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From: Maciej Sobczak <see.my.homepage@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2005 puzzle
Date: Fri, 20 Jul 2012 04:30:27 -0700 (PDT)
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Date: 2012-07-20T04:30:27-07:00
List-Id: <comp.lang.ada>

W dniu pi=C4=85tek, 20 lipca 2012 10:27:15 UTC+2 u=C5=BCytkownik Dmitry A. =
Kazakov napisa=C5=82:

> The problem is that a *new* limited object cannot be result of a function=
,
> this is conceptually broken.

Yes, we have already concluded long time ago that one of the priorities for=
 Ada is to fix the concept of object construction. But in this case I think=
 that Ada has everything that is needed.

> The factory of limited objects passed to
> Insert must provide means to:
>=20
> 1. determine the constraints of the object (using some set of arguments);

This is not the container's business and will usually come with the factory=
 itself - or from the context of the factory. That is why I expect this to =
be used with access to *local* functions, which can drag along all the info=
rmation that is needed to construct the object.

> 2. construct the object in place (using the arguments again);

Same as above - the constructor function can bring the "arguments" from its=
 surrounding context. You can even create wrappers like this:

procedure Insert_From_Arguments
   (C : in out Container;
    X : in Integer; Y : in Integer; Z : in Integer) is

   function Construct return T is
   begin
      return V : T do
         V.X :=3D X;  -- constructing T from arguments X, Y, Z
         -- ...
      end return;
   end Construct;
begin
   C.Insert (Construct'Access);
end Insert_From_Arguments;

Above, the arguments are passed as part of the Construct's environment.

> 3. optionally, construct the class-wide object upon the specific object
> (using the arguments)

I expect that class-wide is what the container sees (for example, by being =
instantiated with such a type) - the rest happens the usual way, by initial=
izing the class-wide placeholder with specific constructed object. This way=
 you could naturally have a container of OO objects.

> 1-3 cannot be effectively fused into single operation

Why? What's wrong (ie. what is ineffective) with the above?

--=20
Maciej Sobczak * http://www.msobczak.com * http://www.inspirel.com