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From: "Alex R. Mosteo" <devnull@mailinator.com>
Newsgroups: comp.lang.ada
Subject: Avoiding use Ada.Tags (was Re: Instantiating private types with
 discriminants?)
Date: Tue, 09 May 2006 17:53:47 +0200
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Date: 2006-05-09T17:53:47+02:00
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rick H wrote:

> (...)
> 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):

>    --  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;

I offer you two alternatives. First one, would be:

with Ada.Tags;

procedure ...

   use type Ada.Tags.Tag;
   --  This makes operators on type Tag visible, without
   --  needing to use the whole package.
begin
   if Var_A'Tag = Type_A'Tag then -- Should work now.

And, the preferred method I guess, without "withing" Ada.Tags:

if Var_A.all in Type_A then -- Is A instance of Type_A?
if Var_A.all in Type_A'Class then -- Is Type_A or descendent?

See this test program:

----8<--------

with Text_Io; use Text_Io;

procedure Test_Tag is
   type X is tagged null record;
   type Y is new X with null record;
   type Z is new Y with null record;

   vz :          Z;
   vn : constant X'Class := vz;
begin
   Put_Line (Boolean'Image (vn in Y));
   Put_Line (Boolean'Image (vn in Y'Class));
end Test_Tag;

----8<--------