Singleton

A singleton is a software design pattern that restricts the instantiation of a class to a single object and provides a global access point to that instance. This ensures that only one instance of the class exists within a given application.

This pattern is useful when exactly one object is needed to coordinate actions across the system, such as a configuration manager, a logging facility, or a shared resource pool. It provides a centralized, consistent point of access for the resource it represents.

Implementation considerations: A singleton is typically implemented by making the class constructor private and providing a static method or property that returns the sole instance. The instance may be created eagerly or lazily on first use. In multi-threaded environments, the creation must be thread-safe; common techniques include synchronization, double-checked locking, or language-specific idioms (for example, the initialization-on-demand holder in Java) or module-level singletons in languages like Python or JavaScript.

Variants: A multiton allows several named instances; a thread-safe singleton; a persistent singleton that survives process restarts is outside the pattern itself but sometimes used. Some languages provide built-in language features to implement singletons via modules.

Criticism and alternatives: Singletons can introduce global state, hinder testing, and mask dependencies, making code harder to understand and maintain. Alternatives include dependency injection, passing shared instances through constructors, or using module-level singletons or registries.

In summary, the singleton enforces a single shared instance for a class and provides a global access point, but it should be used judiciously given potential design drawbacks.