From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Ada generics
Date: Mon, 25 Dec 2006 15:39:03 +0100
Date: 2006-12-25T15:39:01+01:00 [thread overview]
Message-ID: <186qujlcx6rwl.1h6eq4mbdaa5s$.dlg@40tude.net> (raw)
In-Reply-To: DFQjh.1222$oo4.1118@trndny09
On Mon, 25 Dec 2006 13:49:23 GMT, Hyman Rosen wrote:
> Dmitry A. Kazakov wrote:
>> Automatic conversions do not represent any special case. If S is
>> convertible to T, then S is a subtype of T
>
> That simply is not the case in C++. Automatic conversions do not
> define subtypes in any meaningful or useful sense. For example,
> all the numeric types automatically convert to each other. It's
> also possible to write automatic conversions between arbitrary
> classes.
Ah, but it is not required for S<:T & T<:S be wrong. Two types can be
subtypes of each other like int and float are in C. It is a common
misconception about subtypes to believe that either must not hold. Observe,
that it is wrong in Ada either. Consider this:
type T is range 1..100;
subtype S is T;
procedure Exported_To_Base (X : in out S);
X : T;
begin
Exported_To_Base (X); -- It is fine!
See, the operation Exported_To_Base defined on S was exported to the base
type T. In other words T inherits it from S, which is equivalent to say
that T is a subtype of S. T and S are sub-/supertypes of each other.
>> Exactly. The idea of structural matching is wrong.
>
> It's not wrong, and it's not even structural. It is in fact
> matching by name, not by structure.
But in foo<123>, 123 is not a name. It could be in some other language
where I could do:
int 123() const
{
return ::124; // (:-))
}
But that would make many of instantiations ambiguous.
> It's only in the case of
> constant expressions that things go a little awry. But if you
> recall the discussions we've had here on implementing units,
> it lets C++ do things that are impossible in Ada.
You mean automatic instantiation here, but I don't want it. I also don't
want this solution for this problem. I even dare say that this is a bad
solution for *any* problem. Looking a bit more deeply into the issue, there
are two things to address:
1. compile-time computations
2. types computations (types algebra)
Generics and templates offer some quite weird and limited way to have 1.
and 2. Ask yourself, is it the only way?
--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
next prev parent reply other threads:[~2006-12-25 14:39 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-12-21 14:14 Ada generics markww
2006-12-21 15:42 ` Dmitry A. Kazakov
2006-12-22 7:59 ` Martin Krischik
2006-12-22 16:14 ` Hyman Rosen
2006-12-22 7:59 ` Martin Krischik
2006-12-22 16:41 ` Hyman Rosen
2006-12-22 17:33 ` Markus E Leypold
2006-12-22 18:26 ` Hyman Rosen
2006-12-22 20:59 ` Markus E Leypold
2006-12-22 21:01 ` Markus E Leypold
2006-12-23 14:09 ` Marco
2006-12-25 14:23 ` Hyman Rosen
2006-12-29 14:13 ` Marco
2006-12-25 14:20 ` Hyman Rosen
2006-12-23 11:43 ` Dmitry A. Kazakov
2006-12-25 13:49 ` Hyman Rosen
2006-12-25 14:39 ` Dmitry A. Kazakov [this message]
2006-12-26 1:34 ` Hyman Rosen
2006-12-26 9:11 ` Dmitry A. Kazakov
2006-12-26 16:23 ` Hyman Rosen
2006-12-26 19:28 ` Dmitry A. Kazakov
2006-12-27 1:44 ` Hyman Rosen
2006-12-27 9:21 ` Dmitry A. Kazakov
2006-12-27 19:06 ` Hyman Rosen
2006-12-28 10:59 ` Dmitry A. Kazakov
2006-12-28 16:29 ` Hyman Rosen
2006-12-29 11:12 ` Dmitry A. Kazakov
2006-12-29 14:56 ` Hyman Rosen
2006-12-28 17:35 ` Georg Bauhaus
2006-12-29 14:48 ` Dmitry A. Kazakov
2006-12-29 19:39 ` Georg Bauhaus
2006-12-30 9:58 ` Dmitry A. Kazakov
2006-12-30 14:53 ` Georg Bauhaus
2007-01-01 13:00 ` Dmitry A. Kazakov
2007-01-02 10:04 ` Georg Bauhaus
2007-01-02 11:11 ` Dmitry A. Kazakov
2007-01-02 12:33 ` Georg Bauhaus
2007-01-02 13:51 ` Dmitry A. Kazakov
2007-01-02 14:45 ` Georg Bauhaus
2007-01-03 10:10 ` Dmitry A. Kazakov
2007-01-03 14:20 ` Hyman Rosen
2007-01-03 14:55 ` Georg Bauhaus
2007-01-04 10:15 ` Dmitry A. Kazakov
2007-01-03 19:33 ` Alexander E. Kopilovich
2007-01-04 10:27 ` Dmitry A. Kazakov
2007-01-04 15:00 ` Alexander E. Kopilovich
2007-01-05 10:32 ` Dmitry A. Kazakov
2006-12-30 2:25 ` Randy Brukardt
2006-12-30 10:13 ` Dmitry A. Kazakov
2007-01-04 1:09 ` Randy Brukardt
2007-01-04 10:07 ` Dmitry A. Kazakov
2007-01-05 1:32 ` Randy Brukardt
2007-01-05 4:46 ` Randy Brukardt
2007-01-05 9:08 ` Jean-Pierre Rosen
2007-01-05 20:14 ` Georg Bauhaus
2007-01-06 0:14 ` Randy Brukardt
2006-12-29 0:09 ` Randy Brukardt
2006-12-29 11:11 ` Dmitry A. Kazakov
2006-12-30 2:40 ` Randy Brukardt
2006-12-21 16:55 ` Hyman Rosen
2006-12-21 18:22 ` markww
2006-12-22 3:01 ` Steve
replies disabled
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox