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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Finalization of library level tasks
Date: Mon, 30 Apr 2018 19:06:45 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:51845
Date: 2018-04-30T19:06:45+02:00
List-Id: <comp.lang.ada>

On 2018-04-30 18:52, Jeffrey R. Carter wrote:
> On 04/30/2018 02:27 PM, AdaMagica wrote:
>>
>> In short: If you look inside a class-wide object, you'll find inside 
>> an object of a specific type within this class (as Randy said in a 
>> previous post). What's the property of this specific object seen as a 
>> class-wide object that's not a property of the specific object?
> 
> About the only difference is that operations dispatch with a class-wide 
> parameter and don't with a parameter of a specific type.
> 
> Op (Parent'Class (X) );
> 
> and
> 
> Op (X);
> 
> end up executing the same code.

Wrong. Consider this:

    type T1 is tagged null record;
    procedure Op (X : in out T1);

    type T2 is new T1 with null record;
    procedure Op (X : in out T2);

    type T3 is new T2 with null record;
    procedure Op (X : in out T3);

Now this procedure:

    procedure Finalize (X : in out T2);

if you pass T3 to it then

    procedure Finalize (X : in out T2) is
    begin
       Op (T1'Class (X)); -- Dispatches to Op (T3 (X))
       Op (X); -- Statically calls to Op (T2 (X))

The code is not same because dispatch can cross the actual type border 
and go a descendant. This is also why dispatching in Finalize should 
have been illegal. All re-dispatch should have been illegal because it 
breaks typing.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de