Re: Access idiom

[Gene <gene.ressler@gmail.com>]

On Jan 20, 8:01 pm, Ludovic Brenta <ludo...@ludovic-brenta.org> wrote:
> Gene writes:
> > I tentatively have given up on access dispatching entirely,
> > dispatching instead on node types (not access) and copying to form
> > fresh values of a named classwide access type for the return
> > value(s).  This same named type is used for node child pointers.
> > While this okay, there is a lot of useless copying.
>
> > Feels like I'm playing whack-a-mole with the Ada type system.  What's
> > the idomatic way to get this job done without the copying?
>
> Parameters of tagged types being always passed by reference (see ARM
> 6.3.2(5)), there is no copying involved in "dispatching on node types"
> as opposed to access, so I think your solution is fine.  Then again, I
> may have misunderstood your problem.
>

I haven't been clear.  When I dispatch on the tagged record types
rather than access to tagged record, then I have no way of returning a
pointer to the dispatching object or a child pointer.

In concept, what I'd need to do is something like this:

type Node_Ptr_Type is access Node_Type'Class;

procedure Op(P : in Threat_Type; Result : out Node_Ptr_Type) is
begin
   Result := P'Access; -- identity operation
end Op;

... or the other way:

procedure Op(P : access Threat_Type; Result : out Node_Ptr_Type) is
begin
   Result := Node_Ptr_Type(P); -- identity operation
end Op;

Of course neither of these is correct Ada.  Instead I'm doing this:

procedure Op(P : in Threat_Type; Result : out Node_Ptr_Type) is
begin
   Result := Node_Ptr_Type'(new Threat_Type'(P)); -- identity
operation
end Op;

This works, but the copy is unnecessary.