Re: Instantiating private types with discriminants?

[rick H <rik_nntp@dsl.pipex.com>]

Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
> You should have tried it in the same body. Do it as Ludovic did, using
> packages:
> 
> ---------- Simple_Case_Types.ads
> package Simple_Case_Types is
>   type General_T is abstract tagged null record;
>   procedure Put (Item : in General_T) is abstract;
>   
>   type Type_A is new General_T with
>   record
>      Data : Integer;
>   end record;
>   procedure Put (Item : in Type_A);
> 
>   type Type_B is new General_T with
>   record
>      Data : Float;
>   end record;
>   procedure Put (Item : in Type_B);
> 
> end Simple_Case_Types;
> 
> ---------- Simple_Case_Types.adb
> with Ada.Integer_Text_IO;
> with Ada.Float_Text_IO;
> 
> package body Simple_Case_Types is
>   procedure Put (Item : in Type_A) is
>   begin
>      Ada.Integer_Text_IO.Put (Item.Data);
>   end Put;
> 
>   procedure Put (Item : in Type_B) is
>   begin
>      Ada.Float_Text_IO.Put (Item.Data);
>   end Put;
> 
> end Simple_Case_Types;
> 
> ----------- Simple_Case.adb
> with Simple_Case_Types;  use Simple_Case_Types;
> 
> procedure Simple_Case is
>   type Access_T is access General_T'Class;
>   Var_A : Access_T; -- could be "new Type_A" or a "new Type_B"
> begin
>   Var_A := new Type_B' (Data => 100.0);
>   Put (Var_A.all);
> 
> end Simple_Case;
> 
> This will compile as expected.
> 
> BTW, I should use pointers only when needed. Differently to C++, in Ada you
> can create class-wide objects on the stack.
> 

I'm quite happy to abandon pointers in preference to the class-wide
object approach - in all honestly I didn't know that you could declare
variables of a class-wide type - and this is a side-effect of a novice
not having "Programming in Ada" open when coding!

I've now tried declaring Var_A to be...
   Var_A : General_T'Class;

but gnat insists on initialization, and it's here that I tumble.

I can declare Var_A with an initialization...
   Var_A : Type_B' (Data => 100.0);

and then the right Put will work...
   Put (Var_A);

However, I can't work out how to change Var_A dynamically - the compiler
says "dynamically tagged expression required" if I do...
begin
   Var_A := Type_A' (Data => 100);
   Put (Var_A);
end;