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Design pattern: Prototype

Description: "Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype."

This pattern can be used with both classes and prototypes.

Logtalk doesn't provide a built-in object clone functionality that would help in implementing this design pattern. One of the main issues in providing object cloning support is the depth of the copy when an object contains references to other objects. Some languages provide a single cloning primitive while other support both shallow and deep copies (e.g. Java's clone method performs a shallow copy).

The sample implementation uses classes. The cloning operation creates a new instance with the same state as the currently selected prototype. This is a relatively expensive operation do to requiring retrieving the state of the prototype for copying. More efficient solutions can be implemented in most cases by e.g. having customized instance creation predicate definitions.