This being said, let's see why it is still useful. If you are fairly familiar with patterns, the number of books and articles that have been written about them might overwhelm you. You might have heard of a particular pattern, but when you actually need it you might have problems locating it. This is where the The Pattern Almanac helps. If you have an idea of what you are looking for and maybe can even remember the name of the pattern, you can look it up in the almanac; extract the reference to the right book, article, or URL; go to your library; and find the relevant text about the pattern. 

Specifically, you would need some of the following books: 

• Design Patterns: Elements of Reusable Object-Oriented Systems by Erich Gamma et al. (the "GoF book") 
• Pattern Languages of Program Design, Vol. 1?4
• Pattern-Oriented Software Architecture: A System of Patterns by Frank Buschmann et al. 
• Various back issues of C++ Report, JOOP, Java Report, Smalltalk Report, and Communications of the ACM
• Concurrent Programming in Java: Design Principles and Patterns by Doug Lea
• Analysis Patterns by Martin Fowler 

Still, in an almanac we would expect to find a more comprehensive collection of wisdom. For instance, there is a section about C+ idioms, most of which were described by James Coplien, and another one about Java idioms, most of which originate from Doug Lea. 

And that's it? There is more to C++ and Java. The wisdom in some areas of C++ (such as template programming, exception handling, and library usage) is still emerging and might have been published, but not necessarily in the form of patterns. The same holds true for Java. There is a pattern for expressing the immutability of objects in Java. It has not yet been published as a pattern, as far as we know, and for this reason it does not appear in this almanac. The Patterns Almanac lists those insights that have already been published in the form of a pattern and does not aim to identify new patterns where they emerge. The goal here is to provide a directory that points you to already published patterns, and nothing more. 

As a directory, it is well organized. You'll find an alphabetic list of all patterns, grouped into categories, such as behavioral, creational, structural, and C++ Idioms; Java Idioms; Smalltalk Idioms; and also categories such as client-server, concurrent systems, distributed systems, all the way up to topics such as training, testing, refactoring, design process, and writers' workshops. 

Then there is the core of the book, namely the entries, where each pattern is described very, very briefly. To give you an idea of what an entry looks like, here is the entry for the Curiously Recurring Template pattern: 

Category: C++ Idioms, Finite State Machine
Reference: [Coplien95a], C++ Report, Feb. 1995, 24?27
Description: A class is derived from a base class instantiated from a template. The derived class is passed as a parameter to the template instantiation. This pattern captures a circular dependency using inheritance in one direction and templates in the other.
Experience Report: [Piehler99] 

Without knowing what the Curiously Recurring Template pattern is, you would not understand a thing, but if you've already read about it, you'll recognize it. If you also have access to back issues of the C++ Report , you can look it up and use it. (In most cases, the reference is more readily available and does not point to back issues of magazine articles, but instead to pages in a book or to a URL.) 

Now, this pattern and its variations are used in other places as well?among them in Microsoft's ATL library?and you'll find a description of the pattern (but not in the form of a pattern) in the appendix of ATL Internals by Brent Rector, et al. You'll also find the pattern described in early C++ books such as Scientific and Engineering C++ by John Barton and LeeNackman. All of these other references would certainly aid understanding of the pattern. Unfortunately, they are not mentioned in this almanac. True, for understanding the pattern, you would not need further references, but they would illustrate the point, much like the "Experience Report" probably does. 

Depending on how you see it, The Patterns Almanac is not complete but it is certainly great help in finding the relevant information about a given pattern, provided you have access to a fairly comprehensive library of pattern books. 

Angelika Langer develops and teaches classes on Java, C++, multithreading, and internationalization. She is an internationally recognized speaker and served on the ANSI/ISO C++ Committee from 1993 to 1998. Klaus Kreft is a software architect and senior consultant with 15+ years of experience in industrial software development. He currently works for Siemens Business Services in Germany. Langer and Kreft are authors of "Standard C++ IOStreams and Locales" (Addison-Wesley, 2000) and are columnists for the C/C++ Users Journal.