concurring opinion (also called a "concurrence"), a ...

use of multiple instances.Steve Yegge
Singletons considered stupid
September 2004Hevery, Miško,

, ''Clean Code Talks'', 21 November 2008. Singletons also violate the

single-responsibility principle The single-responsibility principle (SRP) is a computer programming principle that states that "A module should be responsible to one, and only one, actor." The term actor refers to a group (consisting of one or more stakeholders or users) that ...

because they are responsible for enforcing their own uniqueness along with performing their normal functions.

References

External links