Re: Self pointer in limited record

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

On Wed, 04 Jul 2007 12:31:08 -0700, Maciej Sobczak wrote:

> Consider:
> 
> type T_Access is access all T;
> type T is  new Ada.Finalization.Limited_Controlled with record
>    Self : T_Access := T'Unchecked_Access;
>    -- more components
>    -- ...
> end record;
> 
> I have seen this pattern repeatedly. What is the use for Self?

The pattern (also called Rosen trick) is used:

1. to having mutable arguments of functions:

   function Search (X : T; K : Key) return Value is
      Object : T renames X.Self.all;
   begin
      ... -- Modify the search cache associated with X via
          -- mutable Object view

2. to re-dispatch from primitive operations:

   type T_Access is access all T'Class;  -- Note 'Class
   type T is  new Ada.Finalization.Limited_Controlled with record
      Self : T_Access := T'Unchecked_Access;

   procedure Bar (X : T); -- Primitive
   procedure Foo (X : T); -- Primitive

   procedure Foo (X : T) is
      Object : T'Class renames X.Self.all;
   begin
      Bar (X); -- This does not dispatch!
      Bar (Object.Self.all); -- This dispatches
      Bar (T'Class (X));  -- This dispatches as well

The later (view conversion) should better be removed from the language, so
I always prefer Rosen trick for such purpose.

Both defeat the type system in some sense and potentially indicate a
design problem.

> And why is it Unchecked?

Because of accessibility rules.

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