Home > Articles > Programming > C/C++

The Standard Containers as Class Templates

  • Print
  • + Share This
Why do generic classes make excellent containers? This excerpt from C++ by Example (Que, 2001) by Steve Donovan shows some of the consequences of not having them.
This article is excerpted from C++ by Example (Que, 2001, ISBN: 0789726769), by Steve Donovan.
This chapter is from the book

How were things handled before templates were available? The first problem with creating containers without templates is that they are not type safe. Imagine if you had only list<void *>; any pointer could be put in such a list. You would need to write code like this:

typedef list<void *> List; // the one & only list...

void draw_shapes(TG& tg, const List& ls)
{
  List::iterator li;
  for(li = ls.begin(); li != ls.end(); ++li)
   static_cast<Shape *>(*li)->draw(tg);
}

This looks nasty, and it is nasty. There is no guarantee at runtime that this list contains only pointers to Shape. A program would inevitably decide to put something odd in the list when it was being used for some crucial operation on the other side of the planet (or, indeed, another planet altogether). It is not possible to check all the possibilities in even a moderate-sized application. Type safety is a guarantee that no surprises with odd types can happen—that they will be caught by the compiler, not the customer.

Traditional object-oriented languages rely on runtime-type information. Every class derives from some polymorphic base class, usually called Object. In Delphi, for instance, deriving from Object is implicit, so the is operator can always work. Of course, this strategy works in C++ as well, as long as the ultimate base class Object has at least one virtual method. The class Shape will have to be derived from Object in some way. Then dynamic_cast() will work. The previous example becomes this:

typedef list<Object *> List; // the one & only list...

void draw_shapes(TG& tg, const List& ls)
{
  List::iterator li;
  for(li = ls.begin(); li != ls.end(); ++li) {
  Shape *ps = dynamic_cast<Shape *>(*li)
  if (ps != NULL) ps->draw(tg);
  }
}

There is now a proper runtime guarantee, at the cost of continuous checking. But what should you do if the object isn't a Shape pointer? Surely you should raise an alarm or make a note somewhere. Although this code is safe, it could be masking an error somewhere else. There should only be Shape pointers in this list; it isn't considered particularly clever to keep different kinds of objects together.

NOTE

Java tends to do things like this. Because there are no parameterized types; typically, code needs plenty of typecasts, which are all dynamic.

So far, we have only talked about pointers. Languages such as Java and Delphi really deal only with references to objects, but they also have to deal with the common case of just wanting a list of integers or doubles. You can either get a whole number of extra container classes or wrap the simple types themselves as objects, which is what Java does (Delphi programmers have a horrible habit of trying to stuff integers into lists of pointers). This is the second problem related to life without templates.

C++ must deal with value-oriented types such as std::string. These types do not fit very well into a pointer-list scheme. There is an object-oriented approach to containers of such types, but it isn't pretty. You define a special base class—which you could call Linkable—that contains a pointer to another Linkable object. Any class that you inherit from Linkable therefore has the ability to link to another Linkable object, rather like the parade of circus elephants holding the tails of their fellows. You can then run through the list by following pointers, until some stopping condition (in the case of a circular list, the elephants keep going around the ring). But this is awkward; it means that you have to decide, as part of your design, that your objects are going to be kept in a list. There would need to be auxiliary types such as linkable strings.

The design of the C++ standard containers answers these two problems, as well as another: C++ standard containers present a very similar interface to the world. With some care, code can switch from using list<T> to vector<T> by changing a single typedef.1 Say you have found that a time-consuming task is iterating through containers; in this case, switching to a vector makes perfect sense. The philosophy is that whether you put widgets in a list, vector, deque, map, or set, it should not be a high-level design decision.

Note an important property of class templates; not every method is compiled when a template class is instantiated; only methods that are referenced in the code, or are needed to implement those methods, are compiled. This can obviously save a lot of dead code (although smart linkers can strip out such baggage). But this is crucial to the design of classes like list<T>. For example, the standard list has a method remove() that is given a specific element value to take out, but not every type has defined what it means to be equal. Similarly, the method sort() assumes that e1 < e2 makes sense. So instantiating only code that is directly needed makes it possible to generate lists of simple structs, which don't define equality or ordering.

The downside of using template containers is that a program might end up containing many different instances of a particular template type. It is common to keep lists of many different kinds of objects, and it is unfortunate if the program has to keep that many (practically identical) copies of the list code. This is often solved by specialization: You can define list<T *> in terms of list<void *>. The specialized template for pointer types becomes a thin "type-safe" wrapper around list<void *>. The code would look something like this:

typedef list<void *> ListP;

template <class T>
 class list<T *>: private ListP {
 ...
 void push_back(T *p)
 { ListP::push_back((void *)p); }

 T * back()    
 { return (T *) ListP::back(); }
 ...
 };

The basic pointer list type ListP is used as a private base class because list<T *> redefines every public member anyway—it is pure "implementation inheritance." This is not the most exciting code to write, but such specializations are fast because they are so easy to inline.

  • + Share This
  • 🔖 Save To Your Account

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.

Newsletters

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.

Cookies and Related Technologies

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.

Links


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.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020