Re: Abstract null value

["Jean-Pierre Rosen" <rosen.adalog@wanadoo.fr>]

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"Ted Dennison" <dennison@telepath.com> a �crit dans le message news: 95uda1$pq6$1@nnrp1.deja.com...
> In article <95sgl4$3c8$2@wanadoo.fr>,
>   "Jean-Pierre Rosen" <rosen.adalog@wanadoo.fr> wrote:
>
> > Sometimes, the constructor is not able to perform the job, and you
> > want to return some "null value" (I know, you can also raise an
> ...
> > package Abstract_Type is
> >    type Root_Type is abstract tagged null record;
> >    procedure Oper (X : Root_Type) is abstract;
> >
> >    Null_Value : constant Root_Type'Class;
>
> For this system to work, all your "constructors" would have to be
> functions returning something like Root_Type'Class, and their targets
> would all have to be class-wide initializations or allocators, right? Or
> is that some kind of unwritten standard of which I'm unaware? A slim
> majority of the time when I'm "constructing" a tagged type object
> there's a specific type the expression is expecting. If I were to do
> that with the above system, then I'd just get Constraint_Error.

The kind of constructor I was thinking about is one that, for example, reads a file and returns various objects (widgets...). If you
have a syntax error, or the end of file, you may want to return a null value. I'm not saying it's applicable to everything, just
that it can be useful in some cases.

--
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           J-P. Rosen (Rosen.Adalog@wanadoo.fr)
Visit Adalog's web site at http://pro.wanadoo.fr/adalog