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From: hreba <hreba@terra.com.br>
Newsgroups: comp.lang.ada
Subject: Accessibility check failed
Date: Sun, 10 Aug 2014 23:12:37 -0300
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Date: 2014-08-10T23:12:37-03:00
List-Id: <comp.lang.ada>

One more beginner's question.

I wrote a linear list of abstract items.
There is an abstract type Iterator with an abstract method Proc which 
can be called with a class wide item type. Procedure Iterate calls Proc 
with each item of the list.

For a test I declared a concrete extension extension of Gen.Lists.ItemD.
I also declare a concrete extension of Iterator called Writer and a 
concrete method Proc.

Everything compiles, but at runtime the line

          i:= Item(itm);

in Proc generates a "accessibility check failed" error.

I checked with a debugger that itm is not null and that its actual 
(dynamic) type is that of the concrete extension.

So what is wrong?

A more complete extract of my program follows.

---------------------

package Gen.Lists is

    type ItemD is abstract tagged private;
    type List is limited private;

    type Iterator is abstract tagged null record;
    procedure Proc (iter: in out Iterator; itm: access ItemD'Class) is 
abstract;
    procedure Iterate (ic: in out Iterator'Class; l: List);

end Gen.Lists;

--------------------

with Gen.Lists;

package test_lists_aux is

    type ItemD is new Gen.Lists.ItemD with
       record
          ch:	Character;
       end record;
    type Item is access all ItemD;

    type Writer is new Gen.Lists.Iterator with null record;
    procedure Proc (w: in out Writer; itm: access Gen.Lists.ItemD'Class);

end test_lists_aux;

-----------------

with Ada.Text_IO;	use Ada.Text_IO;

package body test_lists_aux is

    procedure Proc (w: in out Writer; itm: access Gen.Lists.ItemD'Class) is
       i: Item;
    begin
       if itm.all in ItemD then
          i:= Item(itm);
       end if;
       Put (i.ch);
       --Put (Item(itm).ch);
    end Proc;

end test_lists_aux;


-- 
Frank Hrebabetzky		+55 / 48 / 3235 1106
Florianopolis, Brazil