Al Stevens Interviews Alex Stepanov
This interview appeared in the March 1995 issue of
Dr. Dobb's Journal,
and is reprinted with permission.
Tell us something about your long-term interest in generic
I started thinking about generic programming in the late 70s
when I observed that some algorithms depended not on some
particular implementation of a data structure but only on a few
fundamental semantic properties of the structure. I started
going through many different algorithms, and I found that most
algorithms can be abstracted away from a particular
implementation in such a way that efficiency is not lost.
Efficiency is a fundamental concern of mine. It is silly to
abstract an algorithm in such a way that when you instantiate it
back it becomes inefficient.
At that time I thought that the right way of doing this kind
of research was to develop a programming language, which is what
I started doing with two of my friends, Deepak Kapur, who at
present is a professor at State University of New York, Albany,
and David Musser, professor at Rensselaer Polytechnic Institute.
At that time the three of us worked at the General Electric
Research Center at Schenectady, NY. We started working on a
language called Tecton, which would allow people to describe
algorithms associated with what we called generic structures,
which is just a collection of formal types and properties of
these types. Sort of mathematical stuff. We realized that one
can define an algebra of operations on these structures, you can
refine them, you can enrich them, and do all sorts of things.
There were some interesting ideas, but the research didn't
lead to practical results because Tecton was functional. We
believed Backus's idea that we should liberate programming from
the von Neumann style, and we didn't want to have side effects.
That limited our ability to handle very many algorithms that
require the notion of state and side effects.
The interesting thing about Tecton, which I realized
sometime in the late 70s, was that there was a fundamental
limitation in the accepted notion of an abstract data type.
People usually viewed abstract data types as something which
tells you only about the behavior of an object and the
implementation is totally hidden. It was commonly assumed that
the complexity of an operation is part of implementation and that
abstraction ignores complexity. One of the things that is
central to generic programming as I understand it now, is that
complexity, or at least some general notion of complexity, has to
be associated with an operation.
Let's take an example. Consider an abstract data type
stack. It's not enough to have Push and Pop connected with the
axiom wherein you push something onto the stack and after you pop
the stack you get the same thing back. It is of paramount
importance that pushing the stack is a constant time operation
regardless of the size of the stack. If I implement the stack so
that every time I push it becomes slower and slower, no one will
want to use this stack.
We need to separate the implementation from the interface
but not at the cost of totally ignoring complexity. Complexity
has to be and is a part of the unwritten contract between the
module and its user. The reason for introducing the notion of
abstract data types was to allow interchangeable software
modules. You cannot have interchangeable modules unless these
modules share similar complexity behavior. If I replace one
module with another module with the same functional behavior but
with different complexity tradeoffs, the user of this code will
be unpleasantly surprised. I could tell him anything I like
about data abstraction, and he still would not want to use the
code. Complexity assertions have to be part of the interface.
Around 1983 I moved from GE Research to the faculty of the
Polytechnic University, formerly known as Brooklyn Polytechnic,
in NY. I started working on graph algorithms. My principal
collaborator was Aaron Kershenbaum, now at IBM Yorktown Heights.
He was an expert in graph and network algorithms, and I convinced
him that some of the ideas of high order and generic programming
were applicable to graph algorithms. He had some grants and
provided me with support to start working with him to apply these
ideas to real network algorithms. He was interested in building
a toolbox of high order generic components so that some of these
algorithms could be implemented, because some of the network
algorithms are so complex that while they are theoretically
analyzed, but never implemented. I decided to use a dialect of
Lisp called Scheme to build such a toolbox. Aaron and I
developed a large library of components in Scheme demonstrating
all kinds of programming techniques. Network algorithms were the
primary target. Later Dave Musser, who was still at GE Research,
joined us, and we developed even more components, a fairly large
library. The library was used at the university by graduate
students, but was never used commercially. I realized during
this activity that side effects are important, because you cannot
really do graph operations without side effects. You cannot
replicate a graph every time you want to modify a vertex.
Therefore, the insight at that time was that you can combine high
order techniques when building generic algorithms with
disciplined use of side effects. Side effects are not necessarily
bad; they are bad only when they are misused.
In the summer of 1985 I was invited back to GE Research to
teach a course on high order programming. I demonstrated how you
can construct complex algorithms using this technique. One of
the people who attended was Art Chen, then the manager of the
Information Systems Laboratory. He was sufficiently impressed to
ask me if I could produce an industrial strength library using
these techniques in Ada, provided that I would get support.
Being a poor assistant professor, I said yes, even though I
didn't know any Ada at the time. I collaborated with Dave Musser
in building this Ada library. It was an important undertaking,
because switching from a dynamically typed language, such as
Scheme, to a strongly typed language, such as Ada, allowed me to
realize the importance of strong typing. Everybody realizes that
strong typing helps in catching errors. I discovered that strong
typing, in the context of Ada generics, was also an instrument of
capturing designs. It was not just a tool to catch bugs. It was
also a tool to think. That work led to the idea of orthogonal
decomposition of a component space. I realized that software
components belong to different categories. Object-oriented
programming aficionados think that everything is an object. When
I was working on the Ada generic library, I realized that this
wasn't so. There are things that are objects. Things that have
state and change their state are objects. And then there are
things that are not objects. A binary search is not an object.
It is an algorithm. Moreover, I realized that by decomposing the
component space into several orthogonal dimensions, we can reduce
the number of components, and, more importantly, we can provide a
conceptual framework of how to design things.
Then I was offered a job at Bell Laboratories working in the
C++ group on C++ libraries. They asked me whether I could do it
in C++. Of course, I didn't know C++ and, of course, I said I
could. But I couldn't do it in C++, because in 1987 C++ didn't
have templates, which are essential for enabling this style of
programming. Inheritance was the only mechanism to obtain
genericity and it was not sufficient.
Even now C++ inheritance is not of much use for generic
programming. Let's discuss why. Many people have attempted to
use inheritance to implement data structures and container
classes. As we know now, there were few if any successful
attempts. C++ inheritance, and the programming style associated
with it are dramatically limited. It is impossible to implement a
design which includes as trivial a thing as equality using it.
If you start with a base class X at the root of your hierarchy
and define a virtual equality operator on this class which takes
an argument of the type X, then derive class Y from class X. What
is the interface of the equality? It has equality which compares
Y with X. Using animals as an example (OO people love animals),
define mammal and derive giraffe from mammal. Then define a
member function mate, where animal mates with animal and returns
an animal. Then you derive giraffe from animal and, of course,
it has a function mate where giraffe mates with animal and
returns an animal. It's definitely not what you want. While
mating may not be very important for C++ programmers, equality
is. I do not know a single algorithm where equality of some kind
is not used.
You need templates to deal with such problems. You can have
template class animal which has member function mate which takes
animal and returns animal. When you instantiate giraffe, mate
will do the right thing. The template is a more powerful
mechanism in that respect.
However, I was able to build a rather large library of
algorithms, which later became part of the Unix System Laboratory
Standard Component Library. I learned a lot at Bell Labs by
talking to people like Andy Koenig and Bjarne Stroustrup about
programming. I realized that C/C++ is an important programming
language with some fundamental paradigms that cannot be ignored.
In particular I learned that pointers are very good. I don't
mean dangling pointers. I don't mean pointers to the stack. But
I mean that the general notion of pointer is a powerful tool.
The notion of address is universally used. It is incorrectly
believed that pointers make our thinking sequential. That is not
so. Without some kind of address we cannot describe any parallel
algorithm. If you attempt to describe an addition of n numbers
in parallel, you cannot do it unless you can talk about the first
number being added to the second number, while the third number
is added to the fourth number. You need some kind of indexing.
You need some kind of address to describe any kind of algorithm,
sequential or parallel. The notion of an address or a location
is fundamental in our conceptualizing computational processes---algorithms.
Let's consider now why C is a great language. It is
commonly believed that C is a hack which was successful because
Unix was written in it. I disagree. Over a long period of time
computer architectures evolved, not because of some clever people
figuring how to evolve architectures---as a matter of fact,
clever people were pushing tagged architectures during that
period of time---but because of the demands of different
programmers to solve real problems. Computers that were able to
deal just with numbers evolved into computers with byte-addressable
memory, flat address spaces, and pointers. This
was a natural evolution reflecting the growing set of problems
that people were solving. C, reflecting the genius of Dennis
Ritchie, provided a minimal model of the computer that had
evolved over 30 years. C was not a quick hack. As computers
evolved to handle all kinds of problems, C, being the minimal
model of such a computer, became a very powerful language to
solve all kinds of problems in different domains very
effectively. This is the secret of C's portability: it is the
best representation of an abstract computer that we have. Of
course, the abstraction is done over the set of real computers,
not some imaginary computational devices. Moreover, people could
understand the machine model behind C. It is much easier for an
average engineer to understand the machine model behind C than
the machine model behind Ada or even Scheme. C succeeded because
it was doing the right thing, not because of AT&T promoting it or
Unix being written with it.
C++ is successful because instead of trying to come up with
some machine model invented by just contemplating one's navel,
Bjarne started with C and tried to evolve C further, allowing
more general programming techniques but within the framework of
this machine model. The machine model of C is very simple. You
have the memory where things reside. You have pointers to the
consecutive elements of the memory. It's very easy to
understand. C++ keeps this model, but makes things that reside
in the memory more extensive than in the C machine, because C has
a limited set of data types. It has structures that allow a sort
of an extensible type system, but it does not allow you to define
operations on structures. This limits the extensibility of the
type system. C++ moved C's machine model much further toward a
truly extensible type system.
In 1988 I moved to HP Labs where I was hired to work on
generic libraries. For several years, instead of doing that I
worked on disk drives, which was exciting but was totally
orthogonal to this area of research. I returned to generic
library development in 1992 when Bill Worley, who was my lab
director established an algorithms project with me being its
manager. C++ had templates by then. I discovered that Bjarne had
done a marvelous job at designing templates. I had participated
in several discussions early on at Bell Labs about designing
templates and argued rather violently with Bjarne that he should
make C++ templates as close to Ada generics as possible. I think
that I argued so violently that he decided against that. I
realized the importance of having template functions in C++ and
not just template classes, as some people believed. I thought,
however, that template functions should work like Ada generics,
that is, that they should be explicitly instantiated. Bjarne did
not listen to me and he designed a template function mechanism
where templates are instantiated implicitly using an overloading
mechanism. This particular technique became crucial for my work
because I discovered that it allowed me to do many things that
were not possible in Ada. I view this particular design by
Bjarne as a marvelous piece of work and I'm very happy that he
didn't follow my advice.
When did you first conceive of the STL and what was its original
In 1992, when the project was formed, there were eight people in
it. Gradually the group diminished, eventually becoming two
people, me and Meng Lee. While Meng was new to the area---she
was doing compilers for most of her professional life---she
accepted the overall vision of generic programming research, and
believed that it could lead to changing software development at
the point when very few people shared this belief. I do not
think that I would be able to build STL without her help. (After
all, STL stands for Stepanov and Lee...) We wrote a huge library,
a lot of code with a lot of data structures and algorithms,
function objects, adaptors, and so on. There was a lot of code,
but no documentation. Our work was viewed as a research project
with the goal of demonstrating that you can have algorithms
defined as generically as possible and still extremely efficient.
We spent a lot of time taking measurements, and we found that we
can make these algorithms as generic as they can be, and still
be as efficient as hand-written code. There is no performance
penalty for this style of programming! The library was growing,
but it wasn't clear where it was heading as a project. It took
several fortunate events to lead it toward STL.
When and why did you decide to propose STL as part of the
ANSI/ISO Standard C++ definition?
During the summer of 1993, Andy Koenig came to teach a C++ course
at Stanford. I showed him some of our stuff, and I think he was
genuinely excited about it. He arranged an invitation for me to
give a talk at the November meeting of the ANSI/ISO C++ Standards
Committee in San Jose. I gave a talk entitled "The Science of
C++ Programming." The talk was rather theoretical. The main
point was that there are fundamental laws connecting basic
operations on elements of C++ which have to be obeyed. I showed
a set of laws that connect very primitive operations such as
constructors, assignment, and equality. C++ as a language does
not impose any constraints. You can define your equality
operator to do multiplication. But equality should be equality,
and it should be a reflexive operation. A should be equal to A.
It should be symmetric. If A is equal to B, then B should be
equal to A. And it should be transitive. Standard mathematical
axioms. Equality is essential for other operations. There are
axioms that connect constructors and equality. If you construct
an object with a copy constructor out of another object, the two
objects should be equal. C++ does not mandate this, but this is
one of the fundamental laws that we must obey. Assignment has to
create equal objects. So I presented a bunch of axioms that
connected these basic operations. I talked a little bit about
axioms of iterators and showed some of the generic algorithms
working on iterators. It was a two-hour talk and, I thought,
rather dry. However, it was very well received. I didn't think
at that time about using this thing as a part of the standard
because it was commonly perceived that this was some kind of
advanced programming technique which would not be used in the
"real world". I thought there was no interest at all in any of
this work by practical people.
I gave this talk in November, and I didn't think about ANSI
at all until January. On January 6 I got a mail message from
Andy Koenig, who is the project editor of the standard document,
saying that if I wanted to make my library a part of the
standard, I should submit a proposal by January 25. My answer
was, "Andy, are you crazy?" to which he answered, "Well, yes I am
crazy, but why not try it?"
At that point there was a lot of code but there was no
documentation, much less a formal proposal. Meng and I spent 80-hour
weeks to come up with a proposal in time for the mailing
deadline. During that time the only person who knew it was
coming was Andy. He was the only supporter and he did help a lot
during this period. We sent the proposal out, and waited. While
doing the proposal we defined a lot of things. When you write
things down, especially when you propose them as a standard, you
discover all kinds of flaws with your design. We had to re-implement every single piece of code in the library, several
hundred components, between the January mailing and the next
meeting in March in San Diego. Then we had to revise the
proposal, because while writing the code, we discovered many
Can you characterize the discussions and debate in the committee
following the proposal? Was there immediate support?
We did not believe that anything would come out of it. I
gave a talk, which was very well received. There were a lot of
objections, most of which took this form: this is a huge
proposal, it's way too late, a resolution had been passed at the
previous meeting not to accept any major proposals, and here is
this enormous thing, the largest proposal ever, with a lot of
totally new things. The vote was taken, and, interestingly
enough, an overwhelming majority voted to review the proposal at
the next meeting and put it to a vote at the next meeting in
Bjarne Stroustrup became a strong supporter of STL. A lot
of people helped with suggestions, modifications, and revisions.
Bjarne came here for a week to work with us. Andy helped
constantly. C++ is a complex language, so it is not always clear
what a given construct means. Almost daily I called Andy or
Bjarne to ask whether such-and-such was doable in C++. I should
give Andy special credit. He conceived of STL as part of the
standard library. Bjarne became the main pusher of STL on the
committee. There were other people who were helpful: Mike
Vilot, the head of the library group, Nathan Myers of Rogue Wave,
Larry Podmolik of Andersen Consulting. There were many others.
The STL as we proposed it in San Diego was written in
present C++. We were asked to rewrite it using the new ANSI/ISO
language features, some of which are not implemented. There was
an enormous demand on Bjarne's and Andy's time trying to verify
that we were using these non-implemented features correctly.
People wanted containers independent of the memory model,
which was somewhat excessive because the language doesn't include
memory models. People wanted the library to provide some
mechanism for abstracting memory models. Earlier versions of STL
assumed that the size of the container is expressible as an
integer of type size_t and that the distance between two
iterators is of type ptrdiff_t. And now we were told, why don't
you abstract from that? It's a tall order because the language
does not abstract from that; C and C++ arrays are not
parameterized by these types. We invented a mechanism called
"allocator," which encapsulates information about the memory
model. That caused grave consequences for every component in the
library. You might wonder what memory models have to do with
algorithms or the container interfaces. If you cannot use things
like size_t, you also cannot use things like T* because of
different pointer types (T*, T huge *, etc.). Then you cannot
use references because with different memory models you have
different reference types. There were tremendous ramifications
on the library.
The second major thing was to extend our original set of data
structures with associative data structures. That was easier,
but coming up with a standard is always hard because we needed
something which people would use for years to come for their
containers. STL has from the point of view of containers, a very
clean dichotomy. It provides two fundamental kinds of container
classes: sequences and associative containers. They are like
regular memory and content-addressable memory. It has a clean
semantics explaining what these containers do.
When I arrived at Waterloo, Bjarne spent a lot of time
explaining to me that I shouldn't be concerned, that most likely
it was going to fail, but that we did our best, we tried, and we
should be brave. The level of expectation was low. We expected
major opposition. There was some opposition but it was minor.
When the vote was taken in Waterloo, it was totally surprising
because it was maybe 80% in favor and 20% against. Everybody
expected a battle, everybody expected controversy. There was a
battle, but the vote was overwhelming.
What effect does STL have on the class libraries published in the
ANSI/ISO February 1994 working paper?
STL was incorporated into the working paper in Waterloo.
The STL document is split apart, and put in different places of
the library parts of the working paper. Mile Vilot is
responsible for doing that. I do not take active part in the
editorial activities. I am not a member of the committee but
every time an STL-related change is proposed, it is run by me.
The committee is very considerate.
Several template changes have been accepted by the committee.
Which ones have impact on STL?
Prior to the acceptance of STL there were two changes that
were used by the revised STL. One is the ability to have
template member functions. STL uses them extensively to allow
you to construct any kind of a container from any other kind of a
container. There is a single constructor that allows you to
construct vectors out of lists or out of other containers.
There is a templatized constructor which is templatized on the
iterator, so if you give a pair of iterators to a container
constructor, the container is constructed out of the elements
which are specified by this range. A range is a set of elements
specified by a pair of iterators, generalized pointers, or
addresses. The second significant new feature used in STL was
template arguments which are templates themselves, and that's how
allocators, as originally proposed, were done.
Did the requirements of STL influence any of the proposed
In Valley Forge, Bjarne proposed a significant addition to
templates called "partial specialization," which would allow many
of the algorithms and classes to be much more efficient and which
would address a problem of code size. I worked with Bjarne on
the proposal and it was driven by the need of making STL even
more efficient. Let me explain what partial specialization is.
At present you can have a template function parameterized by
class T called swap(T&, T&) and swaps them. This is the most
generic possible swap. If you want to specialize swap and do
something different for a particular type, you can have a
function swap(int&, int&), and which does integer swapping in
some different way. However it was not possible to have an
intermediate partial specialization, that is, to provide a
template function of the following form:
template <class T> void swap(vector<T>&, vector<T>&);
This form provides a special way to swap vectors. This is an
important problem from an efficiency point of view. If you swap
vectors with the most generic swap, which uses three assignments,
vectors are copied three times, which takes linear time.
However, if we have this partial specialization of swap for
vectors that swap two vectors, then you can have a fast, constant
time operation, that moves a couple of pointers in the vector
headers. That would allow sort, for example, to work on vectors
of vectors much faster. With the present STL, without partial
specialization, the only way to make it work faster is for any
particular kind of vector, such as vector<int>, to define its own
swap, which can be done but which puts a burden on the
programmer. In very many cases, partial specialization would
allow algorithms to be more effective on some generic classes.
You can have the most generic swap, a less generic swap, an even
less generic swap, and a totally specific swap. You can do
partial specialization, and the compiler will find the closest
match. Another example is copy. At present the copy algorithm
just goes through a sequence of elements defined by iterators and
copies them one by one. However, with partial specialization we
can define a template function:
template <class T> T ** copy(T**,T**,T**);
This will efficiently copy a range of pointers by using memcpy,
because when we're copying pointers we don't have to worry about
construction and destruction and we can just move bits with
memcpy. That can be done once and for all in the library and the
user doesn't need to be concerned. We can have particular
specializations of algorithms for some of the types. That was a
very important change, and as far as I know it was favorably
received in Valley Forge and will be part of the Standard.
What kinds of applications beyond the standard class libraries
are best served by STL ?
I have hopes that STL will introduce a style of programming
called generic programming. I believe that this style is
applicable to any kind of application, that is, trying to write
algorithms and data structures in the most generic way.
Specifying families or categories of such structures satisfying
common semantic requirements is a universal paradigm applicable
to anything. It will take a long time before the technology is
understood and developed. STL is a starting point for this type
Eventually we will have standard catalogs of generic
components with well-defined interfaces, with well-defined
complexities. Programmers will stop programming at the micro
level. You will never need to write a binary search routine
again. Even now, STL provides several binary search algorithms
written in the most generic way. Anything that is binary-searchable
can be searched by those algorithms. The minimum
requirements that the algorithm assumes are the only requirements
that the code uses. I hope that the same thing will happen for
all software components. We will have standard catalogs and
people will stop writing these things.
That was Doug McIlroy's dream when he published a famous
paper talking about component factories in 1969. STL is an
example of the programming technology which will enable such
component factories. Of course, a major effort is needed, not
just research effort, but industrial effort to provide
programmers with such catalogs, to have tools which will allow
people to find the components they need, and to glue the
components together, and to verify that their complexity
assumptions are met.
STL does not implement a persistent object container model. The
map and multimap containers are particularly good candidates for
persistent storage containers as inverted indexes into persistent
object databases. Have you done any work in that direction or
can you comment an such implementations?
This point was noticed by many people. STL does not
implement persistence for a good reason. STL is as large as was
conceivable at that time. I don't think that any larger set of
components would have passed through the standards committee.
But persistence is something that several people thought about.
During the design of STL and especially during the design of the
allocator component, Bjarne observed that allocators, which
encapsulate memory models, could be used to encapsulate a
persistent memory model. The insight was Bjarne's, and it is an
important and interesting insight. Several object database
companies are looking at that. In October 1994 I attended a
meeting of the Object Database Management Group. I gave a talk on
STL, and there was strong interest there to make the containers
within their emerging interface to conform to STL. They were not
looking at the allocators as such. Some of the members of the
Group are, however, investigating whether allocators can be used
to implement persistency. I expect that there will be persistent
object stores with STL-conforming interfaces fitting into the STL
framework within the next year.
Set, multiset, map, and multimap are implemented with a red-black
tree data structure. Have you experimented with other structures
such as B*trees?
I don't think that would be quite right for in-memory data
structures, but this is something that needs to be done. The same
interfaces defined by STL need to be implemented with other data
structures---skip lists, splay trees, half-balanced trees, and so
on. It's a major research activity that needs to be done because
STL provides us with a framework where we can compare the
performance of these structures. The interface is fixed. The
basic complexity requirements are fixed. Now we can have
meaningful experiments comparing different data structures to
each other. There were a lot of people from the data structure
community coming up with all kinds of data structures for that
kind of interface. I hope that they would implement them as
generic data structures within the STL framework.
Are compiler vendors working with you to implement STL into their
Yes. I get a lot of calls from compiler vendors. Pete
Becker of Borland was extremely helpful. He helped by writing
some code so that we could implement allocators for all the
memory models of Borland compilers. Symantec is going to release
an STL implementation for their Macintosh compiler. Edison
Design Group has been very helpful. We have had a lot of support
from most compiler vendors.
STL includes templates that support memory models of 16-bit MS-DOS compilers. With the current emphasis on 32-bit, flat model
compilers and operating systems, do you think that the memory-model orientation will continue to be valid?
Irrespective of Intel architecture, memory model is an
object, which encapsulates the information about what is a
pointer, what are the integer size and difference types
associated with this pointer, what is the reference type
associated with this pointer, and so on. Abstracting that is
important if we introduce other kinds of memory such as
persistent memory, shared memory, and so on. A nice feature of
STL is that the only place that mentions the machine-related
types in STL---something that refers to real pointer, real
reference---is encapsulated within roughly 16 lines of code.
Everything else, all the containers, all the algorithms, are
built abstractly without mentioning anything which relates to the
machine. From the point of view of portability, all the
machine-specific things which relate to the notion of address, pointer,
and so on, are encapsulated within a tiny, well-understood
mechanism. Allocators, however, are not essential to STL, not as
essential as the decomposition of fundamental data structures and
The ANSI/ISO C Standards committee treated platform-specific
issues such as memory models as implementation details and did
not attempt to codify them. Will the C++ committee be taking a
different view of these issues? If so, why?
I think that STL is ahead of the C++ standard from the point
of view of memory models. But there is a significant difference
between C and C++. C++ has constructors and operator new, which
deal with memory model and which are part of the language. It
might be important now to look at generalizing things like
operator new to be able to take allocators the way STL containers
take allocators. It is not as important now as it was before STL
was accepted, because STL data structures will eliminate the
majority of the needs for using new. Most people should not
allocate arrays because STL does an effective job in doing so. I
never need to use new in my code, and I pay great attention to
efficiency. The code tends to be more efficient than if I were
to use new. With the acceptance of STL, new will sort of fade
away. STL also solves the problem of deleting because, for
example, in the case of a vector, the destructor will destroy it
on the exit from the block. You don't need to worry about
releasing the storage as you do when you use new. STL can
dramatically minimize the demand for garbage collection.
Disciplined use of containers allows you to do whatever you need
to do without automatic memory management. The STL constructors
and destructors do allocation properly.
The C++ Standard Library subcommittee is defining standard
namespaces and conventions for exception handling. Will STL
classes have namespaces and throw exceptions?
Yes they will. Members of the committee are dealing with
that, and they are doing a great job.
How different from the current STL definition will the eventual
standard definition be? Will the committee influence changes or
is the design under tighter control?
It seems to be a consensus that there should not be any
major changes to STL.
How can programmers gain an early experience with STL in
anticipation of it becoming a standard?
They can download the STL header files from
butler.hpl.hp.com under /stl and use it with Borland or IBM
compiler, or with any other compiler powerful enough to handle
STL The only way to learn some style of programming is by
programming. They need to look at examples and write programs in
You are collaborating with P.J. (Bill) Plauger to write a book
about STL. What will be the emphasis of the book and when is it
scheduled to be published?
It is scheduled to be published in the summer of 1995 and is
going to be an annotated STL implementation. It will be similar
to Bill's books on the Standard C Library and the Draft Standard
C++ Library. He is taking the lead on the book, which will serve
as a standard reference document on the use of the STL. I hope to
write a paper with Bjarne that will address language/library
interactions in the context of C++/STL. It might lead to another
A lot more work needs to be done. For STL to become a
success, people should do research experimenting with this style
of programming. More books and articles need to be written
explaining how to program in this style. Courses need to be
developed. Tutorials need to be written. Tools need to be built
which help people navigate through libraries. STL is a framework
and it would be nice to have a tool with which to browse through
What is the relationship between generic programming and
In one sense, generic programming is a natural continuation
of the fundamental ideas of object-oriented programming---separating
the interface and implementation and polymorphic
behavior of the components. However, there is a radical
difference. Object-oriented programming emphasizes the syntax of
linguistic elements of the program construction. You have to use
inheritance, you have to use classes, you have to use objects,
objects send messages. Generic programming does not start with
the notion of whether you use inheritance or you don't use
inheritance. It starts with an attempt to classify or produce a
taxonomy of what kinds of things are there and how they behave.
That is, what does it mean that two things are equal? What is
the right way to define equality? Not just actions of equality.
You can analyze equality deeper and discover that there is a
generic notion of equality wherein two objects are equal if their
parts, or at least their essential parts are equal. We can have
a generic recipe for an equality operation. We can discuss what
kinds of objects there are. There are sequences. There are
operations on sequences. What are the semantics of these
operations? What types of sequences from the point of view of
complexity tradeoffs should we offer the user? What kinds of
algorithms are there on sequences? What kind of different
sorting functions do we need? And only after we develop that,
after we have the conceptual taxonomy of the components, do we
address the issue of how to implement them. Do we use templates?
Do we use inheritance? Do we use macros? What kind of language
technology do we use? The fundamental idea of generic
programming is to classify abstract software components and their
behavior and come up with a standard taxonomy. The starting
point is with real, efficient algorithms and data structures and
not with the language. Of course, it is always embodied in the
language. You cannot have generic programming outside of a
language. STL is done in C++. You could implement it in Ada.
You could implement it in other languages. They would be
slightly different, but there are some fundamental things that
would be there. Binary search has to be everywhere. Sort has to
be everywhere. That's what people do. There will be some
modification on the semantics of the containers, slight
modifications imposed by the language. In some languages you can
use inheritance more, in some languages you have to use
templates. But the fundamental difference is precisely that
generic programming starts with semantics and semantic
decomposition. For example, we decide that we need a component
called swap. Then we figure out how this particular component
will work in different languages. The emphasis is on the
semantics and semantic classification, while object-orientedness,
especially as it has evolved, places a much stronger emphasis,
and, I think, too much of an emphasis, on precisely how to
develop things, that is, using class hierarchies. OOP tells you
how to build class hierarchies, but it doesn't tell you what
should be inside those class hierarchies.
What do you see as the future of STL and generic programming?
I mentioned before the dream of programmers having standard
repositories of abstract components with interfaces that are well
understood and that conform to common paradigms. To do that
there needs to be a lot more effort to develop the scientific
underpinnings of this style of programming. STL starts it to
some degree by classifying the semantics of some fundamental
components. We need to work more on that. The goal is to
transform software engineering from a craft to an engineering
discipline. It needs a taxonomy of fundamental concepts and some
laws that govern those concepts, which are well understood, which
can be taught, which every programmer knows even if he cannot
state them correctly. Many people know arithmetic even if they
never heard of commutativity. Everybody who graduated from high
school knows that 2+5 is equal to 5+2. Not all of them know that
it is a commutative property of addition. I hope that most
programmers will learn the fundamental semantic properties of
fundamental operations. What does assignment mean? What does
equality mean? How to construct data structures.
At present C++ is the best vehicle for this style of
programming. I have tried different languages and I think that
C++ allows this marvelous combination of abstractness and
efficiency. However, I think that it is possible to design a
language based on C and on many of the insights that C++ brought
into the world, a language which is more suitable to this style
of programming, which lacks some of the deficiencies of C++, in
particular its enormous size. STL deals with things called
concepts. What is an iterator? Not a class. Not a type. It is
a concept. (Or, if we want to be more formal, it is what Bourbaki
calls a structure type, what logicians call a theory, or what
type theory people call a sort.) It is something which doesn't
have a linguistic incarnation in C++. But it could. You could
have a language where you could talk about concepts, refine them,
and then finally form them in a very programmatic kind of way
into classes. (There are, of course, languages that deal with
sorts, but they are not of much use if you want to sort.) We
could have a language where we could define something called
forward iterator, which is just defined as a concept in STL---it
doesn't have a C++ incarnation. Then we can refine forward
iterator into bidirectional iterator. Then random iterator can
be refined from that. It is possible to design a language which
would enable even far greater ease for this style of programming.
I am fully convinced that it has to be as efficient and as close
to the machine as are C and C++. And I do believe that it is
possible to construct a language that allows close approximation
to the machine on the one hand and has the ability to deal with
very abstract entities on the other hand. I think that
abstractness can be even greater than it is in C++ without
creating a gap between underlying machines. I think that generic
programming can influence language research and that we will have
practical languages, which are easy to use and are well suited
for that style of programming. From that you can deduce what I
am planning to work on next.
Copyright © 1995 Dr. Dobb's Journal
1996 Silicon Graphics, Inc. All Rights Reserved.