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).

Contents

1 Applicability 2 Usage examples 3 Consequences 3.1 Pros 3.2 Cons 4 Implementation (in C++) 4.1 Basic implementation (without parameters) 4.2 Passing parameters 4.3 Implementation as a singleton 5 Related patterns

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)

//// Declaration
class Element;
class ElementA;
class ElementB;
class SingleServingVisitor;

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

// Definition
class SingleServingVisitor {
protected:
  SingleServingVisitor();
public:
  ~SingleServingVisitor();

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

// Implementation
void 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 = new SingleServingVisitor(param1, param2, ...);
  elem->accept(ssv);
  delete 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

// Definition
class 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;
}

// Implementation
SingleServingVisitor* 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