/*
 * C++ Design Patterns: Visitor
 * Author: Jakub Vojvoda [github.com/JakubVojvoda]
 * 2016
 *
 * Source code is licensed under MIT License
 * (for more details see LICENSE)
 *
 */

#include <iostream>

class Element;
class ConcreteElementA;
class ConcreteElementB;

/*
 * Visitor
 * declares a Visit operation for each class of ConcreteElement
 * in the object structure
 */
class Visitor {
public:
  virtual void visitElementA(ConcreteElementA *element) = 0;
  virtual void visitElementB(ConcreteElementB *element) = 0;
  // ...
};

/*
 * Concrete Visitors
 * implement each operation declared by Visitor, which implement
 * a fragment of the algorithm defined for the corresponding class
 * of object in the structure
 */
class ConcreteVisitor1 : public Visitor {
public:
  void visitElementA(ConcreteElementA *) {
    std::cout << "Concrete Visitor 1: Element A visited." << std::endl;
  }

  void visitElementB(ConcreteElementB *) {
    std::cout << "Concrete Visitor 1: Element B visited." << std::endl;
  }
  // ...
};

class ConcreteVisitor2 : public Visitor {
public:
  void visitElementA(ConcreteElementA *) {
    std::cout << "Concrete Visitor 2: Element A visited." << std::endl;
  }

  void visitElementB(ConcreteElementB *) {
    std::cout << "Concrete Visitor 2: Element B visited." << std::endl;
  }
  // ...
};

/*
 * Element
 * defines an accept operation that takes a visitor as an argument
 */
class Element {
public:
  virtual void accept(Visitor &visitor) = 0;
  // ...
};

/*
 * Concrete Elements
 * implement an accept operation that takes a visitor as an argument
 */
class ConcreteElementA : public Element {
public:
  void accept(Visitor &visitor) {
    visitor.visitElementA(this);
  }
  // ...
};

class ConcreteElementB : public Element {
public:
  void accept(Visitor &visitor) {
    visitor.visitElementB(this);
  }
  // ...
};


int main()
{
  ConcreteElementA elementA;
  ConcreteElementB elementB;

  ConcreteVisitor1 visitor1;
  ConcreteVisitor2 visitor2;

  elementA.accept(visitor1);
  elementA.accept(visitor2);

  elementB.accept(visitor1);
  elementB.accept(visitor2);

  return 0;
}