Re: how to delete from Ada.Containers.Doubly_Linked_Lists

[Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>]

On 08/18/2014 11:51 AM, Björn Lundin wrote:
> 
> 
> I've got a sample below,
> but I fell it is clumpsy.
> How should a nice solution look like ?

You probably ought to read about "tampering with cursors" in the ARM (A.18.3).
In your case, iterating over the list needs the list to remain unchanged until
iteration finishes. Deleting during iteration would violate this.

This restriction is intended for safety, and sometimes seems onerous. I've had
to iterate over one data structure, storing cursors in another data structure,
then iterate over the 2nd data structure to delete using the stored cursors.

You can delete during iteration if you don't invoke an Iterate subprogram:

   declare
      C    : Example_Pkg.Cursor := List.First;
      Next : Example_Pkg.Cursor;

      use type Example_Pkg.Cursor;
   begin
      loop
         exit when C = Example_Pkg.No_Element;

         declare
            I : constant Example_Type := Example_Pkg.Element (C);
         begin
            if I.A /= 15 then
               Example_Pkg.Next (Position => C);
            else
               Next := Example_Pkg.Next (C);
               List.Delete (Position => C);
               C := Next;
            end if;
         end ;
      end loop;
   end;

-- 
Jeff Carter
"You a big nose have it."
Never Give a Sucker an Even Break
107