# Design Pattern Variations: A Better Visitor

Radu Braniste critiques the Visitor Pattern (VP), reiterating its disadvantages and questioning its value as a pattern in a very exhaustive manner. As usual, this kind of thorough analysis proves to be a fertile ground for new ideas: This article presents a couple of variations of the pattern responding systematically to all the major shortcomings of VP.

The starting point of this article was an extremely interesting critique [1] of the Visitor Pattern (VP), reiterating its disadvantages and questioning its value as a pattern in a very exhaustive manner. As usual, this kind of thorough analysis proves to be a fertile ground for new ideas—this article will present a couple of variations of the pattern responding systematically to all the major shortcomings of VP.

## A Problem Visitor

The following is a short description of the original Visitor Pattern (VP), emphasizing its mechanics and the way in which it affects its perceived usability. We will use the example provided in the aforementioned article to illustrate.

VP is often defined as "a way of separating an algorithm from an object structure upon which it operates." This description implies the existence of three main collaborating parts:

• An algorithm (ALG)
• A structure of objects (SO)
• A way of traversing 2 to apply 1 (TRAV)[1]

The alert reader will observe immediately a similitude with STL and the way it separates data from algorithms. The obvious benefit is that we can freely vary the algorithms working on top of the same data. The difference is that SO is a structure of unrelated objects[2] that can be independently inspected during traversal, whereas in STL we deal with collections of homogeneous components.

Let's consider the following standard example ([1]):

```class Hammer;
class Drill;

class Visitor
{
public:
void visit(Hammer & h) = 0;
void visit(Drill & d) = 0;
};

// root of the given hierarchy
class Tool
{
public:
virtual void accept(Visitor & v) = 0;

// regular operations of Tool omitted
};

class Hammer : public Tool
{
public:
virtual void accept(Visitor & v) { v.visit(*this); }

// regular operations of Hammer omitted
};

class Drill : public Tool
{
public:
virtual void accept(Visitor & v) { v.visit(*this); }

// regular operations of Drill omitted
};

class DoSomethingVisitor : public Visitor
{
public:
void visit(Hammer & h)
{
// do something with the hammer
}

void visit(Drill & d)
{
// do something with the drill
}

};

vector<Tool *> myToolBox; // filled with lots of tools

void doSomethingWithAllTools()
{
DoSomethingVisitor v;

for (size_t i = 0; i != myToolBox.size(); ++i)
{
Tool & t = *(myToolBox[i]);
t.accept(v);
}
}```

Observations:

• Visitor is the family of algorithms ALG that can be applied to an unrelated structure of objects SO(Hammer, Saw)
• In order to traverse SO, VP requires all elements of SO to implement an artificial interface—an acceptor (Tool, in this case). This allows SO to behave polymorphically and be iterated (vector<Tool *> myToolBox). This interface is not necessary otherwise, and its implementation is mechanical.

• VP is intrusive—Hammer, Drill cannot participate in VP without implementing the Tool interface or, more generally, an arbitrary class is not a candidate for VP without having implemented an acceptor.
• VP breaks the Open-Closed Principle (Software entities should be open for extension but closed for modification). We cannot add another tool (Saw, for example) without breaking the existing Visitor interface.
• SO and ALG exist separately (Visitor::drill is not part of the Drill class as sound OO design recommends). This is not always a liability as proved by [2].
• ALG can access only SO public interfaces and doesn't have access to its internals—contrast this with ALG being part of SO (a Drill::drill has access to all Drill's internals). This comes against VP's raison d’être and is considered out of scope for this discussion.

D1 and D2 are major shortcomings, whereas D3 and D4 come at odds with the VP declared intent of adding new algorithms to existing classes, but should still be mentioned because they resonate with a more purist OO point of view.

### InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

## Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

## Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

### Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

### Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

### Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

### Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

### Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

### Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

## Other Collection and Use of Information

### Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

### Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

### Do Not Track

This site currently does not respond to Do Not Track signals.

## Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

## Children

This site is not directed to children under the age of 13.

## Marketing

Pearson may send or direct marketing communications to users, provided that

• Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
• Such marketing is consistent with applicable law and Pearson's legal obligations.
• Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
• Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

## Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

## Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

## Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

## Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

## Sharing and Disclosure

Pearson may disclose personal information, as follows:

• As required by law.
• With the consent of the individual (or their parent, if the individual is a minor)
• In response to a subpoena, court order or legal process, to the extent permitted or required by law
• To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
• In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
• To investigate or address actual or suspected fraud or other illegal activities
• To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
• To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
• To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.