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From: "Paul S. Heidmann" <psh@neta.com>
Subject: Conversion of Access Types Question
Date: 1999/01/14
Message-ID: <369DFFFC.A160D47C@neta.com>#1/1
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Newsgroups: comp.lang.ada
Date: 1999-01-14T00:00:00+00:00
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Greetings fellow Ada lovers!

I'm an experienced Ada83 programmer that is learning to use Ada95.
Things are going very well, but I don't understand the problems
that I'm having converting access types that point to variables of
class wide types.  Consider the following piece of code:

=======================================

procedure Types is

   type T1 is tagged
      record
         I : Integer;
      end record;
   type A_T1 is access all T1'Class;

   type T2 is new T1 with
      record
         I2 : Integer;
      end record;
   type A_T2_Classwide is access all T2'Class;
   type A_T2 is access T2;

   Thing1 : A_T1;
   Thing2 : A_T2_Classwide;
   Thing3 : A_T2;

begin
   Thing1 := new T2;
   Thing2 := A_T2_Classwide (Thing1);
   Thing3 := A_T2 (Thing1);  -- illegal, types are not convertable.
end Types;

=======================================

My question is, why is the last line an illegal conversion, but the
second to last line not?

Thanks!

Paul Heidmann