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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: another way to shoot yourself in the foot?
Newsgroups: comp.lang.ada
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Date: Fri, 20 Jun 2008 12:26:28 +0200
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Date: 2008-06-20T12:26:28+02:00
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On Fri, 20 Jun 2008 03:05:29 -0700 (PDT), christoph.grein@eurocopter.com
wrote:

>> --- Factory ads ---
>> with Named;
>> with Some;
>>
>> package Factory is
>> � �function Create(class: String; name: String) return
>> Named.Object'Class;
>> end Factory;
>>
>> --- Factory.adb ---
>>
>> with Ada.Strings.Fixed; use Ada.Strings.Fixed;
>> with Some;
>>
>> package body Factory is
>> � �--- this one is ugly, but in sake of simplicity ...
>> � �function Create(class: String; name: String) return
>> Named.Object'Class is
>> � �begin
>> � � � if Index(class, "Some") /= 0 then
>> � � � � �return Some.Create(name);
>> � � � end if;
>> � � � raise Program_Error with "Unknown class " & class;
>> � �end;
>> end Factory;
> 
> Since Some.Object is limited, Factory must construct in place. The
> normal return statement produces a copy (I guess there should be a
> warning here - no compiler bug, since the RM has nothing to say about
> warnings).

Are you sure? Then it would be worse, a language design bug. Anyway a
limited object cannot be copied, so it must be an error if an attempt is
made.

> So use the extended return statement as in Some.Create.

It already does. Did you mean in Factory as well? Like this:

function Create (class: String; name: String) return Named.Object'Class is
   begin
      if Index (class, "Some") /= 0 then
         return X : Named.Object'Class := Some.Create (name);
      end if;
      raise Program_Error with "Unknown class " & class;
   end;
end Create;

I bet it would not help.
-------------------------

Honestly I have no idea how it is managed on the language level. A limited
interface can be implemented by a non-limited object. Thus from the
declaration of a factory function, returning a class-wide of some limited
interface, the compiler cannot decide whether the result should be in-place
or not. Therefore, IMO, it must consider the worst case, i.e. an in-place
"return".

Language lawyers?

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