Re: type access Parent'Class

[Yukicanis <yukicanis@googlemail.com>]

On Nov 21, 8:33 pm, Robert A Duff <bobd...@shell01.TheWorld.com>
wrote:
> Yukicanis <yukica...@googlemail.com> writes:
> > Dear Group,
>
> > I'm new to Ada and currently playing around with its OOP features.
> > Now, I have the following source:
>
> > procedure Test is
> >   type Parent is tagged limited null record;
> >   type Parent_Access is access Parent'Class;
> >   A : access Parent'Class;
> >   B : Parent_Access;
> >   procedure Dyn_Disp(X : access Parent'Class) is
> >   begin
> >     A := X;
> >     B := X;
> >   end Dyn_Disp;
> > begin
> >   null;
> > end Test;
>
> > When I try to complie that with GNAT 4.6.1 I get the following error
> > message:
>
> > test.adb:9:10: expected type "Parent_Access" defined at line 3
> > test.adb:9:10: found type access to "Parent'Class" defined at line 6
>
> > which I don't really understand since type "Parent_Access" is type
> > access to Parent'Class, isn't it?
>
> No, there are three different access types -- Parent_Access,
> the anonymous type of A, and the anonymous type of X.  They're all
> "access to Parent'Class", but they are distinct.
>
> There are some implicit conversion rules that make "A := X;" legal.
> But there is no implicit conversion to Parent_Access, so "B := X;"
> is illegal.
>
> You normally want access-to-class-wide types (and many other access
> types) to be "general", which is specified by adding "all":
>
>     type Parent_Access is access all Parent'Class;
>
> If you do that, then you could say "B := Parent_Access(X);"
> or "B := A.all'Access;".
>
> But I suggest you avoid anonymous access types, except in certain
> special cases.  They are confusing.
>
> I also suggest you put most of your code in packages, rather than
> in the main procedure.  That will also help avoid confusion, because
> things behave differently when inside procedures.  For example,
> you can't have any dispatching procedures unless you put the type
> in a package spec.
>
> - Bob

Thanks for your replay. I know that things behave differnetly in
packages. I just wanted the minimal example to be as short es possible
to avoid spamming.