Re: Abstract null value

[Ted Dennison <dennison@telepath.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.

--
T.E.D.

http://www.telepath.com/~dennison/Ted/TED.html


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