Re: Instantiating private types with discriminants?

[Jerry Petrey <jdpetreyAPPENDIX@raytheon.com>]

rick H wrote:

> Thank you, Georg, Ludovic and Dmitri for your replies.
>
> Now I know I won't get shouted at for asking novice questions, I have
> another query for you experts, if you've got the time to have a look:
>
> I'm trying to query the tag of a record, which I can do quite nicely,
> provided I both "with Ada.Tags" and "use Ada.Tags".  Just for the sake
> of it, I tried to remove the "use Ada.Tags" statement, but I can't work
> out how I would then specify the XXX'Tag attribute of a variable.
> Here's an example that illustrates my problem ("if Var_A'Tag..." is the
> problem line):
>
> with Ada.Integer_Text_IO;
> with Ada.Float_Text_IO;
> with Ada.Tags;
> --  use Ada.Tags;
>
> procedure Simple_Case is
>    type General_T is tagged null record;
>
>    type Access_T is access General_T'Class;
>
>    type Type_A is new General_T with
>    record
>       Data : Integer;
>    end record;
>
>    type Type_B is new General_T with
>    record
>       Data : Float;
>    end record;
>
>    Var_A : Access_T; --  could be "new Type_A" or a "new Type_B"
> begin
>    --  Var_A := new Type_A' (Data => 100);
>    Var_A := new Type_B' (Data => 100.0);
>
>    --  problem line:
>    if Var_A'Tag = Type_A'Tag then
>       Ada.Integer_Text_IO.Put ( Type_A (Var_A.all).Data);
>    elsif Var_A'Tag = Type_B'Tag then
>       Ada.Float_Text_IO.Put ( Type_B (Var_A.all).Data);
>    end if;
> end Simple_Case;

You could use function notation for the correct "=" as follows:

   if Ada.Tags."=" (Var_A'Tag, Type_A'Tag) then
      Ada.Integer_Text_IO.Put ( Type_A (Var_A.all).Data);
   elsif Ada.Tags."=" (Var_A'Tag, Type_B'Tag) then
      Ada.Float_Text_IO.Put ( Type_B (Var_A.all).Data);
   end if;


Jerry

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-- Jerry Petrey, GNC Software Engineer, Raytheon Missile Systems
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