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From: ytomino <aghia05@gmail.com>
Newsgroups: comp.lang.ada
Subject: discriminant questions
Date: Sat, 17 Sep 2011 09:30:46 -0700 (PDT)
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Date: 2011-09-17T09:30:46-07:00
List-Id: <comp.lang.ada>

Hello, I'm excited at the containers of Ada 2012.
I tried to write my code with the style of Ada 2012, and I have some
questions.

1) Why is below code error?

procedure q1 is
   generic
      type e is private;
   package the_package_like_ada2012_vector is
      type ref (element : not null access e) is null record;
   end the_package_like_ada2012_vector;
   generic
      type e is private;
      type ref (element : not null access e) is private;
   package the_package_use_ref is
   end the_package_use_ref;
   package inst1 is new the_package_like_ada2012_vector (integer);
   package inst2 is new the_package_use_ref (integer, inst1.ref); --
error
begin
   null;
end q1;

q1.adb:13:65: types of actual discriminants must match formal
q1.adb:13:65: instantiation abandoned

It seems no way to write a generic algorithm independent of a
container.
(without Element or Query_Element of Ada 2005 style)

2) Why non-limited types can not have access discriminant with
default?

procedure q2 is
	type t1 (value : integer := 100) is null record; -- ok
	type int_access is access all integer;
	default : aliased int_access := null;
	type t2 (element : access int_access := default'access) is null
record; -- error
begin
	null;
end q2;

q2.adb:5:56: (Ada 2005) access discriminants of nonlimited types
q2.adb:5:56: cannot have defaults

I would like to write smart pointer and make that it is able to assign-
able type,
as follows:

declare
   a : the_smart_pointer (element => wrap (new Integer'(100)));
   b : the_smart_pointer (element => wrap (new Integer'(200)));
begin
   a.element.all.all := 300; -- a.all := 300; (if compiler supported
Implicit_Dereference)
   b := a; -- it frees element of b on Finalize and add reference
count of a on Assign
end;

Why is it disallowed?

Thank you.