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Single-serving Visitor PatternIn 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. Element* elem; SingleServingVisitor::applyTo(elem); 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
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