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From: dennison@telepath.com
Subject: Re: Why a PROGRAM ERROR?
Date: 1998/12/11
Message-ID: <74relm$okm$1@nnrp1.dejanews.com>#1/1
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References: <3670178F.8751F9B9@nowhere.com> <m3yaofmpon.fsf@mheaney.ni.net>
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Date: 1998-12-11T00:00:00+00:00
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In article <m3yaofmpon.fsf@mheaney.ni.net>,
  Matthew Heaney <matthew_heaney@acm.org> wrote:
> Rusnak <bogus@nowhere.com> writes:
>
> > I have the following code  in a generic package (or code similar to it
> > anyway):
> >
> >
> >       Given a tagged type "My_Instance" and a type defined as
> >
> >           type My_Class_Object_Type is access all My_Instance'Class;
> >
> >     I have a function in the spec/body, defined in the body as
> >
> >           function Unpack(The_Object : access My_Instance'Class)
> >                                       Core_Text : String)
> >          return Command_Object
> >          is
> >             Return_Value :  Command_Object;
> >         begin
> >             . . .
> >                Command_Object.The_Object :=
> > My_Class_Object_Type(The_Object);
> >            . . .
> >          end Unpack;
> >
> >
> >         Any time I try to cast the anonymous access type to
> > My_Instance'Class to the type My_Class_Object_Type,
> >         a program error is raised.  Why is this the case, why is the
> > error not caught at compile time, and how can I resolve
> >        the error?  It seems to only occur  when a cast like this one is
> > made inside an instantiation of a generic package.
> >
> >        Any help is greatly appreciated.
>
> Is it possible that you're following a path that doesn't include a
> return statement (because of an internal exception, perhaps)?  If you're
> getting Program_Error when you execute a function, that's usually the
> reason.

Another very good possibility is that the *actual* parameter passed in was
the "'access" of an object, which was declared at a lower scope level than
the accesss type used by "Command_Object.The_Object" (which may be
My_Class_Object_Type, but we can't tell from this code fragment). I submitted
a (slighty more complete :-)  ) code fragment here with the exact same
problem about 4 months ago...

Perhaps Rusnak should submit an expanded fragment that shows the actual call
made, as well as the declaration of the actual parameters used, and their
enclosing scopes.

--
T.E.D.

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