What is the difference between the Iterator and the Visitor design patterns?

The Iterator and Visitor design patterns are both behavioral design patterns that aim to separate the algorithmic logic from the objects being operated upon. However, they serve different purposes and have distinct differences in their implementation and usage.

1. Purpose:
- Iterator Pattern: The Iterator pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation. It allows clients to traverse the elements of a collection in a consistent manner, regardless of the collection's specific implementation.
- Visitor Pattern: The Visitor pattern separates the algorithmic logic from the objects being operated upon. It allows adding new operations to existing object structures without modifying those structures. The Visitor pattern is useful when there are multiple unrelated operations to be performed on a set of objects, and the objects themselves should not be responsible for defining these operations.

2. Structure:
- Iterator Pattern: The Iterator pattern typically consists of an Iterator interface, which defines the methods for traversing the collection, and a ConcreteIterator class, which implements the Iterator interface and provides the actual traversal logic. The collection class itself may also implement an Iterable interface to provide a way to obtain an iterator object.
- Visitor Pattern: The Visitor pattern involves two main components: the Visitor interface, which declares the visit methods for each element type, and the ConcreteVisitor classes, which implement the visit methods for specific element types. The object structure being visited typically consists of a set of Element classes, each of which accepts a visitor object to perform the desired operation.

3. Usage:
- Iterator Pattern: The Iterator pattern is commonly used when there is a need to traverse elements of a collection in a uniform manner, regardless of the specific collection implementation. It provides a way to access elements sequentially without exposing the underlying structure, making it suitable for scenarios where the collection may change or vary in the future.
- Visitor Pattern: The Visitor pattern is useful when there are multiple unrelated operations to be performed on a set of objects, and the objects themselves should not be responsible for defining these operations. It allows adding new operations without modifying the object structure, making it suitable for scenarios where the object structure is stable but the operations may change or be extended.

4. Relationship:
- Iterator Pattern: The Iterator pattern can be used in conjunction with the Visitor pattern to traverse elements of a collection and apply different operations using visitors. The iterator provides the mechanism to access the elements, while the visitor defines the operations to be performed on those elements.
- Visitor Pattern: The Visitor pattern can be used to implement a visitor that traverses elements using an iterator. The iterator provides the means to access the elements sequentially, while the visitor defines the operations to be performed on each element.

In summary, the Iterator pattern focuses on providing a way to traverse elements of a collection, while the Visitor pattern focuses on separating the algorithmic logic from the objects being operated upon. They have different purposes, structures, and usage scenarios, but can be used together to achieve more complex operations on object structures.