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From: Maciej Sobczak <no.spam@no.spam.com>
Newsgroups: comp.lang.ada
Subject: Re: Structured exception information
Date: Tue, 16 Jan 2007 10:04:48 +0100
Organization: CERN News
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Date: 2007-01-16T10:04:48+01:00
List-Id: <comp.lang.ada>

claude.simon@equipement.gouv.fr wrote:

> when My_Type is an indefinite subtype.
> 
> type My_type (<>) is ... for example
> 
> Then, when you declare an object of that type you must provide an
> initialization expression (a constructor call !).

If I understand correctly:

procedure Hello is

    package P is
       type T(<>) is private;
       function Constructor(S : String) return T;
    private
       type T(I : Integer) is
          record
             null;
          end record;
    end P;

    package body P is
       function Constructor(S : String) return T is
       begin
          return (I => 0);
       end Constructor;
    end P;

    X : P.T := P.Constructor("some params");

begin
    null;
end Hello;

(of course, package is nested for presentation only)

I have initially thought that discriminated types limit me in terms of 
what parameter types can be used for constructing them, but actually 
there is no such limitation - constructor function can have whatever 
parameter types it wants and the discriminant is irrelevant.

It does not feel like a genuine part of the object model (you know, in 
C++ it is much simpler, more readable, maintainable, etc. ;-) ), but 
seems to work.


OK, so putting aside the fact that my compiler can not swallow it, is it 
possible to make T Limited_Controlled as well?

This is my guess:

-- file p.ads
with Ada.Finalization;
package P is
    type T(<>) is limited private;
    function Constructor(S : String) return T;
private
    type T(I : Integer) is new Ada.Finalization.Limited_Controlled with
       record
          null;
       end record;
end P;

-- file p.adb
package body P is
    function Constructor(S : String) return T is
    begin
       return (I => 0);  -- NOTE
    end Constructor;
end P;

-- file hello.adb
with P;
procedure Hello is
    X : P.T := P.Constructor("some params");
begin
    null;
end Hello;

It should be impossible to create X in any other way - this is the exact 
effect that I want to achieve.

My compiler does not like it (but it's not really Ada2005), so I cannot 
verify it. Is the code above correct?

BTW - in the line marked as NOTE the compiler says that aggregate type 
cannot be limited. Is it true for Ada2005?

-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/