Re: Discriminant computation problem

["Nick Roberts" <nick.roberts@acm.org>]

<tmoran@acm.org> wrote in message news:aVamd.100382$R05.30042@attbi_s53...

> Another approach is to make the type controlled, then have the
> Initialize routine do the size calculations based on the discriminant
> and call an allocator for the actual data.  Finalize then deallocates
> the data, of course.

Illustration of this technique:

   with Ada.Finalization;
   package Bit_Vectors is
      ...
      type Bit_Vector (Bit_Max: Bit_Number) is private;
      ...
   private
      ... -- declare Element_Array etc. as before

      type Vector_Payload (Max: Element_Index) is
         record
            ... -- as for Bit_Vector before
         end record;

      type Payload_Access is access Vector_Payload;

      type Bit_Vector (Bit_Max: Bit_Number) is
         new Ada.Finalization.Controlled with
         record
            Payload: Vector_Access;
         end record;

      procedure Initialize (Vector: in out Bit_Vector);
      procedure Finalize   (Vector: in out Bit_Vector);
   end;

   package body Bit_Vectors is
      ...

      procedure Free is
         new Unchecked_Deallocation( Bit_Vector_Payload,
                                    Dynamic_Bit_Vector );

      procedure Initialize (Vector: in out Bit_Vector) is
      begin
         Vector.Payload := new Bit_Vector_Payload'(
            Max => Vector.Bit_Max/Bits_Per_Element,
            ... -- as before
            );
      end;

      procedure Finalize (Vector: in out Bit_Vector) is
      begin
         Free( Vector.Payload );
      end;

      ...
   end Bit_Vectors;

This technique has the disadvantage that the type Bit_Vector is tagged, and 
that controlled types use up some extra memory (to store the list of objects 
needing finalisation). But I would guess that this unlikely to be a problem 
in pratice. It has the advantage of being a little neater to use.

-- 
Nick Roberts