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Design pattern: Multiton

Description: Generalization of the Singleton design pattern where a fixed number of named instances of a class is managed.

This pattern can be used with both classes and prototypes. The description above is from the Wikipedia page on this pattern:

https://en.wikipedia.org/wiki/Multiton_pattern

This pattern is not described in the GoF book. See the Wikipedia page for details and references.

The sample implementation uses classes. To simplify, it assumes a fixed (at compile-time) set of named instances. An alternative would be allow the definition of the set of named instances at runtime by extending the protocol of the multiton with the necessary predicates. Also to simplify, the sample code uses the named instance identifiers as the access keys. This could also be changed to use instead a dictionary.