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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Extending a third party tagged type while adding finalization
Date: Mon, 4 Dec 2017 14:55:18 +0100
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Xref: reader02.eternal-september.org comp.lang.ada:49348
Date: 2017-12-04T14:55:18+01:00
List-Id: <comp.lang.ada>

On 04/12/2017 14:19, AdaMagica wrote:

> There is no Initialize defined for derived S. This is defined then implicitly as:
> 
>    procedure Initialize (X: in out S) is
>    begin
>      Initialize (T (X));
>    end Initialize;
> 
> This is the calling sequence:
> Init S
> Init T
> Foo  S

Which is a wrong sequence per Ada design. Actually it is just

    Initialize of S

This wrong sequence is fixed in practice by applying the pattern:

    procedure Initialize (X : in out Derived) is
    begin
       Parent (X).Initialize; -- Ensure parent's initialization
       <do user initialization of Derived>
    end Initialize;

Using the pattern you get a semantically correct sequence:

       -- enter Initialize of S
       --   enter Initialize of T
    <do user initialization of T>
       --   leave Initialize of T
    <do user initialization of S>
       -- leave Initialize of S

But with Foo it becomes wrong again:

    <do user initialization of T> + call Foo of S
    <do user initialization of S>

Which is catastrophic and there is *no* way to make it right in presence 
of multiple derived types. You cannot announce a call to a deferred 
implementation during initialization of a specific type T. That may 
happen only after initializations of all derived specific types are 
complete.

Semantically it means that it must be called in the initialization of 
T'Class. The proper sequence must be:

    <do user initialization of T>
    <do user initialization of S>
    <do user initialization of S'Class>
    <do user initialization of T'Class> + call Foo of S

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