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Standard C++ IOStreams and Locales

Advanced Programmer's Guide and Reference

Angelika Langer & Klaus Kreft
Addison-Wesley, January 2000
ISBN 0-201-18395-1



Since 1998, the programming language C++ has been formerly specified in the form of the ISO/IEC International Standard 14882, a document that for historical reasons is often referred to as the ANSI C++ Standard . Integral to this standard is a rich set of abstractions known as the C++ Standard Library . In fact, half of the standard is devoted to the library. This book covers two major domains of the standard library: IOStreams and locales.

During the process of standardization from 1989 to 1998, the new language features and the standard library created a fair amount of interest in the C++ community. To address this need for the information, several books were published during and after standardization. Some cover standard C++ in general, typically including a brief introduction to some of the library abstractions; one textbook is devoted exclusively to the standard library. However, the only part of the library that has been discussed in depth so far is the STL, a set of collections and algorithms that was developed at Hewlett-Packard independent of the standardization effort and was later integrated into C++ standard library. While the STL is, without doubt, the most popular part of the standard library, it represents less than a third of the library as a whole (counting the pages in the standards document and considering the time that the committee spent on it), whereas IOStreams and locales form another third of the library.

When we got involved with the standardization of C++ through our professional occupations in 1993, hardly anything had been published about IOStreams, and C++ locales had not yet been invented. The only book on IOStreams was the C++ IOStreams Handbook by Steve Teale, which describes the classic, prestandard IOStreams; and there was a definite lack of information regarding the standardized IOStreams. The situation has not radically changed since then. Even now, at the time of this writing in 1999, little has been published about the standardized IOStreams, and even less about C++ locales. The few books that exist about IOStreams are out of date; they all cover the classic, pre-standard IOStreams. The C++ textbooks provide introductory information about IOStreams but rarely anything about locales. For this reason, we felt the need for a book exclusively devoted to these topics that begins where the tutorials leave off.


This book is a programmer's guide to the standard IOStreams and locales, together with a complete reference of all relevant classes, functions, templates, headers, etc. It is neither a tutorial nor a textbook. It does not aim to teach the reader C++ or the basics of IOStreams. We expect of the readers that they know, at least roughly, what happens when they type a line of C++ code such as
cout << "Hello" << endl;

Hence this book is not for absolute beginners, but rather for C++ programmers who have been studying a C++ textbook, or have comparable practical experience, and who intend to use IOStreams and locales in more than a casual way.

As locales are an abstraction that is new to C++, as opposed to IOStreams, which has been around for more than a decade, we cover locales from the ground up. Some knowledge of locales in C will aid understanding, but it is not required. We do not aim to cover internationalization in a comprehensive way. Internationalization is too broad a topic, and an adequate discussion of it would fill another whole book. However, IOStreams and locales are closely related, and for this reason the book explains the concept of C++ locales, with emphases on usage of locales in conjunction with IOStreams.

Regarding IOStreams, we acknowledge the fact that the classic IOStreams library has been in existence since the early days of C++. We assume that readers are familiar with the basic features as they are explained in every C++ textbook. Instead of repeating the basics, we aim to go beyond that introductory level. For instance, we demonstrate advanced features--such as user-defined shift operators and manipulators, extending streams by use of iword/pword, and derivation of new stream and stream buffer classes--as well as less ambitious topics like format control and error handling.

Overall, the goal of this book is to provide as much information about the general principles as is needed to enable readers to accomplish their concrete programming tasks using IOStreams and locales. The focus is on the underlying concepts and the more advanced programming techniques that IOStreams and locales support, rather than on the details of each and every interface. For this reason we refrain from presenting extensive and lengthy case studies and code examples. IOStreams and locales are general purpose tools and can be used to solve a sheer abundance of problems. It would have been impossible to find a representative and comprehensive set of case studies. Instead, we concentrate on a few condensed examples that we use to explain programming techniques and concepts, and we deliberately refrain from blowing them up to full applications in order to avoid unnecessary distractions. We trust that readers will be astute enough to figure out concrete applications once the principles are clear.


We have received a considerable number of queries such as, "Why is this and that so and so?" seeking an explanation of why IOStreams and locales are designed the way they are. Where we know of an underlying rationale, we explain it. Yet there are inconsistencies and "interesting" design decisions that can be explained only by "historical reasons" or "design by committee." Where we feel that certain features introduce potential pitfalls, we point them out, so that the reader can avoid them. Beyond that, we neither aim to defend the standard nor intend to discuss alternative designs. We describe it as it is.


Writing this book took us more than three years, and during this long period many people helped us to endure and finish the task. As with any book, the authors are only part of the story, and we would like to thank all those people who contributed in one way or another.

At Addison-Wesley, we would like to thank Mike Hendrickson, Deborah Lafferty, and in particular Marina Lang; they believed in the value of this book and accompanied us through the entire process from proposal to print.

We would like to thank all those knowledgeable and patient people at the standards committee and elsewhere who answered our countless and sometimes stupid questions: Nathan Myers, who invented the C++ locales and proposed them to the standards com-mittee, told us everything about locales and helped us understand his proposal as well as any resulting discussions. Jerry Schwarz, who is the "father of IOStreams," that is, the author of the first version of IOStreams (or "streams" as they were called in C++ 1.0), gave us invaluable insights into the intent of many of the IOStreams features, and we thank him for his patience and support. Bill Plauger, author of the Microsoft version of the C++ standard library, helped us find bugs in the implementation resulting from misunder-standings on our side; he was also invaluable in helping us understand and interpret the standard correctly. Philippe LeMouel, a former colleague at RogueWaveSoftware, imple-mented IOStreams there and explained his implementation to us. Joe Delaney worked on RogueWave's implementation of locales. Dietmar Kuehl, worked on the implementation of IOStreams and locales for the gnu compiler. John Spicer of EdisonDesignGroup and Erwin Unruh of Siemens answered questions regarding templates and other language features. Beman Dawes, who maintains the library issue list for the standards committee helped clarify countless open issues in the standard.

Thanks also to our reviewers, some of whom spent a considerable amount of effort and time compiling thorough and helpful comments: (in alphabetical order) Chuck Allison, Stephen Clamage, Mary Dageforde, Amelia Lewis, Stan Lippman, Dietmar Kuehl, Werner Mossner, and Patrick Thompson, as well as others who preferred to stay anonymous.
--Angelika Langer
--Klaus Kreft

I would like to thank Bernd Eggink, author of a book about the classic IOStreams written in German. Our email correspondence about IOStreams spawned the idea of a joint book project on the standard IOStreams. The original idea had been to translate his book into English and upgrade to the standard, but his sudden, serious illness thwarted our plans.

I would like to thank Thomas Keffer, the founder of RogueWaveSoftware, for coming up with the idea of writing a book of my own and for supporting and encouraging me ever since. I had been working at his company when he suggested the book project. I was a German alien working at a US corporation when he proposed that I write a book about internationalization in C++. I would like to thank Roland Hartinger, my former supervisor and head of the C++ compiler construction group at Siemens Nixdorf, who threw me into the library project and encouraged me to join the standards committee.

Last, but not least, I thank Klaus Kreft for joining me on this project. Without him this book would have never been finished, and not much is worth doing without him.
--Angelike Langer

First of all, I would like to thank my parents for recognizing and fostering my interest and talent in mathematics and natural sciences. Without their support I would not be who I am. Next I thank two individuals whom I met through my professional work who were true sources of inspiration and insight: Gerhard Draxler, whom I miss tremendously since he retired from his professional life, and Werner Mossner, with whom I had my first contact with IOStreams. Together we implemented a logging facility by derivation from the IOStreams classes.

Finally, I thank Angelika Langer, who is a constant source of ideas and an overwhelmingly persistent workers. Without her effort this book would not have been finished, yet the significance of her contribution is nothing compared to what she means me to me in private life.
--Klaus Kreft

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