Re: Using Ada.Containers.Vector and Limited Private types

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Thu, 10 Jul 2008 15:59:30 -0700 (PDT), Gene wrote:
 
> procedure List_By_Date is
> 
>    type String_Ptr_Type is access String;
>    type File_Info_Type is
>       record
>          Modification_Time : Time;
>          Simple_Name : String_Ptr_Type; -- Freed with storage pool!
>       end record;
> 
>    package File_Info_Vectors is
>      new Ada.Containers.Vectors (Positive, File_Info_Type);

I am using a different design in such cases, which are a kind of cached
data store with sorted items.

I would make a descriptor type containing all data of an item:

   type File_Info (Length : Natural) is record
      Modification_Time : Time;
      Simple_Name : String (1..Length);
      ...
   end record;

File_Info can be allocated in an arena or mark-and-release pool. That is no
matter. Then comparison operations are defined on the pointers rather than
the descriptors:

   type File_Info_By_Date is access all File_Info;
   function "<" (Left, Right : File_Info_By_Date) return Boolean;

   type File_Info_By_Name is access all File_Info;
   function "<" (Left, Right : File_Info_By_Name) return Boolean;

   type File_Info_By_Size is access all File_Info;
   function "<" (Left, Right : File_Info_By_Size) return Boolean;

etc.

Then I would create ordered sets of File_Info_By_Date, File_Info_By_Name
and so on. All this I put into a controlled object which takes care about
inserting and removing items. Indexes of pointers are kept sorted. This is
exactly the design I used for a persistency layer.

(I don't use Ada.Containers, but I think this approach should work with
them too.)

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de