Single-serving visitor pattern explained

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:

Element* elem; SingleServingVisitor::apply_to(elem);

• With parameters:

Element* elem; TYPE param1, param2; SingleServingVisitor::apply_to(elem, param1, param2);

• Implementation as a singleton:

Element* elem; TYPE param1, param2; SingleServingVisitor::set_param1(param1); SingleServingVisitor::set_param2(param2); SingleServingVisitor::apply_to(elem);

Consequences

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.

Implementation (in C++)

Basic implementation (without parameters)

// Declarationclass Element;class ElementA;class ElementB;class SingleServingVisitor;

... // Same as with the visitor pattern.

// Definitionclass SingleServingVisitor

// Implementationvoid SingleServingVisitor::apply_to(Element* elem)

Passing parameters

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

void SingleServingVisitor::apply_to(Element* elem, TYPE param1, TYPE param2, ...)

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

// Definitionclass SingleServingVisitor

// ImplementationSingleServingVisitor* SingleServingVisitor::instance_ = NULL;

SingleServingVisitor* SingleServingVisitor::get_instance

void SingleServingVisitor::apply_to(Element* elem)

void SingleServingVisitor::set_param1(TYPE param1)

void SingleServingVisitor::set_param2(TYPE param2)

Related patterns

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