Visitor Pattern

What is a visitor pattern?

  • The visitor pattern is a behavioral design pattern. 
  • Visitor pattern allows to separate the data structures and the algorithms to be applied on the data.
  • Both data structure objects and algorithm objects can evolve separately. Makes development and changes easier.
  • Data structure (element) objects have an “accept” method which take in a visitor (algorithmic) object.
  • Algorithmic objects have a “visit” method which take in a data structure object.

    Demonstrate a simple visitor pattern using C++

    #include <iostream>#include <list>using namespace std;// Forwardsclass VisitorIntf;// Abstract interface for Element objectsclass ElementIntf{public:virtual string name() = 0;virtual void accept(VisitorIntf* object) = 0;};// Abstract interface for Visitor objectsclass VisitorIntf{public:virtual void visit(ElementIntf* object) = 0;};// Concrete element objectclass ConcreteElement1 : public ElementIntf{public:string name(){return "ConcreteElement1";}void accept(VisitorIntf *object){object->visit(this);}};// Concrete element objectclass ConcreteElement2 : public ElementIntf{public:string name(){return "ConcreteElement2";}void accept(VisitorIntf *object){object->visit(this);}};// Visitor logic 1class ConcreteVisitor1 : public VisitorIntf{public:void visit(ElementIntf *object){cout << "Visited " << object->name() <<" using ConcreteVisitor1." << endl;}};// Visitor logic 2class ConcreteVisitor2 : public VisitorIntf{public:void visit(ElementIntf *object){cout << "Visited " << object->name() <<" using ConcreteVisitor2." << endl;}};//  Test main programint main(){list<ElementIntf*> elementList1;elementList1.push_back(new ConcreteElement1());elementList1.push_back(new ConcreteElement2());VisitorIntf* visitor1 = new ConcreteVisitor1();while ( ! elementList1.empty() ){ElementIntf* element = elementList1.front();element->accept(visitor1);elementList1.pop_front();}list<ElementIntf*> elementList2;elementList2.push_back(new ConcreteElement1());elementList2.push_back(new ConcreteElement2());VisitorIntf* visitor2 = new ConcreteVisitor2();while ( ! elementList2.empty() ){ElementIntf* element = elementList2.front();element->accept(visitor2);elementList2.pop_front();}delete visitor1;delete visitor2;}


    Visited ConcreteElement1 using ConcreteVisitor1.Visited ConcreteElement2 using ConcreteVisitor1.Visited ConcreteElement1 using ConcreteVisitor2.Visited ConcreteElement2 using ConcreteVisitor2.