Tech-No!: Verification Methodologies and Design Patterns: Singleton Pattern

Definition/Intent: Ensure a class only has one instance, and provide a global point of access to it. It is a creational pattern.

Participants & Implementation:

Singleton:

– is responsible for creating and storing its own unique instance

– defines an Instance operation that lets clients access its unique instance

The “class level” Instance operation will either return or create and return the sole instance; a “class level” attribute will contain either a default indicating there is no instance yet or the sole instance

Applicability:

– 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 subclassing, and clients should be able to use an extended instance without modifying their code.

Benefits:

– Controlled access to sole instance

– Permits a variable number of instances

Usage in OVM:

· OVM_TOP – instance of ovm_root. Only one instance of ovm_top

· OVM Factory - only one instance of ovm_factory

References:

i. http://sourcemaking.com/

ii. http://www.oodesign.com

iii. http://home.gwu.edu/~blankeng/Classes/CSCI253/Class%20page%20(spring%202007).htm

vi. http://www.dofactory.com/Default.aspx