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From: johnson@p.cs.uiuc.edu
Newsgroups: comp.sw.components
Subject: Re: Inheritance vs. component efficienc
Message-ID: <130200004@p.cs.uiuc.edu>
Date: 15 Jun 89 15:21:00 GMT
References: <5682@hubcap.clemson.edu>
Lines: 23
Nf-ID: #R:hubcap.clemson.edu:5682:p.cs.uiuc.edu:130200004:000:1262
Nf-From: p.cs.uiuc.edu!johnson    Jun 15 10:21:00 1989


From billwolf%hazel.c@hubcap.clemson.edu (Bill Wolfe) 

>  Well, wouldn't you know it, a short little program can be written to take
>  an Ada package specification and automatically generate a corresponding
>  package body, with type definitions and the like recast as comments, and
>  procedures and functions rewritten as stubs.  The same thing can be done
>  when you wish to add a layer of abstraction; simply have a program ask you
>  for the name of the new data type(s) and automatically generate a higher-
>  level package which implements itself through calls to the lower-level
>  packages.  Q.E.D., as they say in mathematics...

This is called delegation, which is a common way of implementing
inheritance in object-oriented languages.  One problem is that
if you inherit procedure X, which calls procedure Y, but redefine
Y then the inherited version of X will call the wrong version of Y.
This can be solved by copying all the code in the original class,
not just calling it.  True delegation uses run-time polymophism
to solve this problem, which is extremely useful for code reuse but is
not present in Ada.  However, it can be simulated with a preprocessor.
I assume that is what the various versions of "object-oriented Ada" do.

Ralph Johnson