Singleton

Intent

Ensure a class has only one instance, and provide a global point of access to it.

Motivation

Some classes should have exactly one instance — a filesystem, a window manager, a print spooler. A global variable makes the object accessible but doesn't prevent multiple instantiation. Singleton lets the class itself control access: private constructor, static Instance() method.

Structure

The Singleton class defines a static Instance() operation that returns the unique instance. The constructor is protected or private. Clients access the singleton exclusively through Instance().

Consequences

• Controlled access to sole instance — the class encapsulates its single instance, giving it strict control over how and when clients access it
• Reduced name space — avoids polluting the global namespace with standalone variables
• Permits refinement of operations and representation — you can subclass Singleton and configure the application with an instance of the subclass
• Registry of singletons — instead of hardcoding the subclass in Instance(), register singleton classes by name and look them up at runtime

Thread-Safety Considerations

The classic lazy-initialization idiom (if instance == null) is not thread-safe. Solutions include eager initialization, double-checked locking, or language-level guarantees (e.g., module-scope instances in Python/JS). In most modern contexts, dependency injection containers have replaced hand-rolled singletons.