* Re: Integrating concurrent & O-O programming
1989-11-21 4:09 ` Tim Atkins
@ 1989-11-22 22:08 ` 3929
1989-11-22 22:37 ` Dan Weinreb
` (3 subsequent siblings)
4 siblings, 0 replies; 9+ messages in thread
From: 3929 @ 1989-11-22 22:08 UTC (permalink / raw)
In article <1667@osc.COM> tma@osc.UUCP (Tim Atkins) writes:
>I have heard this before and, frankly, I don't get it. The subclass
>has inherited data structure definition from the parent class. Therefore
>its instances will contain instances of the parent data structure. This
>is totally local to the object! Only the definition was inherited. Why
>is this a bad thing? Why does it break encapsulation? I understood the
>encapsulation wrt data provided by OO techniques to simply state that instance
>data should not be accessed directly but only through a method defined on
>the class. Therefore, inheritance does not seem to break encapsulation.
>
>I would appreciate it if someone could set me straight here.
As I understand your question, here is (one of) the rub(s):
Class A inherits from class X. In many OO programming languages if not
most that means that A gets to access X's local data items. Thus, if X
had a data item 'foo', code from class A could directly access 'foo'.
This means that class doesn't just depend upon the interface of X but
also depends upon the implementation of X. This means that if X changes,
A may have to change (and anything which is a descendant of X including
all of A's descendants). This tends to make those classes near the root
of an inheritance heirarchy become "rigid" very quickly, since changes
can "ripple" down the heirarchy. This is viewed as a bad thing since the
whole point of OO is to avoid "ripples" which propagate through a software
system.
Note that some languages help provide solutions for this. For example, C++
has the concepts of public, protected, and private items in a class, to help
in restricting access to data members. When this is combined with the
ability to inline functions, you can provide interface routines for every
data member that is meant to be accessable in an efficient manner and thus
hide the implementation from sub-classes without taking a major performance
hit. (By the way, I'm still not totally sold on C++ but this aspect is
certainly nice).
Thanks,
--------------------------------------------------------------------------------
Brian R. Gilstrap ...!{ texbell, uunet }!swbatl!uucibg OR uucibg@swbatl.UUCP
One Bell Center +----------------------------------------------------------
Rm 17-G-4 | "Winnie-the-Pooh read the two notices very carefully,
St. Louis, MO 63101 | first from left to right, and afterwards, in case he had
(314) 235-3929 | missed some of it, from right to left." -- A. A. Milne
--------------------------------------------------------------------------------
Disclaimer:
Me, speak for my company? You must be joking. I'm just speaking my mind.
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Integrating concurrent & O-O programming
1989-11-21 4:09 ` Tim Atkins
1989-11-22 22:08 ` 3929
@ 1989-11-22 22:37 ` Dan Weinreb
1989-11-23 2:52 ` Ziaul Masum Hasan
` (2 subsequent siblings)
4 siblings, 0 replies; 9+ messages in thread
From: Dan Weinreb @ 1989-11-22 22:37 UTC (permalink / raw)
In article <1667@osc.COM> tma@osc.COM (Tim Atkins) writes:
In article <7062@hubcap.clemson.edu> wtwolfe@hubcap.clemson.edu (Bill Wolfe) writes:
> ...consider the interference that occurs between class inheritance
> and encapsulation when subclasses are allowed to access freely the
> instance variables of the parent class [Sny86]. In this case we may
> say that support for inheritance diminishes the degree of encapsulation
> that was achieved without inheritance...
I have heard this before and, frankly, I don't get it.
There is definitely a degree of truth in what Wolfe says. To get down
to brass tacks, the problem arises if I write a base class B, and you
make from it a derived class D. (In other jargon, D inherits from B.)
Your function members (methods) on D freely use the data members
(instance variables) in B. Fine. Time passes. I now go back and
modify B, either to add new features or improve performance. In so
doing, I change the internal guts of the implementation of B, which
are supposedly hidden by virtue of encapsulation. This is OK for
outside "users" of B. However, D has "inside information": it knows
about the raw internal guts of B, namely the data members (instance
variables). So my changes to B, which are supposed to be invisible
and hidden by the encapsulation, break D.
Smalltalk-80 really has this problem, and there is no language
mechanism in Smalltalk-80 that deals with this. (I was told this
by one of the great Smalltalk-80 experts.)
C++ provides a feature specifically intended to address this
phenomenon, namely "protected", which allows the writer of B to
specifically distinguish between those data members that are hidden to
writers of derived classes, and those that are visible. This seems to
me like a promising approach. Only many years of experience with many
megalines of code will tell us all how well this works. Perhaps there
are other languages out there in widespread use that try to deal with
this problem.
Dan Weinreb Object Design, Inc. dlw@odi.com
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Integrating concurrent & O-O programming
1989-11-21 4:09 ` Tim Atkins
1989-11-22 22:08 ` 3929
1989-11-22 22:37 ` Dan Weinreb
@ 1989-11-23 2:52 ` Ziaul Masum Hasan
1989-11-23 9:46 ` Re^2: " NIERSTRASZ Oscar
1989-11-23 14:56 ` BJORNERSTEDT Anders
4 siblings, 0 replies; 9+ messages in thread
From: Ziaul Masum Hasan @ 1989-11-23 2:52 UTC (permalink / raw)
In article <1667@osc.COM> tma@osc.UUCP (Tim Atkins) writes:
>In article <7062@hubcap.clemson.edu> wtwolfe@hubcap.clemson.edu (Bill Wolfe) writes:
>> ...consider the interference that occurs between class inheritance
>> and encapsulation when subclasses are allowed to access freely the
>> instance variables of the parent class [Sny86]. In this case we may
>> say that support for inheritance diminishes the degree of encapsulation
>> that was achieved without inheritance...
>
>
>I have heard this before and, frankly, I don't get it. The subclass
>has inherited data structure definition from the parent class. Therefore
>its instances will contain instances of the parent data structure. This
>is totally local to the object! Only the definition was inherited. Why
>is this a bad thing? Why does it break encapsulation? I understood the
>encapsulation wrt data provided by OO techniques to simply state that instance
>data should not be accessed directly but only through a method defined on
>the class. Therefore, inheritance does not seem to break encapsulation.
>
>I would appreciate it if someone could set me straight here.
>
>- Tim
Aren't you confusing runtime behavior with compilation time?
Encapsulation hides representation.
Accessing instance variables means you can directly use or refer to that
variable in any method that you define in the subclass (S) of a class (C).
In other words, the scope of this variable defined in C spans up to the
subclass. Of course, this violates encapsulation (or information hiding).
If the representation of the instance variable is changed in C (say, from
array to linked list), all the methods in all the subclasses down the class
hierarchy that used that variable directly have to be rewritten and recompiled.
Had we used method to access (refer) that variable, modification in one class
was sufficient.
Languages vary in defining what to inherit.
In smalltalk subclasses inherit both instance variables and methods. Any method
can directly use any inherited instance variable. But any other class that is
not a suclass of a class C, but simply uses C by creating an instance of it, can
access variables of C only through methods.
In some other languages a class can access instance vriables of any other
class only through methods.
C++ has a flexible mechanism in defining what to inherit i.e., public, private, friend constructs.
Masum Hasan
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re^2: Integrating concurrent & O-O programming
1989-11-21 4:09 ` Tim Atkins
` (2 preceding siblings ...)
1989-11-23 2:52 ` Ziaul Masum Hasan
@ 1989-11-23 9:46 ` NIERSTRASZ Oscar
1989-11-23 16:18 ` BJORNERSTEDT Anders
1989-11-23 14:56 ` BJORNERSTEDT Anders
4 siblings, 1 reply; 9+ messages in thread
From: NIERSTRASZ Oscar @ 1989-11-23 9:46 UTC (permalink / raw)
tma@osc.COM (Tim Atkins) writes:
>In article <7062@hubcap.clemson.edu> wtwolfe@hubcap.clemson.edu (Bill Wolfe) writes:
>> ...consider the interference that occurs between class inheritance
>> and encapsulation when subclasses are allowed to access freely the
>> instance variables of the parent class [Sny86]. In this case we may
>> say that support for inheritance diminishes the degree of encapsulation
>> that was achieved without inheritance...
>I have heard this before and, frankly, I don't get it. The subclass
>has inherited data structure definition from the parent class. Therefore
>its instances will contain instances of the parent data structure. This
>is totally local to the object! Only the definition was inherited. Why
>is this a bad thing? Why does it break encapsulation? I understood the
>encapsulation wrt data provided by OO techniques to simply state that instance
>data should not be accessed directly but only through a method defined on
>the class. Therefore, inheritance does not seem to break encapsulation.
>I would appreciate it if someone could set me straight here.
At the level of objects, encapsulation protects the client of an object
from needing to know details of the object's representation. One simply
"sends messages", i.e., there is a well-defined interface between the
client and the object. Modifications to the representation should not affect
the client (the service may change somewhat, but the client's code is
still valid).
In object-oriented languages, there are (at least) *two* clients to
object classes: the clients of the object instances, and subclasses
that wish to inherit the code. If you view inheritance as purely a
machanism for share code and interfaces within a collection of object
classes (i.e., if you are a lone programmer) then there is no reason
why subclasses should not see everything they inherit. But if you are
implementing a class library for use by other programmers, or if you
wish yourself to make use (reuse) by inheritance of existing classes,
you would like to protect subclasses from modifications made to
superclasses. That is why some languages permit subclasses to make
use only of instance variables and methods explicitly exported.
What aggravates the situation is that in order to implement useful
subclasses, you often *want* subclass instances to access (hidden)
inherited instance variables and private methods. This means that either:
1. the implementor of a subclass must completely understand not only
the source code of the superclass, but also everything inherited
by the superclass from *its* superclasses etc.
(deciphering the "yoyo" effect in the presence of self and
super can be like trying to read a program loaded with gotos)
or:
2. the interface of the superclass to inheriting subclasses must be
specified as completely as the interface of an object to
its clients.
We would prefer solution 2, but unfortunately we don't know how to do this
very well, although we try sometimes to make do with natural language.
("If you inherit from this class, make sure you don't modify variable x,
unless you ...")
Finally, when we have "concurrent objects", i.e., objects that make
use of concurrency and synchronization mechanisms in their methods,
subclasses inheriting such code must be careful about how they
extend the superclass behaviour to avoid nasty surprises.
Our experience in designing and implementing a concurrent object-oriented
language ("Hybrid" -- see OOPSLA '87) revealed that concurrency mechanisms
that seem quite reasonable at the object level, can pose difficulties
in the presence of inheritance and composition (e.g., using a concurrent
object as an instance variable can create unexpected results). Although
the conflict between concurrency mechanisms and object-oriented features
such as inheritance has been noted by many researchers, clean integration
is still an open problem.
Oscar Nierstrasz
---------------------------------------------------------------------
Dr. O.M. Nierstrasz E-mail: oscar@cuisun.unige.ch
Centre Universitaire d'Informatique Tel: 41 (22) 787.65.80
12 rue du Lac, CH-1207 Geneva Fax: 41 (22) 735.39.05
SWITZERLAND Home: 41 (22) 733.95.68
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^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Re^2: Integrating concurrent & O-O programming
1989-11-23 9:46 ` Re^2: " NIERSTRASZ Oscar
@ 1989-11-23 16:18 ` BJORNERSTEDT Anders
1989-11-26 14:04 ` Markku Sakkinen
0 siblings, 1 reply; 9+ messages in thread
From: BJORNERSTEDT Anders @ 1989-11-23 16:18 UTC (permalink / raw)
In article <477@cui.unige.ch> oscar@cui.unige.ch (NIERSTRASZ Oscar) writes:
>In object-oriented languages, there are (at least) *two* clients to
>object classes: the clients of the object instances, and subclasses
>that wish to inherit the code. If you view inheritance as purely a
Another interface is the interface to other instances of the same class.
Try implementing the class integer with a method add in Smalltalk
without exposing your internal representation.
You can actually distinguish four different interfaces:
1- The interface to self of the same class. No encapsulation wanted here :-)
2- The interface to self of some other (super)class.
3- The interface to some other instance of the same class.
4- The interface to some other instance of some other class.
In the programming language PAL of the Avance prototype (see oopsla88)
we made no distinction between 2 and 3 and had three kinds of operations.
Public operations could be used by 1-4.
Private operations could be used by 1-3.
Hidden operations and instance variables could be used by 1.
The concept of a "friend functions" in C++ is related to this and further
subdivides 4. [I am not a c++ expert though so I may be wrong].
>2. the interface of the superclass to inheriting subclasses must be
> specified as completely as the interface of an object to
> its clients.
>We would prefer solution 2, but unfortunately we don't know how to do this
>very well, although we try sometimes to make do with natural language.
Kind 2 operations give you exactly this.
Kind 3 operations allow you to implement an "algebra" without exposing the
internals of objects.
If you have a distributed system then the distinction of kind 2 and 3
operations becomes important for another reason. Directly accessing
non-local state may not be practical. Some people argue that inheritance
is not useful or even "inconsistent" with distribution. Kind 2 operations
at least makes inheritance "consistent" with distribution. Whether it
is useful can be debated. Still if you drop inheritance from the object
model of a distributed system, operations of kind 3 become important.
For further arguments on this last issue you can read the article on
Emerald in IEEE transactions on software engineering January 1987.
---------------------------------------------------------------------
Anders Bjornerstedt E-mail: anders@cuisun.unige.ch
Centre Universitaire d'Informatique
12 rue du Lac, CH-1207 Geneva
---------------------------------------------------------------------
Tel: 41 (22) 787.65.80-87 Fax: 41 (22) 735.39.05
Home: 41 (22) 735.00.03 Telex: 423 801 UNI CH
---------------------------------------------------------------------
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Re^2: Integrating concurrent & O-O programming
1989-11-23 16:18 ` BJORNERSTEDT Anders
@ 1989-11-26 14:04 ` Markku Sakkinen
0 siblings, 0 replies; 9+ messages in thread
From: Markku Sakkinen @ 1989-11-26 14:04 UTC (permalink / raw)
In article <481@cui.unige.ch> anders@cuisun.unige.ch writes:
>In article <477@cui.unige.ch> oscar@cui.unige.ch (NIERSTRASZ Oscar) writes:
>
>>In object-oriented languages, there are (at least) *two* clients to
>>object classes: the clients of the object instances, and subclasses
>>that wish to inherit the code. If you view inheritance as purely a
>
>Another interface is the interface to other instances of the same class.
>Try implementing the class integer with a method add in Smalltalk
>without exposing your internal representation.
>
>You can actually distinguish four different interfaces:
>
>1- The interface to self of the same class. No encapsulation wanted here :-)
>
>2- The interface to self of some other (super)class.
>
>3- The interface to some other instance of the same class.
>
>4- The interface to some other instance of some other class.
>
>In the programming language PAL of the Avance prototype (see oopsla88)
>we made no distinction between 2 and 3 and had three kinds of operations.
>Public operations could be used by 1-4.
>Private operations could be used by 1-3.
>Hidden operations and instance variables could be used by 1.
I think this is very important and interesting (because I have myself
thought about these matters :-) ).
Note that Smalltalk-80 (TM) and some other OOPL's make no difference
between 3 and 4 nor between 1 and 2,
while perhaps the majority of OOPL's (as well as CLU, Modula-2,
and Ada(R) - for which case 2 is vacuous because they have no inheritance)
make no difference between 1 and 3.
Case 2 can be further subdivided on the basis of whether the other
class is a _sub_ or _super_ class of the class in question.
(In many models of multiple inheritance, classes may even get
indirectly connected via a common subclass.)
Rather many OOPL's allow a class designer to specify that some features
shall not be visible to _sub_classes, but I can't remember reading about any
language that allows the same toward _super_classes.
>The concept of a "friend functions" in C++ is related to this and further
>subdivides 4. [I am not a c++ expert though so I may be wrong].
No, you are right.
> [...]
Markku Sakkinen
Department of Computer Science
University of Jyvaskyla (a's with umlauts)
Seminaarinkatu 15
SF-40100 Jyvaskyla (umlauts again)
Finland
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Integrating concurrent & O-O programming
1989-11-21 4:09 ` Tim Atkins
` (3 preceding siblings ...)
1989-11-23 9:46 ` Re^2: " NIERSTRASZ Oscar
@ 1989-11-23 14:56 ` BJORNERSTEDT Anders
4 siblings, 0 replies; 9+ messages in thread
From: BJORNERSTEDT Anders @ 1989-11-23 14:56 UTC (permalink / raw)
In article <1667@osc.COM> tma@osc.UUCP (Tim Atkins) writes:
>In article <7062@hubcap.clemson.edu> wtwolfe@hubcap.clemson.edu (Bill Wolfe) writes:
>> ...consider the interference that occurs between class inheritance
>> and encapsulation when subclasses are allowed to access freely the
>> instance variables of the parent class [Sny86]. In this case we may
>> say that support for inheritance diminishes the degree of encapsulation
>> that was achieved without inheritance...
>
>I have heard this before and, frankly, I don't get it. The subclass
>has inherited data structure definition from the parent class. Therefore
>its instances will contain instances of the parent data structure. This
>is totally local to the object! Only the definition was inherited. Why
>is this a bad thing? Why does it break encapsulation?
It does break encapsulation.
Whether or not this is desirable is another matter.
If you make the distinction between specification and implementation
of classes and considder inheritance of specifications as the
primary thing then you do want encapsulation. But if inheritance of
implementation (including representation) is the primary thing then you
dont want encapsulation. Note: You may have both inherritance of specification
and inheritance of implementation at the same time, in fact this is what you
get by default in nearly all existing oo-languages. The question then becomes
what you want to emphasize: the inheritance of an abstract datatype or the
inheritance of a concrete datatype ?
The programmer implementing a supertype may not be the same person as
the programmer implementing the subtype and may not want implementors
of subtypes to build in dependencies on, or make assumptions about, the
internal representation of the supertype (same arguments as always
for encapsulation).
Thus, yes the instance contains "instances" of the parent structure,
and yes this is totally local to the object, but you may still want
encapsulation (or partitioning) internally in the object.
If inheritance of specification is the emphasis you may still have
default inheritance of implementation. This just means that the implementor
of the subtype is liberated of the job of providing the implementation
for the supertypes. It does not mean that the implementor of the subtype
can "poke arround" in the implementation of the supertype.
Anders
---------------------------------------------------------------------
Anders Bjornerstedt E-mail: anders@cuisun.unige.ch
Centre Universitaire d'Informatique
12 rue du Lac, CH-1207 Geneva
---------------------------------------------------------------------
Tel: 41 (22) 787.65.80-87 Fax: 41 (22) 735.39.05
Home: 41 (22) 735.00.03 Telex: 423 801 UNI CH
---------------------------------------------------------------------
^ permalink raw reply [flat|nested] 9+ messages in thread