From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Access to function returning class-wide type
Date: Thu, 21 Aug 2008 22:56:32 +0200
Date: 2008-08-21T22:56:35+02:00 [thread overview]
Message-ID: <saffln97j381$.1xderlppcvf4u$.dlg@40tude.net> (raw)
In-Reply-To: g8kg18$48s$1@registered.motzarella.org
On Thu, 21 Aug 2008 21:30:28 +0200, Pawe� 'Nivertius' P�azie�ski wrote:
> Dmitry A. Kazakov wrote:
>
>> So you could replace it with a map condition->type tag and then proceed
>> with Ada.Tags.Generic_Dispatching_Constructor, because the map
>> tag->constructor is for free, here you do not need pointers any more, in
>> Ada 2005. The compiler will force to override the "constructor" for each
>> concrete derived type.
>> -------------
>> Having said that, there still is a difficult problem with construction of
>> the map. I don't know if it is an issue for you, but it is for the
>> persistency layer I designed. The problem is that if the map is
>> open-ended, you need some mechanism of registration of each new type in
>> the map. This is still unresolved in Ada. The pattern I am using is that
>> each package declaring a new type is required to call some class-wide
>> procedure registering the type upon package elaboration. In your case it
>> is the condition associated with the type tag. That cannot be enforced,
>> and it works only at the library level, i.e. for "immortal" types.
>
> I think that the map condition->type tag is in my case same problem as map
> condition->constructor pointer. I need the map registration anyway, because
> it basically needs to be changed at run-time.
Why then N is a parameter? When each type knows its N you don't need to
pass it back.
> Altrough, all the types I
> need to register are library-level types, so the registration mechanism is
> pretty simple.
Then the pattern is (assuming N is unknown in advance):
package A is
type Abstracted is abstract tagged null record;
procedure Register
( N : out Natural; -- Given upon registration
Constructor : not null access
function return Abstracted'Class
);
end A;
package B is
type Derived is new Abstracted with ...;
end B;
package body B is
My_N : Natural;
function Create return Abstracted'Class is
begin
return Derived'(...); -- Returns derived
end Create;
begin
Register (My_N, Create'Access);
end B;
You don't need to declare Create in A at all. Each type will declare it in
order to call to Register anyway.
--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
next prev parent reply other threads:[~2008-08-21 20:56 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-08-21 10:13 Access to function returning class-wide type Paweł 'Nivertius' Płazieński
2008-08-21 10:33 ` Paweł 'Nivertius' Płazieński
2008-08-21 11:02 ` Dmitry A. Kazakov
2008-08-21 11:56 ` Paweł 'Nivertius' Płazieński
2008-08-21 13:34 ` Dmitry A. Kazakov
2008-08-22 4:53 ` Randy Brukardt
2008-08-22 23:50 ` Randy Brukardt
2008-08-21 12:01 ` Georg Bauhaus
2008-08-21 13:01 ` Paweł 'Nivertius' Płazieński
2008-08-21 13:50 ` Dmitry A. Kazakov
2008-08-21 19:30 ` Paweł 'Nivertius' Płazieński
2008-08-21 20:56 ` Dmitry A. Kazakov [this message]
2008-08-22 7:34 ` Paweł 'Nivertius' Płazieński
2008-08-21 17:02 ` Adam Beneschan
2008-08-21 19:22 ` Paweł 'Nivertius' Płazieński
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