The Visitor Design Pattern is a behavioral design pattern that allows you to add new operations to classes of objects without modifying their structure. Instead, the new behavior is implemented in a separate object, called a visitor, that is passed to the objects it needs to operate on.

Intent

• Separate an algorithm from the objects on which it operates.
• Add new operations to existing object structures without altering the classes.
• Simplify extending operations without polluting the existing class hierarchy.

Key Components

1. Visitor Interface:
   • Declares operations to be performed on elements of an object structure. One method per concrete element class.
2. Concrete Visitors:
   • Implement the visitor interface and define specific behaviors for each type of element.
3. Element Interface:
   • Declares a method for accepting a visitor. This method typically takes a visitor as an argument.
4. Concrete Elements:
   • Implement the element interface and provide the accept method that calls the appropriate method on the visitor.
5. Object Structure:
   • A collection or hierarchy of elements that can accept visitors.

When to Use

• When you need to perform many unrelated operations on an object structure.
• When extending object structures is easier than modifying them.
• To maintain the Open/Closed Principle: You can add new behaviors without changing existing classes.