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just hype have been replaced by questions of how to get the benefits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
later). It is tempting to ask if there is any other engineering
discipline that can produce designs of such complexity as software in
such a short time, but first we have to figure out how to measure and
compare complexity. Nevertheless, it is obvious that software designs
get very large rather quickly.
Given that software designs are relatively easy to turn out, and
essentially free to build, an unsurprising revelation is that software
designs tend to be incredibly large and complex. This may seem obvious
but the magnitude of the problem is often ignored. School projects
often end up being several thousand lines of code. There are software
products with 10,000 line designs that are given away by their
designers. We have long since passed the point where simple software is
of much interest. Typical commercial software products have designs
that consist of hundreds of thousands of lines. Many software designs
run into the millions. Additionally, software designs are almost always
constantly evolving. While the current design may only be a few
thousand lines of code, many times that may actually have been written
over the life of the product.
While there are certainly examples of hardware designs that are
arguably as complex as software designs, note two facts about modern
hardware. One, complex hardware engineering efforts are not always as
free of bugs as software critics would have us believe. Major
microprocessors have been shipped with errors in their logic, bridges
collapsed, dams broken, airliners fallen out of the sky, and thousands
of automobiles and other consumer products have been recalled – all
within recent memory and all the result of design errors. Second,
complex hardware designs have correspondingly complex and expensive
build phases. As a result, the ability to manufacture such systems
limits the number of companies that produce truly complex hardware
designs. No such limitations exist for software. There are hundreds of
software organizations, and thousands of very complex software systems
in existence. Both the number and the complexity are growing daily.
This means that the software industry is not likely to find solutions
to its problems by trying to emulate hardware developers. If anything,
as CAD and CAM systems have helped hardware designers to create more
and more complex designs, hardware engineering is becoming more and
more like software development.
Designing software is an exercise in managing complexity. The
complexity exists within the software design itself, within the
software organization of the company, and within the industry as a
whole. Software design is very similar to systems design. It can span
multiple technologies and often involves multiple sub-disciplines.
Software specifications tend to be fluid, and change rapidly and often,
usually while the design process is still going on. Software
development teams also tend to be fluid, likewise often changing in the
middle of the design process. In many ways, software bears more
resemblance to complex social or organic systems than to hardware. All
of this makes software design a difficult and error prone process. None
of this is original thinking, but almost 30 years after the software
engineering revolution began, software development is still seen as an
undisciplined art compared to other engineering professions.
The general consensus is that when real engineers get through with a
design, no matter how complex, they are pretty sure it will work. They
are also pretty sure it can be built using accepted construction
techniques. In order for this to happen, hardware engineers spend a
considerable amount of time validating and refining their designs.
Consider a bridge design, for example. Before such a design is actually
built the engineers do structural analysis; they build computer models
and run simulations; they build scale models and test them in wind
tunnels or other ways. In short, the designers do everything they could
think of to make sure the design is a good design before it is built.
The design of new airliner is even worse; for those, full scale
prototypes must be built and test flown to validate the design
It seems obvious to most people that software designs do not go
through the same rigorous engineering as hardware designs. However, if
we consider source code as design, we see that software designers
actually do a considerable amount of validating and refining their
designs. Software designers do not call it engineering, however, we
call it testing and debugging. Most people do not consider testing and
debugging as real “engineering”; certainly not in the software
business. The reason has more to do with the refusal of the software
industry to accept code as design than with any real engineering
difference. Mock-ups, prototypes, and bread-boards are actually an
accepted part of other engineering disciplines. Software designers do
not have or use more formal methods of validating their designs because
of the simple economics of the software build cycle.
Revelation number two: it is cheaper and simpler to just build the
design and test it than to do anything else. We do not care how many
builds we do—they cost next to nothing in terms of time, and the
resources used can be completely reclaimed later if we discard the
build. Note that testing is not just concerned with getting the current
design correct, it is part of the process of refining the design.
Hardware engineers of complex systems often build models (or at least
they visually render their designs using computer graphics). This
allows them to get a “feel” for the design that is not possible by just
reviewing the design itself. Building such a model is both impossible
and unnecessary with a software design. We just build the product
itself. Even if formal software proofs were as automatic as a compiler,
we would still do build/test cycles. Ergo, formal proofs have never
been of much practical interest to the software industry.
This is the reality of the software development process today. Ever
more complex software designs are being created by an ever increasing
number of people and organizations. These designs will be coded in some
programming language and then validated and refined via the build/test
cycle. The process is error prone and not particularly rigorous to
begin with. The fact that a great many software developers do not want
to believe that this is the way it works compounds the problem
Most current software development processes try to segregate the
different phases of software design into separate pigeon-holes. The top
level design must be completed and frozen before any code is written.
Testing and debugging are necessary just to weed out the construction
mistakes. In between are the programmers, the construction workers of
the software industry. Many believe that if we could just get
programmers to quit “hacking” and “build” the designs as given to them
(and in the process, make fewer errors) then software development might
mature into a true engineering discipline. Not likely to happen as long
as the process ignores the engineering and economic realities.
For example, no other modern industry would tolerate a rework rate
of over 100% in its manufacturing process. A construction worker who
can not build it right the first time, most of the time, is soon out of
a job. In software, even the smallest piece of code is likely to be
revised or completely rewritten during testing and debugging. We accept
this sort of refinement during a creative process like design, not as
part of a manufacturing process. No one expects an engineer to create a
perfect design the first time. Even if she does, it must still be put
through the refinement process just to prove that it was perfect.
If we learn nothing else from Japanese management techniques, we
should learn that it is counter-productive to blame the workers for
errors in the process. Instead of continuing to force software
development to conform to an incorrect process model, we need to revise
the process so that it helps rather than hinders efforts to produce
better software. This is the litmus test of “software engineering.”
Engineering is about how you do the process, not about whether the
final design document needs a CAD system to produce it.
The overwhelming problem with software development is that everything
is part of the design process. Coding is design, testing and debugging
are part of design, and what we typically call software design is still
part of design. Software may be cheap to build, but it is incredibly
expensive to design. Software is so complex that there are plenty of
different design aspects and their resulting design views. The problem
is that all the different aspects interrelate (just like they do in
hardware engineering). It would be nice if top level designers could
ignore the details of module algorithm design. Likewise, it would be
nice if programmers did not have to worry about top level design issues
when designing the internal algorithms of a module. Unfortunately, the
aspects of one design layer intrude into the others. The choice of
algorithms for a given module can be as important to the overall
success of the software system as any of the higher level design
aspects. There is no hierarchy of importance among the different
aspects of a software design. An incorrect design at the lowest module
level can be as fatal as a mistake at the highest level. A software
design must be complete and correct in all its aspects, or all software
builds based on the design will be erroneous.
In order to deal with the complexity, software is designed in
layers. When a programmer is worrying about the detailed design of one
module, there are probably hundreds of other modules and thousands of
other details that he can not possibly worry about at the same time.
For example, there are important aspects of software design that do not
fall cleanly into the categories of data structures and algorithms.
Ideally, programmers should not have to worry about these other aspects
of a design when designing code.
This is not how it works, however, and the reasons start to make
sense. The software design is not complete until it has been coded and
tested. Testing is a fundamental part of the design validation and
refinement process. The high level structural design is not a complete
software design; it is just a structural framework for the detailed
design. We have very limited capabilities for rigorously validating a
high level design. The detailed design will ultimately influence (or should
be allowed to influence) the high level design at least as much as
other factors. Refining all the aspects of a design is a process that
should be happening throughout the design cycle. If any aspect of the
design is frozen out of the refinement process, it is hardly surprising
that the final design will be poor or even unworkable.
It would be nice if high level software design could be a more
rigorous engineering process, but the real world of software systems is
not rigorous. Software is too complex and it depends on too many other
things. Maybe some hardware does not work quite the way the designers
thought it did, or a library routine has an undocumented restriction.
These are the kinds of problems that every software project encounters
sooner or later. These are the kinds of problems discovered during
testing (if we do a good job of testing), for the simple reason that
there was no way to discover them earlier. When they are discovered,
they force a change in the design. If we are lucky, the design changes
are local. More often than not, the changes will ripple through some
significant portion of the entire software design (Murphy’s Law). When
part of the effected design can not change for some reason, then the
other parts of the design will have to be weakened to accommodate. This
often results is what managers perceive as “hacking”, but it is the
reality of software development.
For example, I recently worked on a project where a timing
dependency was discovered between the internals of module A and another
module B. Unfortunately, the internals of module A were hidden behind
an abstraction that did not permit any way to incorporate the
invocation of module B in its proper sequence. Naturally, by the time
the problem was discovered, it was much too late to try to change the
abstraction of A. As expected, what happened was an increasingly
complex set of “fixes” applied to the internal design of A. Before we
finished installing version 1, there was the general feeling that the
design was breaking down. Every new fix was likely to break some older
fix. This is a normal software development project. Eventually, my
colleagues and I argued for a change in the design, but we had to
volunteer free overtime in order to get management to agree.
On any software project of typical size, problems like these are
guaranteed to come up. Despite all attempts to prevent it, important
details will be overlooked. This is the difference between craft and
engineering. Experience can lead us in the right direction. This is
craft. Experience will only take us so far into uncharted territory.
Then we must take what we started with and make it better through a
controlled process of refinement. This is engineering.
As just a small point, all programmers know that writing the
software design documents after the code instead of before, produces
much more accurate documents. The reason is now obvious. Only the final
design, as reflected in code, is the only one refined during the
build/test cycle. The probability of the initial design being unchanged
during this cycle is inversely related to the number of modules and
number of programmers on a project. It rapidly becomes
indistinguishable from zero.
In software engineering, we desperately need good design at all
levels. In particular, we need good top level design. The better the
early design, the easier detailed design will be. Designers should use
anything that helps. Structure charts, Booch diagrams, state tables,
PDL, etc.—if it helps, then use it. We must keep in mind, however, that
these tools and notations are not a software design. Eventually, we
have to create the real software design, and it will be in some
programming language. Therefore, we should not be afraid to code our
designs as we derive them. We simply must be willing to refine them as
There is as yet no design notation equally suited for use in both
top level design and detailed design. Ultimately, the design will end
up coded in some programming language. This means that top level design
notations have to be translated into the target programming language
before detailed design can begin. This translation step takes time and
introduces errors. Rather than translate from a notation that may not
map cleanly into the programming language of choice, programmers often
go back to the requirements and redo the top level design, coding it as
they go. This, too, is part of the reality of software development.
It is probably better to let the original designers write the
original code, rather than have someone else translate a language
independent design later. What we need is a unified design notation
suitable for all levels of design. In other words, we need a
programming language that is also suitable for capturing high level
design concepts. This is where C++ comes in. C++ is a programming
language suitable for real world projects that is also a more
expressive software design language. C++ allows us to directly express
high level information about design components. This makes it easier to
produce the design, and easier to refine it later. With its stronger
type checking, it also helps the process of detecting design errors.
This results in a more robust design, in essence a better engineered
Ultimately, a software design must be represented in some
programming language, and then validated and refined via a build/test
cycle. Any pretense otherwise is just silliness. Consider what software
development tools and techniques have gained popularity. Structured
programming was considered a breakthrough in its time. Pascal
popularized it and in turn became popular. Object oriented design is
the new rage and C++ is at the heart of it. Now think about what has
not worked. CASE tools? Popular, yes; universal, no. Structure charts?
Same thing. Likewise, Warner-Orr diagrams, Booch diagrams, object
diagrams, you name it. Each has its strengths, and a single fundamental
weakness—it really isn’t a software design. In fact the only software
design notation that can be called widespread is PDL, and what does
that look like.
This says that the collective subconscious of the software industry
instinctively knows that improvements in programming techniques and
real world programming languages in particular are overwhelmingly more
important than anything else in the software business. It also says
that programmers are interested in design. When more expressive
programming languages become available, software developers will adopt
Also consider how the process of software development is changing.
Once upon a time we had the waterfall process. Now we talk of spiral
development and rapid prototyping. While such techniques are often
justified with terms like “risk abatement” and “shortened product
delivery times”, they are really just excuses to start coding earlier
in the life cycle. This is good. This allows the build/test cycle to
start validating and refining the design earlier. It also means that it
is more likely that the software designers that developed the top level
design are still around to do the detailed design.
As noted above—engineering is more about how you do the process than
it is about what the final product looks like. We in the software
business are close to being engineers, but we need a couple of
perceptual changes. Programming and the build/test cycle are central to
the process of engineering software. We need to manage them as such.
The economics of the build/test cycle, plus the fact that a software
system can represent practically anything, makes it very unlikely that
we will find any general purpose methods for validating a software
design. We can improve this process, but we can not escape it.
One final point: the goal of any engineering design project is the
production of some documentation. Obviously, the actual design
documents are the most important, but they are not the only ones that
must be produced. Someone is eventually expected to use the software.
It is also likely that the system will have to be modified and enhanced
at a later time. This means that auxiliary documentation is as
important for a software project as it is for a hardware project.
Ignoring for now users manuals, installation guides, and other
documents not directly associated with the design process, there are
still two important needs that must be solved with auxiliary design
The first use of auxiliary documentation is to capture important
information from the problem space that did not make it directly into
the design. Software design involves inventing software concepts to
model concepts in a problem space. This process requires developing an
understanding of the problem space concepts. Usually this understanding
will include information that does not directly end up being modeled in
the software space, but which nevertheless helped the designer
determine what the essential concepts were, and how best to model them.
This information should be captured somewhere in case the model needs
to be changed at a later time.
The second important need for auxiliary documentation is to document
those aspects of the design that are difficult to extract directly from
the design itself. These can include both high level and low level
aspects. Many of these aspects are best depicted graphically. This
makes them hard to include as comments in the source code. This is not
an argument for a graphical software design notation instead of a
programming language. This is no different from the need for textual
descriptions to accompany the graphical design documents of hardware
disciplines. Never forget that the source code determines what the
actual design really is, not the auxiliary documentation. Ideally,
software tools would be available that post processed a source code
design and generated the auxiliary documentation. That may be too much
to expect. The next best thing might be some tools that let programmers
(or technical writers) extract specific information from the source
code that can then be documented in some other way. Undoubtedly,
keeping such documentation up to date manually is difficult. This is
another argument for the need for more expressive programming
languages. It is also an argument for keeping such auxiliary
documentation to a minimum and keeping it as informal as possible until
as late in the project as possible. Again, we could use some better
tools, otherwise we end up falling back on pencil, paper, and chalk
- Real software runs on computers. It is a sequence of ones
and zeros that is stored on some magnetic media. It is not a program
listing in C++ (or any other programming language).
- A program listing is a document that represents a software design. Compilers and linkers actually build software designs.
- Real software is incredibly cheap to build, and getting cheaper all the time as computers get faster.
software is incredibly expensive to design. This is true because
software is incredibly complex and because practically all the steps of
a software project are part of the design process.
is a design activity—a good software design process recognizes this and
does not hesitate to code when coding makes sense.
actually makes sense more often than believed. Often the process of
rendering the design in code will reveal oversights and the need for
additional design effort. The earlier this occurs, the better the
design will be.
- Since software is so cheap to build, formal
engineering validation methods are not of much use in real world
software development. It is easier and cheaper to just build the design
and test it than to try to prove it.
- Testing and debugging
are design activities—they are the software equivalent of the design
validation and refinement processes of other engineering disciplines. A
good software design process recognizes this and does not try to short
change the steps.
- There are other design activities—call
them top level design, module design, structural design, architectural
design, or whatever. A good software design process recognizes this and
deliberately includes the steps.
- All design activities
interact. A good software design process recognizes this and allows the
design to change, sometimes radically, as various design steps reveal
- Many different software design notations are
potentially useful—as auxiliary documentation and as tools to help
facilitate the design process. They are not a software design.
development is still more a craft than an engineering discipline. This
is primarily because of a lack of rigor in the critical processes of
validating and improving a design.
- Ultimately, real
advances in software development depend upon advances in programming
techniques, which in turn mean advances in programming languages. C++
is such an advance. It has exploded in popularity because it is a
mainstream programming language that directly supports better software
- C++ is a step in the right direction, but still more advances are needed.
This essay first appeared in C++ Journal in
the Fall, 1992 issue. Copyright ©1992 by Jack W. Reeves. developer.* is
grateful to Mr. Reeves for granting the right of this publication. All
future rights owned and reserved by Jack W. Reeves. Reprint or
distribute only with written permission of the author.
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.