Re: Conditional holder for a type with unknown discriminant

[AdaMagica <christ-usch.grein@t-online.de>]

Can't see why this shouldn't do the trick:

package Owl_Holder is new Ada.Containers.Indefinite_Holders (T);
use Owl_Holder;

type R (Has_Item: Boolean) is record
  case Has_Item is
    when False => null;
    when True  => I: Holder;
  end case;
end record; 


Y: R := F (...);

case Y.Has_Item is
  when False => null;               -- Do nothing
  when True  => P (Y.I.Element);    -- Do whatever you want
                P (Element (Y.I);   -- ditto
                P (Y.I.Reference);  -- ditto using Ada 2012 reference type
                Y.I.Reference       := New_Item;   -- via reference type or
                Y.I.Replace_Element   (New_Item);  -- in traditional way
end case;

(Haven't compiled, but you get the idea:-)