Large-Scale C++ Software Design
As a member of the IC Division at Mentor Graphics Corporation, I am fortunate to have worked with many bright, talented software engineers, developing very large systems.Back in 1985, Mentor Graphics became one of the first companies to attempt a truly large project in C++. Back then no one knew how to do that, and no one could have anticipated the cost overruns, slipped schedules, huge executables, poor performance, and incredibly expensive build times that a naive approach would inevitably produce.Many valuable lessons were learned along the way - knowledge obtained through bitter experience. There were no books to help guide the design process; object-oriented designs on this scale had never before been attempted.Ten years later, with a wealth of valuable experience under its belt, Mentor Graphics has produced several large software systems written in C++, and in doing so has paved the way for others to do the same without having to pay such a high price for the privilege.During my 13 years as a C (turned C++) Computer-Aided Design (CAD) software developer, I have seen over and over again that planning ahead invariably produces a higher-quality, more maintainable product. My emphasis at Mentor Graphics has been on helping to ensure that quality is an integral part of the design process from the very start.In 1990 I developed the graduate course "Object-Oriented Design and Programming" at Columbia University. As the instructor of this course since 1991, I have had the opportunity to share many of the insights that we at Mentor Graphics gained during our industrial-strength software development efforts. Questions and feedback from literally hundreds of graduate students and professional programmers have helped me to crystallize many important concepts. This book is a direct result of that experience. To my knowledge, this is the first text that identifies development and quality issues that arise only in large C++ projects. I hope that this information will be as useful in your work as it is in mine.Audience Large-Scale C++ Software Design was written explicitly for experienced C++ software developers, system architects, and proactive quality-assurance professionals. This book is particularly appropriate for those involved in large development efforts such as databases, operating systems, compilers, and frameworks.Developing a large-scale software system in C++ requires more than just a sound understanding of the logical design issues covered in most books on C++ programming. Effective design also requires a grasp of physical design concepts that, although closely tied to the technical aspects of development, include a dimension with which even expert professional software developers may have little or no experience.Yet most of the advice presented in this book also applies to small projects. It is typical for a person to start with a small project and then begin to take on larger and more challenging enterprises. Often the scope of a particular project will expand, and what starts out as a small project becomes a major undertaking. The immediate consequences of disregarding good practice in a large project, however, are far more severe than they are for disregarding good practice in a smaller project.This book unites high-level design concepts with specific C++ programming details to satisfy two needs:An object-oriented design book geared specifically to practical aspects of the C++ programming language.  A C++ programming book describing how to use the C++ programming language to develop very large systems.Make no mistake, this is an advanced text. This is not the book from which to learn C++ syntax for the first time, nor is it likely to expose you to the dark corners of the language. Instead, this book will show you how to use the full power of the C++ language in ways that scale well to very large systems.In short, if you feel that you know C++ well, but would like to understand more about how to use the language effectively on large projects, this book is for you.Examples in this Text Most people learn by example. In general, I have supplied examples that illustrate real-world designs. I have avoided examples that illustrate one point but have blatant errors in other aspects of the design. I have also tried to avoid examples that illustrate a detail of the language but serve no other useful purpose.Except where otherwise indicated, all examples in this text are intended to represent "good design." Examples presented in earlier chapters are therefore consistent with all practices recommended throughout the book. A disadvantage of this approach is that you may see code that is written differently from the code you are used to seeing, without yet knowing exactly why. I feel that being able to use all of the examples in the book for reference compensates for this drawback.There are two notable exceptions to this practice: comments and package prefixes. Comments for many of the examples in this text have simply been omitted for lack of space. Where they are presented, they are at best minimal. Unfortunately, this is one place where the reader is asked to "do as I say, not as I do" -- at least in this book. Let the reader be assured that in practice I am scrupulous about commenting all interfaces as I write them (not after).The second exception is the inconsistent use of package prefixes in the early examples of the book. In a large project environment package prefixes are required, but they are awkward at first and take some getting used to. I have elected to omit the consistent use of registered package prefixes until after they are formally presented in Chapter 7, so as not to detract from the presentation of other important fundamental material.Many texts note that inline functions are used in examples for textual brevity when illustrating intended functionality. Since much of this book is directly related to organizational issues such as when to inline, my tendency will be to avoid inline functions in examples. If a function is declared inline, there is a justification for it beyond notational convenience.Developing large systems in C++ is a constant series of engineering trade-offs. There are almost no absolutes. It is tempting to make statements using words such as never and always. Such statements allow for a simplified presentation of the material. For the level of C++ programmers whom I expect will read this book, such sweeping statements would be challenged - and rightly so. To avoid getting side-tracked in such situations, I will state what is (almost) always true, and then provide a footnote or a pointer to the exceptional case.There are a variety of popular file name extensions used to distinguish C++ header files and C++ implementation files. For example:Header File Extensions:.h  .hxx  .H  .h++  .hh  .hppImplementation File Extensions:  .c  .cxx  .C  .c++  .cc  .cppThroughout the examples we consistently use the .h extension to identify C++ header files and the .c extension to identify C++ implementation files. In the text, we will frequently refer to header files as .h files and to implementation files as .c files. Finally, all of the examples in this text have been compiled and are syntactically correct using SUN's version of CFRONT 3.0 running on SUN SPARC stations, as well as on HP700 series machines running their native C++ compiler. Of course, any errors are the sole responsibility of the author.A Road Map There is a lot of material to cover in this book. Not all readers will have the same background. I have therefore provided some basic (but essential) material in Chapter 1 to help level the field. Expert C++ programmers may choose to skim this section or simply refer to it if needed. Chapter 2 contains a modest collection of software design rules that I would hope every experienced developer will quickly ratify.Chapter 0: Introduction. An overview of what lies in wait for the large-scale C++ software developer.PART I: BASICSChapter 1: Preliminaries. A review of basic language information, common design patterns, and style conventions used in this book.Chapter 2:  Ground Rules. Important design practices that should be followed in any C++ project.The remainder of the text is divided into two main sections. The first, entitled "Physical Design Concepts," presents a sequence of important topics related to the physical structure of large systems. The material in these chapters (3 through 7) focuses on aspects of programming that will be entirely new to many readers, and cuts right to the bone of large program design. This section is presented "bottom up," with each chapter drawing on information developed in previous chapters.PART II:  PHYSICAL DESIGN CONCEPTSChapter 3:  Components. The fundamental physical building blocks of a system.Chapter 4:  Physical Hierarchy. The importance of creating a hierarchy of components with acyclic physical dependencies for testing, maintainability, and reuse.Chapter 5:  Levelization.  Specific techniques for reducing lin