Re: class-wide objects

[Francois Godme <fgodme@magic.fr>]

Matthew Heaney <matthew_heaney@acm.org> writes:

> Your type will probably have to have some sort of dispatching clone method:
>
> generic
>   ...
> package Stacks is
>
>    type Stack_Type is tagged limited private;
>
>    type Stack_Access is access all Stack_Type'Class;
>
>    function Copy (Stack : Stack_Type) return Stack_Access;
> ...
> end Stacks;
>
> which you'd use something like:
>
> procedure Op (Stack : in Stack_Type'Class) is
>
>   Copy_Of_Stack : Stack_Access := Copy (Stack);
>
> begin

The clone method is inventive but it is a very odd method in the sense
that when you inherit it, it is always wrong for you. So, you have to
redefine it.

Usually, when you redefine a method, you call the inherited method on
the superclass part of you and finish the job yourself for the added
part. Here, the inherited method is not reusable because it clones
objects of your superclass not of yourself. You are the only one able
to clone yourself as your father knows nothing of you.

You are not forced to override this method by the language like the
language would do for an abstract method, so you can forget to do
it. You are on your own.

It's like a genetic disease that affects all generation. It can be
cured easily in one generation but the next generation will be
affected the same way. The disease cannot be stopped. It reappears
from generation to generation. Some generation are sane, some are
are sterile (they cannot reproduce themselves, they reproduce as their
parents.)

The cure is always the same. It can be as simple as an Emacs Macro.

   function Copy (Stack : Stack_Type) return Stack_Access is
      use type Ada.Tags.Tag;
   begin
      -- tries to be kind with sick children
      if Stack_Type'Class (Stack)'Tag /= Stack_Type'Tag then
         raise ...; -- Is that kindness ? I think it's better than return
null.
                    -- The family takes on responsabilities for its sick
                    -- children.

      else
  return new Stack_Type;
      end if;
   end Copy;

The attribute I was asking for does almost the same thing as the clone method
but without the associated problems.