Re: private type discriminants ignored?

["Samuel T. Harris" <sam_harris@hso.link.com>]

Patrick D. Rogers wrote:
> 
> Samuel T. Harris wrote:
> >
> > Robert A Duff wrote:
> > >
> > > In article <rz84szje1ol.fsf@cs.williams.edu>,
> > > Jay Sachs  <sachs@cs.williams.edu> wrote:
> > > >       type Stack(Size : Integer := 100) is private;
> > >
> > > >...  What is the reason that the default initialization
> > > >seems to be ignored in the discriminant for the private type?
> > >
> > > The default isn't being ignored; Size is being correctly initialized to
> > > 100.  However, the mere presence of a default means that the
> > > discriminant can change.  That's confusing, and is a language design
> > > flaw, IMHO.  There should have been a separate keyword or some other
> > > separate syntax to mean "the discrim might change".  But as it is in
> > > Ada, an unconstrained variable of type Stack can later have a discrim of
> > > Integer'Last, even though it defaults initially to 100.  And many
> > > compilers take the (quite reasonable) attitude that implicit heap
> > > allocation is evil, so they allocate the maximum possible size for these
> > > things.  And in this case, that's enormous, and will typically raise
> > > Storage_Error; hence the warning.
> >
> > Not entirely accurate. The mere presense of the default value does
> > not mean the discriminant can change. It depends on how objects
> > are declared.
> 
> Sam,
> 
> Since we are in pedantic mode, :) Bob said "... unconstrained variable
> ...", so he is indeed entirely accurate.
> 
> :)
> 
> --pat
> 
> Patrick Rogers

To one-up the pedantic theme :) Bob's "... unconstrained variable"
appears several lines after "a default means that the discriminant
can change". Perhaps I should have said "Not entirely complete."

Bob covers unconstrained variables exclusively.
I simply sought to cover constrained variables and complete the picture.

The subtle differences are lost on many of our (my company's)
Ada initiates and I have spent some time explaining the nuances
of this construction, especially as applied to polymorphic variant
records.

Now, how's that for a recovery ;)

-- 
Samuel T. Harris, Principal Engineer
Raytheon Training Incorporated
"If you can make it, We can fake it!"