What Is Software Design? by Jack W. Reeves - developer.*, Developer Dot Star

The Independent Magazine<br>for Software Developers

Books<br>Articles<br>Blogs<br>Subscribe<br>d.* Gear<br>About<br>Home

What Is Software Design?

By Jack W. Reeves

Published February 23, 2005

Discussion<br>Printable<br>PDF

This is Part One of Code As Design: Three Essays by Jack W. Reeves. Click here for the introduction. This essay first appeared in the Fall, 1992 issue of C++ Journal.

Object oriented techniques, and C++ in particular, seem to be taking the software world by storm. Numerous articles and books have appeared describing how to apply the new techniques. In general, the questions of whether O-O techniques are just hype have been replaced by questions of how to get the benefits with the least amount of pain. Object oriented techniques have been around for some time, but this exploding popularity seems a bit unusual. Why the sudden interest? All kinds of explanations have been offered. In truth, there is probably no single reason. Probably, a combination of factors has finally reached critical mass and things are taking off. Nevertheless, it seems that C++ itself is a major factor in this latest phase of the software revolution. Again, there are probably a number of reasons why, but I want to suggest an answer from a slightly different perspective: C++ has become popular because it makes it easier to design software and program at the same time.

If that comment seems a bit unusual, it is deliberate. What I want to do in this article is take a look at the relationship between programming and software design. For almost 10 years I have felt that the software industry collectively misses a subtle point about the difference between developing a software design and what a software design really is. I think there is a profound lesson in the growing popularity of C++ about what we can do to become better software engineers, if only we see it. This lesson is that programming is not about building software; programming is about designing software.

Years ago I was attending a seminar where the question came up of whether software development is an engineering discipline or not. While I do not remember the resulting discussion, I do remember how it catalyzed my own thinking that the software industry has created some false parallels with hardware engineering while missing some perfectly valid parallels. In essence, I concluded that we are not software engineers because we do not realize what a software design really is. I am even more convinced of that today.

The final goal of any engineering activity is the some type of documentation. When a design effort is complete, the design documentation is turned over to the manufacturing team. This is a completely different group with completely different skills from the design team. If the design documents truly represent a complete design, the manufacturing team can proceed to build the product. In fact, they can proceed to build lots of the product, all without any further intervention of the designers. After reviewing the software development life cycle as I understood it, I concluded that the only software documentation that actually seems to satisfy the criteria of an engineering design is the source code listings.

There are probably enough arguments both for and against this premise to fill numerous articles. This article assumes that final source code is the real software design and then examines some of the consequences of that assumption. I may not be able to prove that this point of view is correct, but I hope to shown that it does explain some of the observed facts of the software industry, including the popularity of C++.

There is one consequence of considering code as software design that completely overwhelms all others. It is so important and so obvious that it is a total blind spot for most software organizations. This is the fact that software is cheap to build. It does not qualify as inexpensive; it is so cheap it is almost free. If source code is a software design, then actually building software is done by compilers and linkers. We often refer to the process of compiling and linking a complete software system as "doing a build". The capital investment in software construction equipment is low—all it really takes is a computer, an editor, a compiler, and a linker. Once a build environment is available, then actually doing a software build just takes a little time. Compiling a 50,000 line C++ program may seem to take forever, but how long would it take to build a hardware system that had a design of the same complexity as 50,000 lines of C++.

Another consequence of considering source code as software design is the fact that a software design is relatively easy to create, at least in the mechanical sense. Writing (i.e., designing) a typical software module of 50 to 100 lines of code is usually only a couple of day's effort (getting it fully debugged is another story, but more on that...