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From: julius_bip@yahoo.com (Julio Cano)
Newsgroups: comp.lang.ada
Subject: Re: Persistence of limited tagged types
Date: 10 Apr 2003 04:41:49 -0700
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Actualy I already posted a creational pattern like that when Jano (I
think it was him) asked for the first time how to instantiate the
types from tags.


mheaney@on2.com (Matthew Heaney) wrote in message news:<1ec946d1.0304080558.2df9c8c8@posting.google.com>...
> 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.