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Newsgroups: comp.lang.ada
Subject: Re: Generic Collection
From: Georg Bauhaus <bauhaus@futureapps.de>
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Date: Fri, 11 May 2007 23:28:34 +0200
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Date: 2007-05-11T23:28:35+02:00
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On Fri, 2007-05-11 at 13:41 -0700, andrew wrote:
> On May 11, 3:10 am, Georg Bauhaus <bauh...@futureapps.de> wrote:
> > On Thu, 2007-05-10 at 15:48 -0700, andrew wrote:
> > > On May 10, 2:31 pm, Simon Wright <simon.j.wri...@mac.com> wrote:
> > > > > So, I'm thinking that many of the "capabilities" or operations I
> > > > > desire of the collection would have to be "dispatching" to the type of
> > > > > the collection object.
> >
> > When is the type of the collection object determined? How?
> The statement I made may be misleading.  It's not dispatching on the
> type of the collection object but on the type of the object stored in
> the collection.  Does that make better sense?

OK.

tuples: collection;
tables: collection;

declare
 item: ? := get(tables, key);

If you don't care that there is only one type of collection for
all kinds of objects, then indeed references to DB_thing'class
objects seem a plausible choice for storing table objects in
collection. Where

   type DB_thing is abstract tagged private;
   type DB_thing_ref is access DB_thing'class;

   type attribute is new DB_thing with private;
   type tuple is new DB_thing with private; 
   ...

   package DB_collections is new Containers.Hashed_Sets
      (Element_Type => DB_thing_ref, ...);

   subtype collection is DB_collections.Set;

Is there really a sufficiently common algorithm for schemas,
tables, tuples, and attributes?


> > If you want a collection that doesn't collect either Apples
> > or Oranges, but instead is capable of holding Apples, Oranges,
> > Cabbage, Motor_Oil, Whatever, you can have that, too. Is this
> > what you want?
> Yes, but I wouldn't really be storing apples, oranges, cabbage, ... I
> would be storing a common object type like "Object".  Right?

Right.


> > > I don't understand why I would have run-time errors.  Please expand
> > > that thought.
> >
> > Just think of the consequences of Java pre-1.5 collections delivering
> > nothing but Object objects.
> There was a problem with delivering objects of type Object?

Yes, there was a problem with everything being of type Object.
For example, retrieving references from collections forces
type checking at run time and throwing ClassCastException
when there is a mismatch.
    some_ref = (IKnowWhatType) container.get(some_key);
That is a reason why Java now has generics.