Re: Dynamich Dispatching...

["Matthew Heaney" <mheaney@on2.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:uv6ftd235dgf$.1px68p9wf1tjb.dlg@40tude.net...
> On Mon, 04 Oct 2004 21:02:44 +1000, Brian May wrote:
>
> Then if one really needs a contravariant result, then Create should be
> class-wide. It is as simple as:
>
>    function Create return X'Class;

The issue is not whether one needs a contravariant result.  Rather, the
issue is whether the constructor is primitive for the type or not.

In general, constructors should *not* be primitive.  One way to do that is
to declare the constructor in a nested package:

package P is
   type X is tagged ...;
   package Constructors is
      function Create return X;
   end;
end P;

A constructor should always return a specific type, not a class-wide type.
Your example above is wrong, since it would require the declaration of an
object whose type is class-wide, and hence force the use of dispatching.
Clearly that's wrong, when you know the specific type of object you want to
create.

The only time is makes sense for a constructor to return a class-wide type
is when this is a factory method pattern.  In that case, the operation is
primitive for parameter type:

package Q is
   type X is abstract tagged ...;
   type Y is abstract tagged ...;

   function Create (O : X) return Y'Class is abstract;
end Q;