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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Invoking parental methods (was: Java vs Ada 95)
Date: 1996/11/08
Message-ID: <EACHUS.96Nov8183418@spectre.mitre.org>#1/1
X-Deja-AN: 195381747
references: <E0ED6w.24D.0.-s@inmet.camb.inmet.com>
 <E0EIM2.A3p.0.-s@inmet.camb.inmet.com>
organization: The Mitre Corp., Bedford, MA.
newsgroups: comp.lang.ada
Date: 1996-11-08T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <55tm4r$8al@news.ida.org> wheeler@aphrodite (David Wheeler) writes:

  > I actually recommended such an attribute (I named it 'Parent) as
  > part of the Ada95 revision. My suggestion was struck down for the
  > reason you see here (you can emulate it with "subtype").  I still
  > think it's a good idea, even though there's a simple workaround,
  > because it simplifies maintenance and reduces possible errors
  > (e.g. forgetting to redefine subtype Parent when the hieararchy
  > changes).

   Once upon a time in Ada 9X you could emulate it fairly easily, if
you knew the name of the parent.  But with the current (and very
correct) rules on dispatching, it can be VERY difficult to get the
effect you want.  (A view conversion to the parent type that
dispatches dynamically or statically depending on context.)  I argued
for the attribute at more than one WG9 workshop, and got regularly put
aside with "it is easy" to emulate.  Now we have the rules for
dispatching right but it can be very hard to get the dynamic behavior
to work.

    I guess most people thought about the static case, and didn't
realize that there are cases where you want the dynamic behavior.  For
example, suppose I want to find out how deep an inheritance is.

    I want to write:

       function Count_Depth(Bar: in Foo'CLASS) return Integer is
       begin
         if Foo'Tag = Bar'Tag then return 0;
         else return Count_Depth(Parent(Bar)) + 1; end if;
       end Count_Depth;

    This is a semi-silly example, but think about a typical window
system where I want to add gadgets by inheritance.  Most of the time,
the behavior of the child statically calling the parent's method is
fine, but often you want to walk the inheritance list, doing something
at each level.  You either define all these methods on the root class,
which requires an impossible amount of forethough since you didn't
write that package, or you make them classwide, and need the dynamic
behavior.

--

					Robert I. Eachus

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