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From: lutz@iks-jena.de (Lutz Donnerhacke)
Newsgroups: comp.lang.ada
Subject: Re: Sugestion to Multiple Inheritance
Date: Mon, 21 Jan 2002 17:00:36 +0000 (UTC)
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* Brian Rogoff wrote:
>Yes, generics can do exactly that. Do you understand the signature idiom
>in Ada 95?

Now I start to understand. Thanx.

>What you don't get, that tagged types give you, is the same kind of
>polymorphism at run time. You pay for that run time capability with run
>time dispatch and the overhead of tagging.

Of course. I was looking for an abstraction require a full instantiation.
Generics does this a small step later than abtract tagged types. (Besides
using tagged types for saving multiple implementations.) Without class wide
programming there is no run-time overhead, but a storage one.

>So you parameterize over a forward iterator signature, which looks
>something like this
>
>generic
>    type Value_Type (<>) is limited private;
>    type Pointer_Type is access Value_Type;
>    type Iterator_Type is private;
>
>    with procedure Incr (Iterator : in out Iterator_Type) is <>;
>    with function Succ (Iterator : in Iterator_Type)
>                       return Iterator_Type is <>;
>    with function Get_Value (Iterator : Iterator_Type)
>                            return Value_Type is <>;
>    with procedure Set_Value (Iterator : in out Iterator_Type;
>                              Value : in Value_Type) is <>;
>    with function Get_Pointer (Iterator : Iterator_Type)
>                              return Pointer_Type is <>;
>    with function "=" (X, Y : Iterator_Type)
>                        return Boolean is <>;
>package Forward_Iterator_Signature is
>-- Look, no body! This is a signature package.
>end Forward_Iterator_Signature;

Understand. Thank you. This seems to be the way Booch is working.