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From: ms <m.siedlarek@nctz.net>
Newsgroups: comp.lang.ada
Subject: Abstraction over container iterators
Date: Fri, 3 Aug 2012 05:01:41 -0700 (PDT)
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Date: 2012-08-03T05:01:41-07:00
List-Id: <comp.lang.ada>

I'll let the code speak first:

-----------------------------------------------
with Ada.Containers.Hashed_Maps;                                           =
    =20
with Ada.Strings.Unbounded;                                                =
    =20
with Ada.Strings.Unbounded.Hash_Case_Insensitive;                          =
    =20
with Ada.Strings.Unbounded.Equal_Case_Insensitive;                         =
    =20

package Environments is                                                    =
    =20

   type Environment is tagged private;                                     =
    =20

   function Variable (                                                     =
    =20
      E    : in Environment;                                               =
    =20
      Name : in Ada.Strings.Unbounded.Unbounded_String                     =
    =20
   )                                                                       =
    =20
      return Ada.Strings.Unbounded.Unbounded_String                        =
    =20
      with Inline;                                                         =
    =20

   procedure Set_Variable (                                                =
    =20
      E     : in out Environment;                                          =
    =20
      Name  : in Ada.Strings.Unbounded.Unbounded_String;                   =
    =20
      Value : in Ada.Strings.Unbounded.Unbounded_String                    =
    =20
   )                                                                       =
    =20
      with Inline;                                                         =
    =20

private                                                                    =
    =20

   package Variable_Maps is new Ada.Containers.Hashed_Maps (               =
    =20
      Key_Type        =3D> Ada.Strings.Unbounded.Unbounded_String,         =
      =20
      Element_Type    =3D> Ada.Strings.Unbounded.Unbounded_String,         =
      =20
      Hash            =3D> Ada.Strings.Unbounded.Hash_Case_Insensitive,    =
      =20
      Equivalent_Keys =3D> Ada.Strings.Unbounded.Equal_Case_Insensitive,   =
      =20
      "=3D"             =3D> Ada.Strings.Unbounded."=3D"                   =
          =20
   );                                                                      =
    =20

   type Environment is tagged record                                       =
    =20
      Variables : Variable_Maps.Map;                                       =
    =20
   end record;                                                             =
    =20

end Environments;
-----------------------------------------------

What we have here is an example package fairly well illustrating my problem=
. I'm storing some environment variables in Hashed_Map, but I want to build=
 a abstraction layer over the standard container, so I can in future change=
 the underlaying container without changing any code in my package's custom=
ers.

Getting and setting variables is easy - as declared above. The real problem=
 is iterating. I'd like to let my package's customers to iterate over the e=
nvironment and get both key and value for each element easily.

As I'm using Ada 2012 the best way would be to use iterators, but how? I co=
uld return a cursor to the underlaying container, but again, this cursor's =
interface would be container-dependent.

What would you say is the best way to achieve such abstraction over standar=
d container iteration?