analogy

The Visitor pattern is a behavioral pattern that separates an algorithm from an object structure on which it operates. The pattern allows you to add new operations or algorithms to an object structure without modifying the structure itself. The pattern achieves this by defining a separate object called a visitor, which can traverse an object structure and apply a specific operation to each element of the structure.

The main idea behind the Visitor pattern is to provide a way to separate the concerns of an algorithm from the object structure it operates on. This allows you to add new operations or algorithms to an object structure without modifying the structure itself.

Example

Here’s an example of the Visitor pattern in Python:

# Element interface
class Element:
    def accept(self, visitor):
        pass

# Concrete element classes
class ConcreteElementA(Element):
    def accept(self, visitor):
        visitor.visit_concrete_element_a(self)

    def operation_a(self):
        pass

class ConcreteElementB(Element):
    def accept(self, visitor):
        visitor.visit_concrete_element_b(self)

    def operation_b(self):
        pass

# Visitor interface
class Visitor:
    def visit_concrete_element_a(self, element):
        pass

    def visit_concrete_element_b(self, element):
        pass

# Concrete visitor class
class ConcreteVisitor(Visitor):
    def visit_concrete_element_a(self, element):
        print("ConcreteVisitor visiting ConcreteElementA.")
        element.operation_a()

    def visit_concrete_element_b(self, element):
        print("ConcreteVisitor visiting ConcreteElementB.")
        element.operation_b()

# Object structure class
class ObjectStructure:
    def __init__(self):
        self._elements = []

    def attach(self, element):
        self._elements.append(element)

    def detach(self, element):
        self._elements.remove(element)

    def accept(self, visitor):
        for element in self._elements:
            element.accept(visitor)

# Client code
def client_code():
    object_structure = ObjectStructure()
    object_structure.attach(ConcreteElementA())
    object_structure.attach(ConcreteElementB())

    visitor = ConcreteVisitor()
    object_structure.accept(visitor)

# Usage
client_code()

In this example, we have an Element interface that defines the common interface for all the concrete element classes. The Element object represents an element of the object structure that can be visited by a visitor.

The ConcreteElementA and ConcreteElementB classes are concrete element classes that implement the Element interface. Each ConcreteElement object represents an element of the object structure that can be visited by a visitor.

The Visitor interface defines the common interface for all the concrete visitor classes. The Visitor object represents an algorithm that can be applied to each element of the object structure.

The ConcreteVisitor class is a concrete visitor class that implements the Visitor interface. The ConcreteVisitor object represents a specific algorithm that can be applied to each element of the object structure.

The ObjectStructure class represents the object structure that can be visited by a visitor. The ObjectStructure object maintains a list of elements and allows visitors to visit each element in the list.

When we create an ObjectStructure object, add some ConcreteElement objects to it, create a ConcreteVisitor object, and apply the visitor to the object structure using the accept method, we get the output:

ConcreteVisitor visiting ConcreteElementA.
ConcreteVisitor visiting ConcreteElementB.

As you can see, the ConcreteVisitor object visits each ConcreteElement object in the ObjectStructure and applies the algorithm defined in the visit_concrete_element_a and visit_concrete_element_b methods. This example demonstrates how the Visitor pattern can be used to separate an algorithm from an object structure on which it operates.

Pros of the Visitor pattern

  • It allows you to add new operations to existing object structures without modifying the objects themselves, promoting the Open-Closed Principle.
  • It centralizes the operations in the visitor classes, making it easier to manage and maintain the codebase.
  • It separates the concerns of the objects and the operations, leading to better code organization and modularity.

Cons of the Visitor pattern

  • Adding new elements to the object structure requires modifying the visitor interface and all its implementations, which can be cumbersome when the structure grows.
  • It can increase the complexity of the code, especially if the object structure and the number of operations are large.
  1. Composite: The Visitor pattern is often used with the Composite pattern to apply operations to a hierarchical structure of objects.
  2. Iterator: The Visitor pattern can be combined with the Iterator pattern to traverse and perform operations on a collection of objects.
  3. Strategy: The Visitor pattern can be used in conjunction with the Strategy pattern to provide different algorithms or strategies for visiting objects.

These related patterns can be applied in conjunction with the Visitor pattern based on the specific requirements and design goals of the system.