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I have rarely invested as much energy in any endeavor as in naming this book. As the manuscript evolved, its title evolved to emphasize one or two concepts at any given time from the set of basic elements Domain, Engineering, Multi-Paradigm, Analysis, Design, Programming, and C++. The publisher was afraid that the unfamiliar term "domain engineering" would fail to engage the target market. One of the reviewers, Tim Budd, was concerned about confusion between his use of "multi-paradigm" and the way the term is used in this book. I was concerned about using terms such as "analysis" because of my desire to put the book into the hands of everyday programmers, whose problems it strives to address. Tim Budd graciously offered that our discipline is diverse enough to accommodate a broad spectrum of definitions for "multi-paradigm," and I insisted on a title that emphasized the role of the programmer and not that of the methodologist. That led to a happy convergence on the current title, Multi-Paradigm Design for C++.
I never considered titles containing the words pattern, object, CORBA, component, or Java. Multi-paradigm design tries to dig deeper than any single technology or technique to address fundamental questions of software abstraction and design. What is a paradigm? What is the relationship between analysis, design, and implementation? These questions go to the foundations of abstraction that underlie the basic paradigms of programming.
One of the most basic questions is, what is abstraction? Abstraction is one of the key tools of software design; it is necessary for managing the immense and ever-growing complexity of computer systems. The common answer to this question usually has something to do with objects, thereby reflecting the large body of literature and tools that have emerged over the past decade or two to support object-oriented techniques. But this response ignores common design structures that programmers use every day and that are not object-oriented: templates, families of overloaded functions, modules, generic functions, and others. Such use is particularly common in the C++ community, though it is by no means unique to that community.
There are principles of abstraction common to all of these techniques. Each technique is a different way of grouping abstractions according to properties they share, including regularities in the way individual entities vary from each other. To some, commonality captures the recurring external properties of a system that are germane to its domain. To others, commonality helps regularize implicit structure that analysis uncovers in the recurring solutions for a domain. Multi-paradigm design honors both perspectives. For example, the technique called object-oriented design groups objects into classes that characterize the common structure and behaviors of those objects. It groups classes into hierarchies or graphs that reflect commonality in structure and behavior, while at the same time allowing for regular variations in structure and in the algorithms that implement a given behavior. One can describe templates using a different characterization of commonality and variation. Commonality and variation provide a broad, simple model of abstraction, broader than objects and classes and broad enough to handle most design and programming techniques.
Commonality and variation aren't new to software design models. Parnas's concept of software families Parnas1976 goes back at least two decades. Families are collections of software elements related by their commonalities, with individual family members differentiated by their variations. The design ideas that have emerged from software families have often found expression in widely accepted programming languages; good examples are modules, classes and objects, and generic constructs. The work of Lai and Weiss on environments for application-specific languages takes this idea to its limits Weiss1999. The so-called analysis activities that focus on the discovery of software families and the so-called coding activities that focus on how to express these abstractions have always been closely intertwined. Multi-paradigm design explicitly recognizes the close tie between language, design, and domain structure and the way they express commonality and variation.
We discover software families in an activity called domain analysis, which is another field with a long history Neighbors1980. Software reuse was the original goal of domain analysis, and this goal fits nicely with software families. Multi-paradigm design explicitly focuses on issues that are important for reuse. To help the designer think about adapting software to a spectrum of anticipated markets, multi-paradigm design explicitly separates commonalities--assumptions that don't change--from variabilities--assumptions that do change. We strive for domain analysis, not just analysis. We design families of abstractions, not just abstractions. Done well, this approach to design leads in the long term to easier maintenance (if we predict the variabilities well) and to a more resilient architecture (we don't have to dig up the foundations every time we make a change). Of course, multi-paradigm development is just one tool that helps support the technical end of reuse. Effective reuse can happen only in the larger context of organizational issues, marketing issues, and software economics.
We use these foundations of commonality and variation to formalize the concept of paradigm. A paradigm, as the term is popularly used in contemporary software design, is a way of organizing system abstractions around properties of commonality and variation. The object paradigm organizes systems around abstractions based on commonality in structure and behavior and variation in structure and algorithm. The template paradigm is based on structural commonality across family members, with variations explicitly factored out into template parameters. Overloaded functions form families whose members share the same name and semantics, and in which each family member is differentiated by its formal parameter types.
C++ is a programming language that supports multiple paradigms: classes, overloaded functions, templates, modules, ordinary procedural programming, and others. Bjarne Stroustrup, the creator of C++, intended it that way. Most programmers use the C++ features that go beyond objects (though some abuse them to excess and others force designs into an object-oriented mold when they should be using more natural expressions of design provided by other language features). The powerful template code of John Barton and Lee Nackman Barton1994 is perhaps the height of tasteful multi-paradigm design.
Even though Stroustrup designated C++ as a multi-paradigm language, there have been no serious attempts to create a design method suited to the richness of C++ features. And though C++ provides a particularly rich and crisp example of multi-paradigm programming, the opportunity for multi-paradigm development generalizes to other programming languages. There is a gap between the current design literature and the intended use of C++ features that is reflected in current practice. This book bridges that gap, using simple notations and vocabulary to help developers combine multiple paradigms instructively.
During a lecture I gave at DePaul University in September 1995, the department chair, Dr. Helmut Epp, suggested the term meta-design for this work because its first concern is to identify design techniques suitable to the domain for which software is being developed. That is a useful perspective on the approach taken in this book and in fact describes how most developers approach design. One must first decide what paradigms to use; then one can apply the rules and tools of each paradigm for the
