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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Run-Time Type Assignment
References: <mailman.1030551002.7393.comp.lang.ada@ada.eu.org>
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Date: Thu, 29 Aug 2002 00:37:41 GMT
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Date: 2002-08-29T00:37:41+00:00
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>   type Level_One is new Base_Type with record
>...
>   type Level_Two is new Level_One with record
>...
>   type Level_Three is new Level_Two with record
  Seems the most straightforward to me.

But in <mailman.1030517642.16877.comp.lang.ada@ada.eu.org> Bob Leif said:
> I do not know how to select a subtype of a class at run-time.
    function Read_Whatever return Base_Type'class is
    begin
      -- read and find out what kind of record you have, then return it
      if figured_out_its_Level_One then
        declare
          Result : Level_One;
        begin
          -- fill Result with info
          return Result;
        end;
      elsif figured_out_its_Level_Two then
        declare
          Result : Level_Two;
        begin
          -- fill Result with info
          return Result;
        end;
      ...
    end Read_Whatever;

    One way to handle the different subtypes is

    procedure Process is
      Data : Base_Type'class := Read_Whatever;
    begin
      -- process the Level_One info that's always present
      if Data in Level_One then return;end if; -- that's all in this case
      -- now process the Level_Two stuff that's present in this case
      if Data in Level_Two then return;end if; -- that's all
      -- now process the Level_Three stuff that must be present in this case
      ...

    or, more in OO style:

    procedure Process(Data : in Level_One) is ...
    procedure Process(Data : in Level_Two) is ...
    procedure Process(Data : in Level_Three) is ...

    and then

    declare
      Data : Base_Type'class := Read_Whatever;
    begin
      Process(Base_Type'class(Data));  -- call the appropriate Process routine
    end;

    Since these things are different sizes, you can't put them in an
array, all of whose elements must be the same size.  But you can put
in access values.
    type Base_Type_Access is access Base_Type'class;
    Everything : array(1 .. 1_000_000) of Base_Type_Access;
    Count : Natural := 0;

    while not EOF loop
      Count := Count+1;
      Everything(Count) := new Base_Type'class'(Read_Whatever);
    end loop;
    -- Now Everything(1 .. Count) contains pointers to the records.
    -- The records are stored on the heap, each taking only as
    -- much room as it needs.
    for i in 1 .. Count loop                       -- for each record,
      Process(Base_Type'class(Everything(i).all)); -- do appropriate Process
    end loop;
---
Code not tested!