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Visitor

Visitor in C++

Visitor is a behavioral design pattern that allows adding new behaviors to existing class hierarchy without altering any existing code.

Read why Visitors can’t be simply replaced with method overloading in our article Visitor and Double Dispatch.

Usage of the pattern in C++

Complexity:

Popularity:

Usage examples: Visitor isn’t a very common pattern because of its complexity and narrow applicability.

Conceptual Example

This example illustrates the structure of the Visitor design pattern. It focuses on answering these questions:

  • What classes does it consist of?
  • What roles do these classes play?
  • In what way the elements of the pattern are related?

main.cc: Conceptual example

/**
 * The Visitor Interface declares a set of visiting methods that correspond to
 * component classes. The signature of a visiting method allows the visitor to
 * identify the exact class of the component that it's dealing with.
 */
class ConcreteComponentA;
class ConcreteComponentB;

class Visitor {
 public:
  virtual void VisitConcreteComponentA(const ConcreteComponentA *element) const = 0;
  virtual void VisitConcreteComponentB(const ConcreteComponentB *element) const = 0;
};

/**
 * The Component interface declares an `accept` method that should take the base
 * visitor interface as an argument.
 */

class Component {
 public:
  virtual ~Component() {}
  virtual void Accept(Visitor *visitor) const = 0;
};

/**
 * Each Concrete Component must implement the `Accept` method in such a way that
 * it calls the visitor's method corresponding to the component's class.
 */
class ConcreteComponentA : public Component {
  /**
   * Note that we're calling `visitConcreteComponentA`, which matches the
   * current class name. This way we let the visitor know the class of the
   * component it works with.
   */
 public:
  void Accept(Visitor *visitor) const override {
    visitor->VisitConcreteComponentA(this);
  }
  /**
   * Concrete Components may have special methods that don't exist in their base
   * class or interface. The Visitor is still able to use these methods since
   * it's aware of the component's concrete class.
   */
  std::string ExclusiveMethodOfConcreteComponentA() const {
    return "A";
  }
};

class ConcreteComponentB : public Component {
  /**
   * Same here: visitConcreteComponentB => ConcreteComponentB
   */
 public:
  void Accept(Visitor *visitor) const override {
    visitor->VisitConcreteComponentB(this);
  }
  std::string SpecialMethodOfConcreteComponentB() const {
    return "B";
  }
};

/**
 * Concrete Visitors implement several versions of the same algorithm, which can
 * work with all concrete component classes.
 *
 * You can experience the biggest benefit of the Visitor pattern when using it
 * with a complex object structure, such as a Composite tree. In this case, it
 * might be helpful to store some intermediate state of the algorithm while
 * executing visitor's methods over various objects of the structure.
 */
class ConcreteVisitor1 : public Visitor {
 public:
  void VisitConcreteComponentA(const ConcreteComponentA *element) const override {
    std::cout << element->ExclusiveMethodOfConcreteComponentA() << " + ConcreteVisitor1\n";
  }

  void VisitConcreteComponentB(const ConcreteComponentB *element) const override {
    std::cout << element->SpecialMethodOfConcreteComponentB() << " + ConcreteVisitor1\n";
  }
};

class ConcreteVisitor2 : public Visitor {
 public:
  void VisitConcreteComponentA(const ConcreteComponentA *element) const override {
    std::cout << element->ExclusiveMethodOfConcreteComponentA() << " + ConcreteVisitor2\n";
  }
  void VisitConcreteComponentB(const ConcreteComponentB *element) const override {
    std::cout << element->SpecialMethodOfConcreteComponentB() << " + ConcreteVisitor2\n";
  }
};
/**
 * The client code can run visitor operations over any set of elements without
 * figuring out their concrete classes. The accept operation directs a call to
 * the appropriate operation in the visitor object.
 */
void ClientCode(std::array<const Component *, 2> components, Visitor *visitor) {
  // ...
  for (const Component *comp : components) {
    comp->Accept(visitor);
  }
  // ...
}

int main() {
  std::array<const Component *, 2> components = {new ConcreteComponentA, new ConcreteComponentB};
  std::cout << "The client code works with all visitors via the base Visitor interface:\n";
  ConcreteVisitor1 *visitor1 = new ConcreteVisitor1;
  ClientCode(components, visitor1);
  std::cout << "\n";
  std::cout << "It allows the same client code to work with different types of visitors:\n";
  ConcreteVisitor2 *visitor2 = new ConcreteVisitor2;
  ClientCode(components, visitor2);

  for (const Component *comp : components) {
    delete comp;
  }
  delete visitor1;
  delete visitor2;

  return 0;
}

Output.txt: Execution result

The client code works with all visitors via the base Visitor interface:
A + ConcreteVisitor1
B + ConcreteVisitor1

It allows the same client code to work with different types of visitors:
A + ConcreteVisitor2
B + ConcreteVisitor2

Visitor in Other Languages

Design Patterns: Visitor in Java Design Patterns: Visitor in C# Design Patterns: Visitor in PHP Design Patterns: Visitor in Python Design Patterns: Visitor in Ruby Design Patterns: Visitor in Swift Design Patterns: Visitor in TypeScript Design Patterns: Visitor in Go