Xref: utzoo comp.lang.smalltalk:1857 comp.lang.misc:4826
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!wuarchive!udel!new
From: new@udel.EDU (Darren New)
Newsgroups: comp.lang.smalltalk,comp.lang.misc
Subject: Languages that allow extensions to classes
Message-ID: <16560@estelle.udel.EDU>
Date: 11 Apr 90 19:16:51 GMT
Reply-To: new@ee.udel.edu (Darren New)
Followup-To: comp.lang.smalltalk
Organization: University of Delaware
Lines: 28

One feature of Smalltalk that I really like and use is the ability to
put methods from one class into several files.  This allows, for example,
for the extention of already-defined classes with new methods.

Example: I wish to extend Point and Rectangle to include a "scaleTo:"
method.  In Smalltalk, that method can be included into my application
without disturbing the sources for the system and without recompilation
of anything in the system. Even addition of new instance variables
is possible (in ParcPlace systems, at least).  I find this a MAJOR factor
in software reusability.  

In C++, I (think) I have to change the header file, thereby requiring me
to recompile all sources that reference that object. In Objective-C,
I don't know if this is possible, especially without the sources
to the other methods.  In Eiffel (sp?) I understand that each class is
defined within a single file, forcing me to -change- at least one system
file (thereby maybe corrupting other hopefully-non-conflicting applications).
I think adding overloaded functions for Ada types requires changes to
the definition file, does it not?  Or is that only for opaque types?

In 2OL (my own experimental language) overloaded functions and messages 
automatically include themselves into the correct tables when compiled,
thereby requiring no changes to somebody else's sources.

Is my analysis of C++, Objective-C, and Eiffel correct?  Does anybody know
of other languages which allow the user to extend functionality of system
classes/libraries/etc without changing files where the old stuff is
defined?  How about Ada