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From: mheaney@on2.com (Matthew Heaney)
Newsgroups: comp.lang.ada
Subject: Re: Persistence of limited tagged types
Date: 8 Apr 2003 06:58:56 -0700
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Jano <nono@celes.unizar.es> wrote in message news:<MPG.18fc158bd901913d9896db@News.CIS.DFN.DE>...
> Hyman Rosen dice...
> > 
> > This is known as the Factory pattern. Instead of changing the
> > function for each new tag, you can have the package that defines
> > the type enroll a read method into a table of methods. Then the
> > factory reads the external tag representation, looks it up in
> > the table, and calls the method if it's present.
> 
> I like this solution. With very little overwork.

This will be the subject of the Dear Ada column in next issue of Ada
Letters.

Basically, each type will register a factory function like this:

  function New_T return T'Class;

Implemented something like this (say, for NT, which derives from T):

  function New_T return T'Class is
  begin
     return NT'(T with I, J, K);
  end;

Actually, your type is limited, so it would be something like:

  function New_T return T_Class_Access is
     O : constant NT_Access := new NT;
  begin
     --init O
     return T_Class_Access (O);
  end;

You have a map whose keys are type string, and whose element is a
pointer to the factory function:

   type Factory_Type is access function return T_Class_Access;

   package Table_Types is
     new Charles.Maps.Sorted.Strings.Unbounded (Factory_Type);

   Table : Table_Types.Container_Type;

Now a type registers its factory function:

   procedure Register 
     (Key : Ada.Tags.Tag;
      Factory : Factory_Type) is
   begin
      Insert (Table, External_Tag (Key), Factory);
   end;

Now, given a tag, you can look up the factory function, and create a
new object, e.g.

   function New_T (Key : Ada.Tags.Tag) return T_Class_Access is
      I : constant Iterator_Type := Find (Table, External_Tag (Key));
   begin
      if I = Back (Table) then
        raise Tag_Error;
      end if;

      declare
         Factory : constant Factory_Type := Element (I);
      begin
         return Factory.all;  --invoke factory function
      end;
   end New_T;

If you're streaming off of disk, then the tag is probably stored in
the stream in its external form, so one possibility is:

   function New_T (Stream : access Root_Stream_Type'Class) 
      return T_Class_Access is

      Key : constant String := Read (Stream);  --or whatever
      I : constant Iterator_Type := Find (Table, Key);
   begin
      ...
      declare
         Factory : constant Factory_Type := Element (I);
      begin
         return Factory (Stream); --invoke factory function
      end;
   end New_T;

Here, we pass the stream to the factory function, and let it finish
reading the object out of the stream.

The map container in the example is from the Charles library.

http://home.earthlink.net/~matthewjheaney/charles/

You can use either the sorted or hashed version of the map, that has
type String as the key.