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Date: Sat, 09 Feb 2008 04:48:25 +0100
From: Georg Bauhaus <see.reply.to@maps.futureapps.de>
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Subject: Re: Loosing tagged type via linked list
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Date: 2008-02-09T04:48:25+01:00
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jason.height@gmail.com wrote:

Looks like an issue with copying into a record component
in the generic list. Copying would be unlike parameter passing.
While tagged objects are passed by reference, assigning to a
variable of a specific type copies a value of the target
type. Only the relevant components of the target type stay.

Consider this:

with Ada.Tags;
with Ada.Text_IO;

procedure Main is

    package Type_Hier is

        type A_Type is tagged null record;

        procedure DoPrint(The_Item: A_Type);
        -- print The_Items's actual tag

        type B_Type is new A_Type with null record;

    end Type_Hier;


    package body Type_Hier is

        procedure DoPrint(The_Item: A_Type) is
            use Ada;
            Whoami: A_Type'Class renames A_Type'Class(The_Item);
        begin
            Text_IO.put_line("Tag is " &
                Tags.Expanded_Name(Whoami'Tag));
        end DoPrint;

    end Type_Hier;



    use Type_Hier;


    Object: A_Type;
    -- this would be the Item_Type component of Item_Record

begin  -- Main

    DoPrint(A_Type'(null record));
    DoPrint(A_Type(B_Type'(null record)));  -- A_Type view

    Object := A_Type(B_Type'(null record));
      -- not a reference, neither just an A_Type view, but a copy
      -- of the A_Type parts

    DoPrint(Object);

end Main;

$ ./main
Tag is MAIN.TYPE_HIER.A_TYPE
Tag is MAIN.TYPE_HIER.B_TYPE
Tag is MAIN.TYPE_HIER.A_TYPE

One solution would be to use references to classwide types,
somewhat like Java.


> I am attempting this in Ada95, with my own Linked_List container impl,
> which seems to be destroying the type information. If anyone knows a
> Ada95 list implementation that can store items of different types
> (derived from a base type) then please let me know.

The Booch components will let you do this, using indefinite
generic formal types.
http://sourceforge.net/project/showfiles.php?group_id=135616.
(The WiKi seems out of order at this time; doc is contained in
an accompanying archive.)

The Charles library is the predecessor of Ada 2005's container
library, http://charles.tigris.org/ . There is a subdirectory
ai302 which has an Ada 95 implementation of containers for
indefinite types, too.

HTH,
 -- Georg