sdl@MITRE-BEDFORD.ARPA (Litvintchouk) (08/17/86)
If I have read the ADA Reference Manual correctly, it seems that
ADA(r) does support inheritance of the form used in Smalltalk-80 and
other object oriented languages. The inheritance occurs when one
declares a new derived type which then inherits all the operations
already existing for the type from which it was derived.
It's not quite that simple. One problem is what happens to the
"instance variables" of the class (type); that is, the "private
memory" or internal implementation of each instance of the class.
In Smalltalk-80, a subclass inherits everything from its parent
superclass, including its instance variables. For example, if there
were a class STACK implemented as an array, any subclass of STACK
would also inherit that implementation. You could then add additional
operations (messages) that manipulate that implementation, as
further refinements to the subclass.
In Ada, on the other hand, if you define an abstract data type (ADT)
in a package specification, you must make its implementation visible,
if you want it to be inherited by any type derived from that ADT.
(That violates the principle of information hiding, unfortunately.)
You cannot make the ADT a private type, because then its
implementation is not accessible from any derived type. Ada has this
unfortunate conflict between encapsulation (via private types) and
full inheritance. Smalltalk doesn't need encapsulation to prevent
access to the "private memory" or internal implementation of an
object; there simply are no language constructs to directly access an
object's private memory; all you can do is send messages to the object
that it can decide how to deal with.
One thing you could do is create an "inheritance hierarchy" of derived
types, all of which had visible implementations, and then create a
final set of private types (whose implementations were derived from
these inherited types) for use by end-users. But this would require
having two types (one private and one visible) for every type in the
hierarchy; a clumsy solution at best.
Steven Litvintchouk
MITRE Corporation
Burlington Road
Bedford, MA 01730
(617)271-7753
ARPA: sdl@mitre-bedford
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