Visitor Pattern

Purpose

• Allowing one or more operations to be applied to a set of objects at runtime
• Decoupling the operations from the object structure(=the set of objests)

 

Use when

• An object structure must have many unrelated operations performed upon it.
• The object structure can't cahnge but operations on it can
• Operations must be performed on the concrete classes of an object structure
• Operations should be albe to operate on multiple object structures that implement the same interface sets.

 

More operation types are expected

• There will be one class for every single thing that needs to be performed.
• Therefore, distrubuting operations among various node classes is hard to understand, maintain and change

 

Motivations and Benefits

• Motivations
  - An obeject structure contains many classes of objects
  - Many distinct and unrelated operations on these obejcts
  - Classes defining the object structure rarely change, but operations chage frequently
• Benefits
  - Makes adding new operations easy.
  - Gathers related operations and separates unrelated operations
  - Visitors can visit objects that don't have a common parent class.

• Compare the normalized pattern above with the Trash module below.
• It is used when the characteristics of the operation in the existing class and the newly added operation are different.

• A client must create a ConcreteVisitor object and then traverse the object structre, visiting each element with the visiotor   
• When the elemment is visited, it calls the Visitor operation that corresponds to its class.
• The element supplies itself as an argument to this operation to let the visitor acccess its state, if necessary

 

Method Overloading

• Method overloading, however, is done at compile time.
  - using "name mangling" where the internal name of the method has the argument's type encoded in it
  - Different from method overriding (run-time polymorphism)

 

Technical Details

• Double Dispatch- Speical form of multiple dispatch
  - a mechanism that dispatches a method call to different concrete methods depending on the runtime types of two objects involved in the call.
• Single Dispatch
  - In most object-oriented systems, the concrete method that is called from a method call in the code depends on the dynamic type of a single object and therefore they are known as single dispatch calls or simply virtual function(method) calls.

 

Double Dispatch

• Double Dispatch- Speical form of multiple dispatch
  - a mechanism that dispatches a method call to different concrete methods depending on the runtime types of two objects involved in the call.
• Single Dispatch
  - In most object-oriented systems, the concrete method that is called from a method call in the code depends on the dynamic type of a single object and therefore they are known as single dispatch calls or simply virtual function(method) calls.

 

Visitor Pattern and Double Dispatch

• Visitor pattern requires a programming language that supports double dispatch.
• An element ahs an accept method that can take the visitor as an argument
• The accept method calls a visit method of the visitor. The element passes itself as an argument to the visit method

 

Benefits

• Vistor makes adding new operations easy
• A visitor gathers related operations and separates unrelated ones

 

Liabilities

• Adding new ConcreteElement classes is hard
  - Each new ConcreteElement gives rise to a new abstract operation on Visitor and a corresponding implementation in every ConcreteVisitor class
• Breaking encapsulation
  - The pattern often forces you to provide public operations that access an element's internal state, which may compromise its encapsulation