Re: Conversion of Access Types Question

["David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>]

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;