Design Pattern - Singleton

http://en.wikipedia.org/wiki/Singleton_pattern

Intent:

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

Applicability:

1. Exactly N instance(s) of a class needed:
1. must be accessible to clients
2. a well-known access point
2. When class of sole instance should be extensible

Structure:

Participants:

1. Singleton defines an Instance operation that lets clients access its unique instance.
2. Instance is a class operation (e.e. static member funciton in C++).
3. May be responsible for creating its own unique instance.

Overview:
Client accss a Singleton instance solely through Singleton's instance operation

1. Controlled access to sole instance.
2. Reduced name space.
3. Permoits refinement of operations and representation.
4. Permits a variable number of instances.
5. More flexible than class operations.

Applications:

1. Settings
2. Configuration Data
3. Read-only Data / Cache
4. Print spooler