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From: "Matthew Heaney" <mheaney@on2.com>
Newsgroups: comp.lang.ada
Subject: Re: Questions - Polimorphism/Dynamic Binding
Date: Tue, 30 Oct 2001 17:28:02 -0500
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Date: 2001-10-30T17:28:02-05:00
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"Eric Merritt" <cyberlync@yahoo.com> wrote in message
news:mailman.1004456694.3472.comp.lang.ada@ada.eu.org...
> I started out in Java and C++ and recently found the
> one true langauge (Ada95, lol). In any case, one thing
> that I found very usefull in Java was the Interface. I
> don't believe that Ada has this at all. I was
> wondering if there is a way to impliment this type of
> functionality in Ada

Not directly.  For now the closest thing you can do is use the "multiple
views" idiom.  Suppose you have an "interface" type:

package P is

   type T is tagged limited null record;
   procedure Op (O : in out T) is abstract;

end P;

Now what we want is to have a tagged type that "conforms" to this interface.
One way to do this is like this (I haven't tried compiling this, but you'll
get the idea):

with P;
package Q is

   type T_Public;

   type TI is new P.T  (O : access T_Public) with null record;
   procedure Op (O : in out TI);

   type T_Public is abstract tagged limited record
      I : aliased TI;
   end record;

   type T is new T_Public with private;
   procedure Op2 (O : in T);
...
end Q;

This idea is that if you have an object of type T, then you can either view
it as a T, or use its (public) I component to view it as the interface type
P.T:

declare
   O : Q.T;
begin
   Op2 (O);  --view as a Q.T
   Op (O.I);  --view as "interface" P.T
end;

The interface part of T (its component I) can view the outer type through
its access discriminant.


> If you have a tagged type and extend that type with a
> new type.

package P is
   type T is limited null record;
   procedure Op (O : in out T);
end;

package P.C is
   type NT is new T with null record;
   procedure Op (O : in out NT);
end;


> Then pass the new type to a procedure that
> accepts its the new types parent, the original type.

Hmmm... Is the call static or dynamically bound?  Do you mean:

declare
   O : P.C.NT;
begin
   P.Op (O);  --?
end;

I don't think this will compile (this is a statically bound call), because
procedure P.Op takes type P.T, not P.C.NT.


> Are procedures associated with the child type
> (methods) that override it's parent types procedures
> called in this instance, even though the procedure
> that  this type is passed into is unaware of the new
> child type and has not withed it's package?

Not sure what you mean.  The correct procedure will always get called in a
dispatching (dynamically bound) call:

declare
   O : P.T'Class := P.C.NT'(null record);  --check my syntax
begin
   Op (O);  --try P.Op and P.C.Op and see how your compiler reacts
end;

Here, P.C.Op will get called, because O's tag has the value P.C.NT.  Or
where you expecting something else?