Visitor pattern's purpose with examples

Question

I'm really confused about the visitor pattern and its uses. I can't really seem to visualize the benefits of using this pattern or its purpose. If someone could explain with examples if possible that would be great.

Answer 1

So you've probably read a bajillion different explanations of the visitor pattern, and you're probably still saying "but when would you use it!"

Traditionally, visitors are used to implement type-testing without sacrificing type-safety, so long as your types are well-defined up front and known in advance. Let's say we have a few classes as follows:

abstract class Fruit { }
class Orange : Fruit { }
class Apple : Fruit { }
class Banana : Fruit { }

And let's say we create a Fruit[]:

var fruits = new Fruit[]
    { new Orange(), new Apple(), new Banana(),
      new Banana(), new Banana(), new Orange() };

I want to partition the list in to three lists, each containing oranges, apples, or bananas. How would you do it? Well, the easy solution would be a type-test:

List<Orange> oranges = new List<Orange>();
List<Apple> apples = new List<Apple>();
List<Banana> bananas = new List<Banana>();
foreach (Fruit fruit in fruits)
{
    if (fruit is Orange)
        oranges.Add((Orange)fruit);
    else if (fruit is Apple)
        apples.Add((Apple)fruit);
    else if (fruit is Banana)
        bananas.Add((Banana)fruit);
}

It works, but there are lots of problems with this code:

• For a start, its ugly.
• Its not type-safe, we won't catch type errors until runtime.
• Its not maintainable. If we add a new derived instance of Fruit, we need to do a global search for every place which performs a fruit type-test, otherwise we might miss types.

Visitor pattern solves the problem elegantly. Start by modifying our base Fruit class:

interface IFruitVisitor
{
    void Visit(Orange fruit);
    void Visit(Apple fruit);
    void Visit(Banana fruit);
}

abstract class Fruit { public abstract void Accept(IFruitVisitor visitor); }
class Orange : Fruit { public override void Accept(IFruitVisitor visitor) { visitor.Visit(this); } }
class Apple : Fruit { public override void Accept(IFruitVisitor visitor) { visitor.Visit(this); } }
class Banana : Fruit { public override void Accept(IFruitVisitor visitor) { visitor.Visit(this); } }

It looks like we're copy pasting code, but note the derived classes are all calling different overloads (the Apple calls Visit(Apple), the Banana calls Visit(Banana), and so on).

Implement the visitor:

class FruitPartitioner : IFruitVisitor
{
    public List<Orange> Oranges { get; private set; }
    public List<Apple> Apples { get; private set; }
    public List<Banana> Bananas { get; private set; }

    public FruitPartitioner()
    {
        Oranges = new List<Orange>();
        Apples = new List<Apple>();
        Bananas = new List<Banana>();
    }

    public void Visit(Orange fruit) { Oranges.Add(fruit); }
    public void Visit(Apple fruit) { Apples.Add(fruit); }
    public void Visit(Banana fruit) { Bananas.Add(fruit); }
}

Now you can partition your fruits without a type-test:

FruitPartitioner partitioner = new FruitPartitioner();
foreach (Fruit fruit in fruits)
{
    fruit.Accept(partitioner);
}
Console.WriteLine("Oranges.Count: {0}", partitioner.Oranges.Count);
Console.WriteLine("Apples.Count: {0}", partitioner.Apples.Count);
Console.WriteLine("Bananas.Count: {0}", partitioner.Bananas.Count);

This has the advantages of:

• Being relatively clean, easy to read code.
• Type-safety, type errors are caught at compile time.
• Maintainability. If I add or remove a concrete Fruit class, I could modify my IFruitVisitor interface to handle the type accordingly, and the compiler will immediately find all places where we implement the interface so we can make the appropriate modifications.

---

With that said, visitors are usually overkill, and they have a tendency to grossly complicate APIs, and it can be very cumbersome to define a new visitor for every new kind of behavior.

Usually, simpler patterns like inheritance should be used in place of visitors. For example, in principle I could write a class like:

class FruitPricer : IFruitVisitor
{
    public double Price { get; private set; }
    public void Visit(Orange fruit) { Price = 0.69; }
    public void Visit(Apple fruit) { Price = 0.89; }
    public void Visit(Banana fruit) { Price = 1.11; }
}

It works, but what's the advantage over this trivial modification:

abstract class Fruit
{
    public abstract void Accept(IFruitVisitor visitor);
    public abstract double Price { get; }
}

So, you should use visitors when the following conditions hold:

• You have a well-defined, known set of classes which will be visited.

• Operations on said classes are not well-defined or known in advance. For example, if someone is consuming your API and you want to give consumers a way to add new ad-hoc functionality to objects. They're also a convenient way to extend sealed classes with ad-hoc functionaity.

• You perform operations of a class of objects and want to avoid run-time type testing. This is usually the case when you traverse a hierarchy of disparate objects having different properties.

Don't use visitors when:

• You support operations on a class of objects whose derived types are not known in advance.

• Operations on objects are well-defined in advance, particularly if they can be inherited from a base class or defined in an interface.

• Its easier for clients to add new functionality to classes using inheritance.

• You are traversing a hierarchy of objects which have the same properties or interface.

• You want a relatively simple API.