Despite the widespread adoption of object-oriented terminology in
the various areas of computer science and software
development practice, there is considerable confusion about
the precise meaning of the terms employed and the (true)
nature of object-oriented computing.
In an attempt to resolve this confusion, [Wegner87]
(in the landmark paper Dimensions of object-based language design)
introduces the distinction between object-based
and object-oriented.
See slide [5-object-based].
This distinction comes down to, roughly, the distinction between
languages providing only encapsulation (object-based) or encapsulation plus
inheritance (object-oriented). See section [object-based].
Another issue in the debate about object orientation
is the relation between classes
and types.
[Wegner87] concludes that the notions of objects, classes
and inheritance (that constitute the classical object model)
are highly interrelated, and instead
proposes an orthogonal approach by outlining the various dimensions
along which to design an object-oriented language.
These dimensions may be characterized by the phrases: objects,
types, delegation and abstraction.
In this section we will look at the arguments presented in [Wegner87]
in somewhat more detail.
Also, we will look at the viability of combining seemingly
disparate paradigms (such as the logic programming paradigm)
with the object-oriented language paradigm.
In the sections that follow, we will discuss some alternatives and extensions to
the object model.
Object Oriented Language Design
\nl
{\footnotesize
object: state + operations\n
class: template for object creation\n
inheritance: super/base and subclasses
}
[D
object-\=\+oriented =
objects + classes + inheritance
D]
data abstraction
-- state accessible by operations
strong typing
-- compile time checking
slide: Object-based versus object-oriented
Object-based versus object-oriented
How would you characterize Ada83? See [Barnes94].
Is Ada object-oriented?
And Modula-2?
See [Wirth83].
The answer is no and no.
And Ada9X and Modula-3?
See [Barnes94] and [Card89].
The answer is yes and yes.
In the past there has been some confusion
as to when to characterize a language as
object-oriented.
For example, [Booch86] characterizes Ada as
object-oriented and motivates this by saying that Ada
can be used as an implementation language in
an object-oriented approach to program development.
Clearly, Ada supports some notion of objects (which
are defined as packages).
However, although Ada supports objects and generic descriptions
of objects (by generic packages), it does not support
code sharing by inheritance.
In a later work, [Booch91] revises his original (faulty)
opinion, in response to
[Wegner87], who proposed considering inheritance
as an essential characteristic of object orientation.
Similarly, despite the support that Modula-2 offers
for defining (object-like) abstract data types,
consisting of
an interface specification and an implementation
(which may be hidden), Modula-2 does not support
the creation of derived (sub)types that share
the behavior of their base (super)type.
See also section [adt-modules].
Classes versus types
Another confusion that frequently arises
is due to the ill-defined relationship between
the notion of a class and the notion of a type.
The notion of types is already familiar
from procedural programming languages such as
Pascal and (in an ill-famed way) from C.
The type of variables and functions may be profitably
used to check for (syntactical) errors.
Strong static type checking may prevent errors
ranging from simple typos to using undefined
(or wrongly defined) functions.
The notion of a class originally has a more
operational meaning.
Operationally, a class is a template for object creation.
In other words, a class is a description of the collection
of its instances, that is the objects that are
created using the class description as a recipe.
Related to this notion of a class, inheritance was
originally defined as a means to share (parts of)
a description.
Sharing by (inheritance) derivation is, pragmatically,
very convenient.
It provides a more controlled way of code sharing
than, for example, the use of macros and file inclusion
(as were popular in the C community).
Since [Wegner87] published his original analysis
of the dimensions of object-oriented language design,
the phrase object-oriented has been commonly understood
as involving objects, classes and inheritance.
This is the traditional object model as embodied
by Smalltalk and, to a large extent, by Eiffel and C++.
However, unlike Smalltalk, both Eiffel and C++ have also been
strongly influenced by the abstract data type approach
to programming.
Consequently, in Eiffel and C++ classes have been identified
with types and derivation by inheritance with subtyping.
Unfortunately, derivation by inheritance need not necessarily
result
in the creation of proper subtypes,
that is classes whose instances conform to the behavior
specified by their base class.
In effect, derived classes may be only distantly related
to their base classes when inheritance is only
used as a code sharing device.
For example, a window manager class may inherit
from a list container class (an idiom used in Meyer, 1988).
For those inclined to a more formal approach,
the phrase object-oriented has gradually acquired
the meaning of abstract data types
and polymorphism.
From the perspective of language design, however,
this characterization merely serves as a constraint
that (the application of) particular constructs must satisfy
and as an invitation to explore language constructs
that enable the use of abstractions and flexible
behavioral specifications.
Towards an orthogonal approach {\em -- type extensions}
According to [Wegner87], much of the confusion
around the various features of object-oriented
programming languages arises from the fact that
these features are largely interdependent,
as for instance the notion of object and class on
the one hand,
and the notion of class and inheritance on the other.
To resolve this confusion, [Wegner87] proposes
a more orthogonal approach to characterize the
various features of object-oriented languages,
according to dimensions that are to a large extent independent.
See slide [5-orthogonal].
Orthogonal approach
\ldeno{}{\footnotesize
objects -- {\em modular computing \c{agents}}\n
types -- expression classification\n
delegation -- resource sharing\n
abstraction -- {\em interface \c{specification}}\n
}\nl
type: behavior specification\n
delegation: ancestors \c{(with dynamic self-reference)}
\nl\ifsli{\vspace{-1cm}}{}
slide: Orthogonal dimensions
The features that constitute an object-oriented
programming language in an orthogonal way are,
according to [Wegner87]: objects, types,
delegation and abstraction.
Objects
are in essence modular computing agents.
They correspond to the need for encapsulation in design,
that is the construction of modular units to
which a principle of locality applies (due to
combining data and operations).
Object-oriented languages may, however, differ in
the degree to which they support encapsulation.
For example, in a distributed environment a
high degree of encapsulation must be offered,
prohibiting attempts to alter global variables
(from within an object) or local instance
variables (from without).
Moreover, the runtime object support system must
allow for what may best be called remote method invocation.
As far as parallel activity is concerned, only a few languages provide
constructs to define concurrently active objects.
See chapter [communication] for a more detailed discussion.
Whether objects support reactiveness,
that is sufficient flexibility to respond safely
to a message, depends largely upon (program) design.
[Meyer88], for instance, advocates a {\em shopping list
approach} to designing the interface of an object,
to allow for a high degree of (temporal) independence
between method calls.
Types
may be understood as a mechanism for expression classification.
From this perspective, Smalltalk may be regarded
as having a dynamic typing system:
dynamic, in the sense that the inability to evaluate an expression
will lead to a runtime error.
The existence of types obviates the need to have classes,
since a type may be considered as a more abstract description of the
behavior of an object.
Furthermore, subclasses (as may be derived through inheritance)
are more safely defined as subtypes in a polymorphic
type system. See section [flavors].
At the opposite side of the type dimension we find the statically typed languages,
that allow us to determine the type of
the designation of a variable at compile-time.
In that case, the runtime support system need not carry any type information,
except a dispatch table to locate virtual functions.
Delegation
(in its most generic sense)
is a mechanism for resource sharing.
As has been shown in [Lieber], delegation subsumes inheritance,
since the resource sharing effected by inheritance may easily be mimicked
by delegating messages to the object's ancestors
by means of an appropriate dispatching mechanism.
In a narrower sense, delegation is usually understood as a more
dynamic mechanism that allows the redirection of control dynamically.
In addition, languages supporting dynamic delegation
(such as Self, see section [prototypes]) do
not sacrifice dynamic self-reference.
This means that when the object executing
a method refers to itself, the actual context
will be the delegating object.
See section [prototypes] for a more detailed discussion.
In contrast, inheritance (as usually understood in the context of classes)
is a far more static mechanism.
Inheritance may be understood as (statically) copying the code
from an ancestor class to the inheriting class
(with perhaps some modifications),
whereas delegation is truly dynamic in that messages are dispatched
to objects that have a life-span independent of the dispatching object.
Abstraction
(although to some extent related to types)
is a mechanism that may be independently applied to provide
an interface specification for an object.
For example, in the presence of active objects
(that may execute in parallel)
we may need to be able to restrict dynamically the interface of an object as specified by its type
in order to maintain the object in a consistent state.
Also for purely sequential objects we may impose a particular protocol of interaction
(as may, for example, be expressed by a contract) to be able to
guarantee correct behavior.
Another important aspect of abstraction is protection.
Object-oriented languages may provide (at least) two kinds
of protection.
First, a language may have facilities to
protect the object from illegal access by a client (from without).
This is effected by annotations such as private and protected.
And secondly, a language may have facilities to protect the object
(as it were from within) from illegal access through delegation
(that is by instances of derived object classes).
Most languages support the first kind of protection.
Only few languages, however (among which are C++), support the second kind too.
The independence of abstraction and typing may further
be argued by pointing out that languages
supporting strong typing need not
enforce the use of abstract data types having a well-defined
behavior.
Discussion
Currently there is no single language providing
what in any sense can be called an ideal or
optimal combination of features.
At least there is no consensus as to whether
Smalltalk, Eiffel, C++ or any of the other
languages mentioned is the likely candidate
for this position.
A critique on many of these languages is that they
contain too many features or try to serve too many goals.
Think of Smalltalk's meta classes (see section [meta]),
Eiffel's attempt to be satisfying both from
a practical and formal point of view
(which severely limits the power of the assertion mechanism)
and the intricate type construction and coercion
operators of C++ (which make it flexible, but complex).
An interesting example of what may be called
minimal language design
is presented by the language Oberon
that features so-called type extensions
encompassing (in a minimal way) both the notion
of object and class inheritance.
The Oberon language has been developed
as the (object-oriented) successor of Modula-2.
According to [BPS89], simplicity was the prime goal
of its design.
Oberon is a procedural language (like Modula-2)
with strong static type checking.
It provides only simple data types, arrays and records.
A class concept is lacking.
Oberon, however, supports records that may
contain (so-called installed) procedures.
And, more importantly, records can be extended.
Extension works like inheritance,
in that a record type can be defined that
includes all the components of the extended
(read inherited) record type.
In Oberon a record that is the result
of extending a given record type
is called compatible with the original
record type,
which means that it may be assigned to a variable holding
a record of that (base) type.
When such an assignment is made, the extended
record is projected onto the original
record (type),
ignoring the fields that are added due to the extension.
In a similar way as Modula-2,
Oberon supports separate compilation by requiring
a definition part (containing an interface specification)
and an implementation part.
Modules may have private classes (that is records),
just by omitting them from the interface specification.
Oberon allows the dynamic creation of (record) objects.
A garbage collector takes care of their elimination.
As far as inheritance is concerned, Oberon only supports single
inheritance.
In other words, record extension only operates
on a single (base) record.
Moreover, inheritance in Oberon is strict,
due to the restriction that no overriding is allowed.
Viewed from the principle small is beautiful,
Oberon indeed is a very nice language, containing a
minimal set of features to support object-oriented
programming.
However, from a programming perspective,
as a vehicle to implement concrete data types
(see section [canonical]) it leaves
much to be desired.
Not surprising, then, that even purported
object-oriented extensions to Oberon have been proposed.
See [BPS89] for a further discussion.
Multi-paradigm languages
Object-oriented programming has evolved as a new and
strong paradigm of programming.
Has it?
Of the languages mentioned, only Smalltalk
has what may be considered a radically new language design
(and to some extent also the language Self, that we will
discuss in the next section).
Most of the other languages, including Eiffel, C++
(and for that matter also CLOS and Oberon),
may be considered as object-oriented extensions of already
existing languages, or to put it more broadly, language
paradigms.
Most popular are, evidently, object-oriented extensions
based on procedural language paradigms,
closely followed by the (Lisp-based) extensions of the
functional language paradigm.
Less well-known are extensions based on the logic programming
paradigm, of which DLP is my favorite example.
In [Wegner92], it is argued that the logic programming
paradigm does not fit in with an object-oriented approach.
I strongly disagree with this position.
However, the arguments given in [Wegner92] to defend it
are worthwhile, in that they make explicit what desiderata
we may impose on object-oriented languages.
Remaining within the confines of a classical object model,
the basic ingredients for an object-oriented extension
of any language (paradigm) are:
objects, classes and inheritance.
Although the exact meaning of these notions is
open for discussion,
language designers seem to have no difficulty in
applying these concepts to extend (or design) a programming language.
}
Open systems
- reactive -- {\em \c{flexible (dynamic)} choice of actions}
- modular -- {\em (static) scalability}
Dimensions of modularity
- encapsulation boundary -- {\em interface \c{to client}}
- distribution boundary -- {\em \c{visibility} from within \c{objects}}
- concurrency boundary -- {\em threads \c{per object, synchronization}}
slide: Dimensions of modularity
According to [Wegner92],
the principal argument against combining logic programming
and object-oriented programming is that such a combination
does not support the development of open systems
without compromising the logical nature of logic programming.
Openness may be considered as one of the prime goals of
object-orientation.
See slide [5-open].
A software system is said to be open
if its behavior can be easily modified and extended.
[Wegner92] distinguishes between two mechanisms to achieve
openness; dynamically through reactiveness,
and statically through modularity.
Reactiveness allows a program to choose dynamically between
potential actions.
For sequential object-oriented languages, late binding
(that is, the dispatching mechanism underlying virtual function calls)
is one of the mechanisms used to effect
the dynamic selection of alternatives.
Concurrent object-oriented languages usually offer an additional
construct, in the form of a guard
or accept statement, to determine dynamically which method call
to answer.
In both cases, the answer depends upon the nature of the object
and (especially in the latter case) the state of the object
(and its willingness to answer).
Openness through modularity means that a system can safely be extended by adding
(statically) new components.
The issue of openness in the latter sense is immediately related
to the notion of scalability,
that is the degree to which a particular component
can be safely embedded in a larger environment
and extended to include new functionality.
At first sight, classes and inheritance strongly contribute
to achieving such (static) openness.
However, there is more to modularity than the encapsulation
provided by classes only.
From a modeling perspective, encapsulation (as provided
by objects and classes) is the basic mechanism to define the
elements or entities of a model.
The declarative nature of an object-oriented approach
resides exactly in the opportunity to define such entities
and their relations through inheritance.
However, encapsulation (as typically understood in the
context of a classical object model) only provides
protection from illegal access from without.
As such, it is a one-sided boundary.
The other side, the extent to which the outside world
is visible for the object (from within),
may be called the distribution boundary.
Many languages, including Smalltalk and C++,
violate the distribution boundary by allowing
the use of (class-wide) global variables.
(See also section [meta].)
Evidently, this may lead to problems when objects reside on distinct
processors, as may be the case in distributed systems.
Typically, the message passing metaphor
(commonly used to characterize the interaction between objects)
contains the suggestion that objects may be physically
distributed (across a network of processors).
Also (because of the notion of encapsulation),
objects are often regarded as autonomous entities,
that in principle may have independent activity.
However, most of the languages mentioned do not (immediately)
fulfill the additional requirements needed for actual physical
distribution or parallel (multi-threaded) activity.
In section [active] we will look at these issues in more detail.
Object-oriented logic programming
Logic programming is often characterized as relational
programming, since it allows the exhaustive
exploration of a search space defined by logical relations
(for instance, by backtracking as in Prolog).
The advantage of logic programming, from a modeling point of view,
is that it allows us to specify in a logical manner
(that is by logical clauses) the relations between
the entities of a particular domain.
A number of efforts to combine logic programming with
object-oriented features have been undertaken,
among which is the development of the language Vulcan
(that was mentioned in section [classification]).
Vulcan is based on the Concurrent Prolog language
and relies on a way of implementing objects as
perpetual processes.
Without going into detail, the idea (originally proposed in
[ST83]) is that an object may be implemented as
a process defined by one or more (recursive) clauses.
An object may accept messages in the form of a predicate call.
The state of an object is maintained by parameters of the
predicate, which are (possibly modified by the method call)
passed to the recursive invocation of one of the clauses
defining the object.
To communicate, an object (defined as a process)
waits until a client asks for the execution of a method.
The clauses defining the object are then evaluated to
check which one is appropriate for that particular method call.
If there are multiple candidate clauses, one is selected and evaluated.
The other candidate clauses are discarded.
Since the clauses defining an object are recursive, after the evaluation
of a method the object is ready to accept another message.
The model of (object) interaction supported by Concurrent Prolog
requires fine-grained concurrency, which is possible
due to the side-effect free nature of logical clauses.
However, to restrict the number of processes created during the evaluation of a goal,
Concurrent Prolog enforces a committed choice between
candidate clauses, thus throwing away alternative solutions.
[Wegner92] observes, rightly I think,
that the notion of committed choice is in conflict
with the relational nature of logic programming.
Indeed, Concurrent Prolog absolves logical completeness
in the form of backtracking, to remain within the confines
of the process model adopted.
[Wegner92], however, goes a step further and states
that reactiveness and backtracking are irreconcilable features.
That these features may fruitfully be incorporated in a single
language framework is demonstrated by the language DLP.
However, to support backtracking and objects,
a more elaborate process model is needed than
the process model supported by Concurrent Prolog
(which in a way identifies an object with a process).
With such a model (that will be sketched in appendix D),
there seems to be no reason to be against the marriage
of logic programming and object orientation.