Xref: utzoo comp.lang.misc:1097 comp.lang.pascal:635
Path: utzoo!mnetor!uunet!mcvax!inria!crin!tombre
From: tombre@crin.crin.fr (Karl Tombre)
Newsgroups: comp.lang.misc,comp.lang.pascal
Subject: Object Pascal and dynamic binding
Message-ID: <416@crin.crin.fr>
Date: 25 Feb 88 11:38:25 GMT
Reply-To: tombre@crin.crin.fr (Karl Tombre)
Organization: CRIN - INRIA Lorraine, Nancy, France
Lines: 40

I have a question about Object Pascal, the object extension to Pascal
written by Apple for the MAcintosh. I have read Kurt Schmucker's book
(Object-Oriented Programming for the Macintosh) without finding an answer.

Many object languages (Smalltalk, Objective-C for instance) use dynamic
typing. On the contrary, Simula-67, C++, Eiffel and some others use static
typing, and achieve dynamic binding by means of the virtual function
mechanism. Schmucker says several times that Object Pascal also uses static
typing (more precisely compile-time type checking), but does not explain how
dynamic binding is obtained.

Some excerpts from his book (I don't copy the integral text, as americans seem
to be crazy about copyrights ;-) :

p. 67 : he speaks of examples of what Object Pascal cannot determine until
run-time and says that this gives the impression that a great deal of
decisions must be delayed until then. But according to him, Object Pascal
makes very few decisions at run-time... and so on.

But no word on how dynamic binding is made possible.

p. 323 : in the chapter on Clascal, he notes that Object Pascal has no
ABSTRACT and DEFAULT keywords and that Apple found a "better method"...

My question : is this method a trade secret, or is it the good old virtual
mechanism of Simula ?

Said in another way : does Object Pascal detect which functions must be
dynamically bound (as they are overriden in several classes) and implement
this by a pointer to a function associated with the instance, or something
like that ? Or is there another mechanism for achieving this ? Or is there
really a method lookup function searching at run-time for the right
procedure to call ?

Can somebody give an answer to this ? Thanks in advance,

--- Karl Tombre @ CRIN (Centre de Recherche en Informatique de Nancy)
EMAIL : tombre@crin.crin.fr  --   tombre@crin.UUCP
POST  : Karl Tombre, CRIN, B.P. 239, 54506 VANDOEUVRE CEDEX, France
PHONE : +33  83.91.21.25