Single-serving visitor pattern

In computer programming, the single-serving visitor pattern is a design pattern. Its intent is to optimise the implementation of a visitor that is allocated, used only once, and then deleted (which is the case of most visitors).

Applicability
The single-serving visitor pattern should be used when visitors do not need to remain in memory. This is often the case when visiting a hierarchy of objects (such as when the visitor pattern is used together with the composite pattern) to perform a single task on it, for example counting the number of cameras in a 3D scene.

The regular visitor pattern should be used when the visitor must remain in memory. This occurs when the visitor is configured with a number of parameters that must be kept in memory for a later use of the visitor (for example, for storing the rendering options of a 3D scene renderer).

However, if there should be only one instance of such a visitor in a whole program, it can be a good idea to implement it both as a single-serving visitor and as a singleton. In doing so, it is ensured that the single-serving visitor can be called later with its parameters unchanged (in this particular case "single-serving visitor" is an abuse of language since the visitor can be used several times).

Usage examples
The single-serving visitor is called through the intermediate of static methods.


 * Without parameters:
 * With parameters:
 * Implementation as a singleton:

Pros

 * No "zombie" objects. With a single-serving visitor, it is ensured that visitors are allocated when needed and destroyed once useless.
 * A simpler interface than visitor. The visitor is created, used and free by the sole call of the apply_to static method.

Cons

 * Repeated allocation. At each call of the apply_to method, a single-serving visitor is created then discarded, which is time-consuming. In contrast, the singleton only performs one allocation.

Passing parameters
If the single-serving visitor has to be initialised, the parameters have to be passed through the static method:

Implementation as a singleton
This implementation ensures:
 * that there is at most one instance of the single-serving visitor
 * that the visitor can be accessed later

Related patterns

 * Visitor pattern, from which this pattern derives
 * Composite pattern: single-serving visitor is often applied to hierarchies of elements
 * Singleton pattern