Re: Re^2: Integrating concurrent & O-O programming

[anders@cui.unige.ch (BJORNERSTEDT Anders)]

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.

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