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From: "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>
Subject: Re: Conversion of Access Types Question
Date: 1999/01/14
Message-ID: <77l492$b5s@hobbes.crc.com>#1/1
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References: <369DFFFC.A160D47C@neta.com>
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Organization: Coleman Research Corporation
Newsgroups: comp.lang.ada
Date: 1999-01-14T00:00:00+00:00
List-Id: <comp.lang.ada>


Paul S. Heidmann wrote in message <369DFFFC.A160D47C@neta.com>...
>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?
>
Compiling your code with gnat results in the following message:

  [hoosd@VISNT021 d:/gnat_examples]$ gcc -c types.adb
  types.adb:23:14: target type must be general access type
  types.adb:23:14: add "all" to type "A_T2" defined at line 14

Making the change the compiler directs solves the problem.

i.e., change line 14 to read:
   type A_T2 is access all T2;