Home > Articles > Programming > C/C++

Generic Programming and the C++ Standard Library

One of C++'s most powerful features is its support for generic programming. In this sample chapter, Herb Sutter shows some of the power and flexibility of the C++ standard library.
Herb Sutter is the author of Exceptional C++ (Addison-Wesley, 1999, ISBN 0-201-61562-2) and More Exceptional C++ (Addison-Wesley, 2001, ISBN 0-201-70434-X).
This chapter is from the book

One of C++'s most powerful features is its support for generic programming. This power is reflected directly in the flexibility of the C++ standard library, especially in its containers, iterators, and algorithms portion, originally known as the standard template library (STL).

This opening section focuses on how to make the best use of the C++ standard library, particularly the STL. When and how can you make best use of std::vector and std::deque? What pitfalls might you encounter when using std::map and std::set, and how can you safely avoid them? Why doesn't std::remove() actually remove anything?

This section also highlights some useful techniques, as well as pitfalls, that occur when writing generic code of your own, including code that's meant to work with and extend the STL. What kinds of predicates are safe to use with the STL? What kinds aren't, and why? What techniques are available for writing powerful generic template code that can change its own behavior based on the capabilities of the types it's given to work with? How can you switch easily between different kinds of input and output streams? How does template specialization and overloading work? And what's with this funny typename keyword, anyway?

This and more, as we delve into topics related to generic programming and the C++ standard library.

Item 1: Switching Streams (Difficulty: 2)

What's the best way to dynamically use different stream sources and targets, including the standard console streams and files?

  1. What are the types of std::cin and std::cout?

  2. Write an ECHO program that simply echoes its input and that can be invoked equivalently in the two following ways:

ECHO <infile >outfile

ECHO infile outfile

In most popular command-line environments, the first command assumes that the program takes input from cin and sends output to cout. The second command tells the program to take its input from the file named infile and to produce output in the file named outfile. The program should be able to support all of the above input/output options.


  1. What are the types of std::cin and std::cout?

    The short answer is that cin boils down to this:

    std::basic_istream<char, std::char_traits<char> >

    and cout boils down to this:

    std::basic_ostream<char, std::char_traits<char> >

    The longer answer shows the connection by following some standard typedefs and templates. First, cin and cout have type std::istream and std::ostream, respectively. In turn, those are typdef'd as std::basic_istream<char> and std::basic_ostream<char>. Finally, after accounting for the default template arguments, we get the above.


    If you are using a pre-standard implementation of the iostreams subsystem, you might still see intermediate classes, such as istream_with_assign. Those classes do not appear in the standard.

  2. Write an ECHO program that simply echoes its input and that can be invoked equivalently in the two following ways:

    ECHO <infile >outfile

    ECHO infile outfile

The Tersest Solution

For those who like terse code, the tersest solution is a program containing just a single statement:

// Example 1-1: A one-statement wonder
#include <fstream>
#include <iostream>

int main( int argc, char* argv[] )
 using namespace std;

 (argc > 2
  ? ofstream(argv[2], ios::out | ios::binary)
  : cout)
 (argc > 1
  ? ifstream(argv[1], ios::in | ios::binary)
  : cin)

This works because of two cooperating facilities: First, basic_ios provides a convenient rdbuf() member function that returns the streambuf used inside a given stream object, in this case either cin or a temporary ifstream, both of which are derived from basic_ios. Second, basic_ostream provides an operator<<() that accepts just such a basic_streambuf object as its input, which it then happily reads to exhaustion. As the French would say, "C'est ça" ("and that's it").

Toward More-Flexible Solutions

The approach in Example 1-1 has two major drawbacks: First, the terseness is borderline, and extreme terseness is not suitable for production code.


Prefer readability. Avoid writing terse code (brief, but difficult to understand and maintain). Eschew obfuscation.

Second, although Example 1-1 answers the immediate question, it's only good when you want to copy the input verbatim. That may be enough today, but what if tomorrow you need to do other processing on the input, such as converting it to uppercase or calculating a total or removing every third character? That may well be a reasonable thing to want to do in the future, so it would be better right now to encapsulate the processing work in a separate function that can use the right kind of input or output object polymorphically:

#include <fstream>
#include <iostream>

 int main( int argc, char* argv[] )
  using namespace std;

  fstream in, out;
  if( argc > 1 ) in.open ( argv[1], ios::in | ios::binary );
  if( argc > 2 ) out.open( argv[2], ios::out | ios::binary );

  Process( in.is_open() ? in : cin,
      out.is_open() ? out : cout );

But how do we implement Process()? In C++, there are four major ways to get polymorphic behavior: virtual functions, templates, overloading, and conversions. The first two methods are directly applicable here to express the kind of polymorphism we need.

Method A: Templates (Compile-Time Polymorphism)

The first way is to use compile-time polymorphism using templates, which merely requires the passed objects to have a suitable interface (such as a member function named rdbuf()):

// Example 1-2(a): A templatized Process()
template<typename In, typename Out>
void Process( In& in, Out& out )
 // ... do something more sophisticated,
 //   or just plain "out << in.rdbuf();"...
Method B: Virtual Functions (Run-Time Polymorphism)

The second way is to use run-time polymorphism, which makes use of the fact that there is a common base class with a suitable interface:

// Example 1-2(b): First attempt, sort of okay
void Process( basic_istream<char>& in,
       basic_ostream<char>& out )
 // ... do something more sophisticated,
 //   or just plain "out << in.rdbuf();"...

Note that in Example 1-2(b), the parameters to Process() are not of type basic_ios<char>& because that wouldn't permit the use of operator<<().

Of course, the approach in Example 1-2(b) depends on the input and output streams being derived from basic_istream<char> and basic_ostream<char>. That happens to be good enough for our example, but not all streams are based on plain chars or even on char_traits<char>. For example, wide character streams are based on wchar_t, and Exceptional C++ Items 2 and 3 showed the potential usefulness of user-defined traits with different behavior (in those cases, ci_char_traits provided case insensitivity).

So even Method B ought to use templates and let the compiler deduce the arguments appropriately:

// Example 1-2(c): Better solution
template<typename C = char, typename T = char_traits<C> >
void Process( basic_istream<C,T>& in,
       basic_ostream<C,T>& out )
 // ... do something more sophisticated,
 //   or just plain "out << in.rdbuf();"...
Sound Engineering Principles

All of these answers are "right" as far as they go, but in this situation I personally tend to prefer Method A. This is because of two valuable guidelines. The first is this:


Prefer extensibility.

Avoid writing code that solves only the immediate problem. Writing an extensible solution is almost always better—as long as we don't go overboard, of course.

Balanced judgment is one hallmark of the experienced programmer. In particular, experienced programmers understand how to strike the right balance between writing special-purpose code that solves only the immediate problem (shortsighted, hard to extend) and writing a grandiose general framework to solve what should be a simple problem (rabid overdesign).

Compared with the approach in Example 1-1, Method A has about the same overall complexity but it's easier to understand and more extensible, to boot. Compared with Method B, Method A is at once simpler and more flexible; it is more adaptable to new situations because it avoids being hardwired to work with the iostreams hierarchy only.

So if two options require about the same effort to design and implement and are about equally clear and maintainable, prefer extensibility. This advice is not intended as an open license to go overboard and overdesign what ought to be a simple system; we all do that too much already. This advice is, however, encouragement to do more than just solve the immediate problem, when a little thought lets you discover that the problem you're solving is a special case of a more general problem. This is especially true because designing for extensibility often implicitly means designing for encapsulation.


Prefer encapsulation. Separate concerns.

As far as possible, one piece of code—function or class—should know about and be responsible for one thing.

Arguably best of all, Method A exhibits good separation of concerns. The code that knows about the possible differences in input/output sources and sinks is separated from the code that knows how to actually do the work. This separation also makes the intent of the code clearer, easier for a human to read and digest. Good separation of concerns is a second hallmark of sound engineering, and one we'll see time and again in these Items.

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.


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.


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.

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.


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


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


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.


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.

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