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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: How to hide inherited implementation of a public interface?
Date: Fri, 21 Mar 2014 18:02:28 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2014-03-21T18:02:28-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:1goz7i2pg8860.1kma3d5vz1fl$.dlg@40tude.net...
> On Fri, 21 Mar 2014 10:58:54 -0700, Jeffrey Carter wrote:
>
>> On 03/21/2014 05:53 AM, Natasha Kerensikova wrote:
>>>
>>>     package I is
>>>        type T is interface;
>>>        procedure P (Object : in out T) is abstract;
>>>     end I;
>>
>> "IMHO, Interfaces are worthless."
>
> Interface is an abstract type. Abstract types are not worthless.
>
> Worthless is the Java's idea that multiple inheritance could become more
> useful, less flawed, choose what you want, when restricted to interfaces.

Exactly. "Abstract types" (which can have components, implementations, etc.) 
are not worthless. The restrictions on interfaces make them worth little. 
("Worthless" is going a bit far, of course, but it makes a good sound bite.) 
The costs of multiple inheritance (which are considerable) make them not 
worth the effort.

Full multiple inheritance CAN be implemented, but it's expensive enough in 
compiler and language complexity that the costs outweight the value. The 
halfway Java-like solution is easier to implement but makes no one happy. We 
should have told the multiple inheritance nuts to forget it, because it 
makes no sense for Ada.

                                                      Randy.