Re: controlled type in generic package?

["Pat Rogers" <progers@NOclasswideSPAM.com>]

"Ted Dennison" <dennison@telepath.com> wrote in message
news:95rpj5$gq7$1@nnrp1.deja.com...
> In article <95q1fa$9e$1@usenet.rational.com>,
>   "Mark Lundquist" <mark@rational.com> wrote:
> >
> > Ted Dennison <dennison@telepath.com> wrote in message
> > > Also, any instantiations of this generic will have to be done at the
> > > library level, since Limited_Controlled is declared at the library
> > > level, and types can't be declared at a lower level of scope than
> > > their parent types.
> ...
> > > (As a user, I hate that rule.)
> >
> > Do you hate it as much as what might happen if it weren't for the
> > rule? :-)
>
> I have to admit I'm a little unclear as to what nasty things could
> happen to me if I were allowed to declare a derived tagged type at a
> lower scope level than their parent type.

It is really very similar to the issue of an access value designating a
declared (aliased, on the stack) object.  For example, one could have an
access-to-classwide value designating an allocated value of a type derived
within a nested declarative region.  When that region "goes away", the type,
and hence, the primitive operations go away, and you don't want to try to
dispatch to them using that dangling pointer.

Consider:

package Graphics is

    type Shape is tagged
        record
            ...
    end record;

    type Any_Shape is access Shape'Class;

    procedure Dispatch( O : in Shape );

end Graphics;


with Graphics;  use Graphics;
procedure Illegal_Demo is

    Global : Any_Shape;

    procedure Bogus is

        package Shapes is
            type Circle is new Shape with ...
            procedure Dispatch( C : in Circle );
        end Shapes;

        package body Shapes is
            procedure Dispatch( C : in Circle ) is
            begin
                ...
            end Dispatch;
        end Shapes;

    begin
        Global := new Shapes.Circle;
    end Bogus;

begin
    Bogus;
    Dispatch( Global.all );
end Illegal_Demo;



Hoe this helps...

---
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