add Visitor pattern

Author: JakubVojvoda

visitor/README.md

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+## Visitor
+
+Visitor represents an operation to be performed on the elements of an object
+structure. It lets you define a new operation without changing the classes of 
+the elements on which it operates. The pattern has behavioral purpose and applies 
+to the objects.
+
+### When to use
+
+* an object structure contains many classes of objects with differing interfaces, 
+and you want to perform operations on these objects that depend on their concrete classes
+* many distinct and unrelated operations need to be performed on objects in an object structure, 
+and you want to avoid "polluting" their classes with these operations
+* the classes defining the object structure rarely change, but you often want
+to define new operations over the structure

visitor/Visitor.cpp

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+/*
+ * 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;
+}