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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada requires too much typing!
Date: Thu, 10 Jun 2010 17:17:11 -0500
Organization: Jacob Sparre Andersen
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Xref: g2news2.google.com comp.lang.ada:12589
Date: 2010-06-10T17:17:11-05:00
List-Id: <comp.lang.ada>

"Georg Bauhaus" <rm.dash-bauhaus@futureapps.de> wrote in message 
news:4c10baa2$0$6974$9b4e6d93@newsspool4.arcor-online.net...
> On 10.06.10 10:57, Vadim Godunko wrote:
>> On Jun 10, 11:20 am, AdaMagica <christoph.gr...@eurocopter.com> wrote:
>>> On 10 Jun., 08:34, Vadim Godunko <vgodu...@gmail.com> wrote:
>>>
>>>> I want to known more about how to use OOP in Ada without access types!
>>>
>>> type Element is tagged ...
>>> subtype Any_Element is Element'Class;
>>>
>>> procedure Show (E: Element);
>>>
>> Thank you for example, this is example of dispatching but not use of
>> Ada in heavy OOP style of programming. :-)
>
> Will it, in theory, be possible to write a Qt-like library
> in Ada such that client programs declare variables like
>
>   W : Some_Window'Class := Some_Lib_Pack.Make (Typ => Some_Id);
>
> That is, programs do not use access Some_Window'Class?

We could have done this in Claw, but we found little need for "generic 
windows". At some point, you have to declare the specific window object that 
you need, and at that point you might as well just declare an object of that 
type.

Claw allows window objects to be assigned, so you could definitely do 
something like:

   declare
      My_Window : Claw.Basic_Window.Window;
   begin
      Global_Window := Root_Window_Type'Class(My_Window);
   end;

Unfortunately, Ada doesn't allow the changing of the tags of declared 
classwide objects. (This is a language bug in my view, but hardly anyone 
elses.) So you have to use an Ada 2012 holder container:

   declare
      My_Window : Claw.Basic_Window.Window;
   begin
      Global_Window.Replace_Element (Root_Window_Type'Class(My_Window));
   end;

where Global_Window is defined as:

    package Global_Holder is new Ada.Containers.Indefinite_Holder 
(Claw.Root_Window_Type'Class);

    Global_Window : Global_Holder;


                                 Randy.