Applicability

Use Singleton pattern when:

• There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point.
• When the sole instance should be extensible by subclassis, and client should be able to use an extended instance without modifying their code.

Structure

Participants

• Singleton: defines an instance operation that lets clients access it's unique instance. May be responsible for creating it's own unique instance.

Collaborations

Clients access a Singleton instance solely through Singleton's Instances operation.

Consequences

The singleton pattern has several benefits:

• Controlled access to sole instance.
• Reduced name space.
• Permits refinement of operations and representations.
• Permts a variable number of instances.
• More flexible than class operations.

Sample Code

Singleton

Related Patterns

Many patterns can be implemented using the Singleton pattern - Abstract Factory, Builder, Prototype.