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:

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

* With parameters:

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

* Implementation as a singleton:

Element* elem; TYPE param1, param2; SingleServingVisitor::setParam1(param1); SingleServingVisitor::setParam2(param2); SingleServingVisitor::applyTo(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 "applyTo" static method.

Cons

* "Repeated allocation". At each call of the "applyTo" 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 {protected: SingleServingVisitor();public: ~SingleServingVisitor();

static void applyTo(Element*); virtual void visitElementA(ElementA*) = 0; virtual void visitElementB(ElementB*) = 0;}

// Implementationvoid SingleServingVisitor::applyTo(Element* elem){ SingleServingVisitor* ssv = new SingleServingVisitor(); elem->accept(ssv); delete ssv;}

Passing parameters

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

void SingleServingVisitor::applyTo(Element* elem, TYPE param1, TYPE param2, ...){ SingleServingVisitor ssv(param1, param2, ...); elem->accept(&ssv);}

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 {protected: static SingleServingVisitor* _instance; TYPE _param1; TYPE _param2;

SingleServingVisitor();

static SingleServingVisitor* getInstance(); // Note: getInstance method does not need to be public

public: ~SingleServingVisitor();

static void applyTo(Element*);

// static methods to access parameters static void setParam1(TYPE); static void setParam2(TYPE);

virtual void visitElementA(ElementA*) = 0; virtual void visitElementB(ElementB*) = 0;}

// ImplementationSingleServingVisitor* SingleServingVisitor::_instance = NULL;

SingleServingVisitor* SingleServingVisitor::getInstance() { if (_instance = NULL) _instance = new SingleServingVisitor(); return _instance;}

void SingleServingVisitor::applyTo(Element* elem){ elem->accept( getInstance() );}

void SingleServingVisitor::setParam1(TYPE param1){ getInstance()->_param1 = param1;}

void SingleServingVisitor::setParam2(TYPE param2){ getInstance()->_param2 = param2;}

Related patterns

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