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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada Annoyances
Date: Sat, 24 Jun 2017 11:07:48 +0200
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Xref: news.eternal-september.org comp.lang.ada:47098
Date: 2017-06-24T11:07:48+02:00
List-Id: <comp.lang.ada>

On 2017-06-24 10:35, Maciej Sobczak wrote:
> 
>> A class-wide object of type S derived from T must be constructed in this
>> order:
>>
>>      T
>>      S
>>      S'Class
>>      T'Class
>>
>> For each stage a user-defined hook called.
> 
> I see no practical benefit from this. Can you provide some motivating example?

Dispatch upon object construction.

    type T is tagged ...
    procedure Foo (X : in out T);
    procedure Bar (X : in out T) is abstract;

    procedure T'Initialize (X : in out T) is
    begin
       Foo (X); -- This is OK
       Bar (T'Class (X)); -- This is not OK
    end T'Initialize;

    procedure T'Class'Initialize (X : in out T'Class) is
    begin
       Bar (X); -- This is OK, all descendants constructed
    end T'Class'Initialize;

In C++ you can have T and S parts, plus a language hack that kills 
dispatching while there.

In Ada you can override T + S + S'Class + T'Class as a whole for a 
controlled type.

Neither is complete nor safe and causes a lot of problem. E.g. in Ada it 
is the latest stage of construction which apart from enforcing an 
error-prone practice calling parent's Initialize prevents us from having 
task components.

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