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From: stt@inmet.inmet.com
Newsgroups: comp.object
Subject: Types vs. Classes
Message-ID: <60700001@inmet>
Date: 25 Sep 90 14:05:00 GMT
Lines: 44
Nf-ID: #N:inmet:60700001:000:1771
Nf-From: inmet.inmet.com!stt    Sep 25 10:05:00 1990


I would like to begin a discussion on the distinction,
if any, between "types" and "classes".  There seems to be
no consistent distinction made between these two in the OOP literature.  
I have seen a type considered an implementation of a class, a class
as an implementation of a type, the two considered
synonyms, etc.  

Here is a proposed distinction:

1) Each value has a *unique* "type"
2) A value may be a member of one or more "class"es, determined
   by the type of the value.

Essentially, the type structure is considered a "partitioning"
of the value space (i.e. types are non-overlapping), 
whereas the class structure is a taxonomy (i.e. classes overlap
and enclose one another).

A class comprises a (potentially open-ended) set of types, 
and a subclass is a subset of a class, comprising a subset 
of the types in the superclass.  

In formal terms, it is meaningless to ask "what is the class" of
a given value.  However, this can be reinterpreted as
"what is the smallest class which contains the type" of a given value.

-------------------------
I prefer these definitions, because I think they are consistent
with the use of the term "type" in strongly-typed non-OOP languages,
and consistent with the use of the term "class" in OOP languages.

Actually, I think OOP languages tend to be a bit schizophrenic
about the term "class" already, since the term is used both
for parameters/references which may in fact refer to a value
in any subclass of the class, and to identify *the* class of a value (which
only makes sense if you use the interpretation suggested above
of "smallest class which contains the value").

Comments, flames, etc. welcome...

S. Tucker Taft      stt@inmet.inmet.com   uunet!inmet!stt
Intermetrics, Inc.
Cambridge, MA  02138