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From: nospam@thanks.com.au (Don Harrison)
Subject: Re: Can't export object of private type
Date: 1999/03/03
Message-ID: <F7zu92.Iwu@syd.csa.com.au>#1/1
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References: <m3emn876zf.fsf@mheaney.ni.net>
Organization: CSC Australia, Sydney
Reply-To: nospam@thanks.com.au
Newsgroups: comp.lang.ada
Date: 1999-03-03T00:00:00+00:00
List-Id: <comp.lang.ada>

Matt Heaney wrote:

[Matt's example]

Thanks. Your pattern for a hierarchy of singletons is exactly what I'm after. 
Here is my test program based on your solution:


package T is

  type T_Type (<>) is abstract tagged private;
  type Access_Class_T_Type is access all T_Type'Class;

  procedure Display (T: access T_Type) is abstract;
  function Solo_T return Access_Class_T_Type is abstract;

private
  type T_Type is abstract tagged null record;
end;


package T.Child is

  type T_Child_Type (<>) is new T_Type with private;
  type Access_Class_T_Child_Type is access all T_Child_Type'Class;

  procedure Display (TC: access T_Child_Type);
  function Solo_T return Access_Class_T_Child_Type;

private
  type T_Child_Type is new T_Type with null record;
  Self: aliased T_Child_Type;
end;

with Text_IO; use Text_IO;
package body T.Child is

  procedure Display (TC: access T_Child_Type) is
  begin
    Put_Line ("I am a T_Child object");
  end;

  function Solo_T return Access_Class_T_Child_Type is
  begin
    return Self'Access;
  end;

end;


package T.Other_Child is 
  ...  -- similar
end;

package body T.Other_Child is 
  ...  -- similar
end;


with T; use T;
with T.Child; use T.Child;
with T.Other_Child; use T.Other_Child;
procedure Use_T is

  type T_Index_Type is new Integer range 1 .. 2;
  type T_Array_Type is array (T_Index_Type) of Access_Class_T_Type;

  T_Array: T_Array_Type := (
    1 => T.Child.Solo_T.all'Access,          -- 1)
    2 => T.OTher_Child.Solo_T.all'Access);   -- 1)

begin
  for I in T_Array'Range loop
    Display (T_Array(I) );                   -- 2)
  end loop;
end;


1) Can this ugly stuff be avoided?
2) Dispatching call.


:We have used the features the language provides us to design a type
:hierarchy such that, the client must use the singleton instance(s).  He
:has no other choice in the matter, which is the intended effect.

That's what I'm after.


-- 
Don (Harrison).   donh at syd.csa.com.au