Re: Re-exporting primitive operations of a private type (like "-", and 'min)

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sat, 29 Oct 2005 21:19:27 GMT, Anonymous Coward wrote:

> I'm bothered by the fact that the primitive operations of a parent are
> not inherited in a private type.

They are but in the view where inheritance happens. In your example it
happens in the private view.

> In particular, the basic arithmetic
> operations are lost, like "+", "-", 'min, 'max, etc.

"+' and "-" are rather hidden.

> There seems to
> be no clean way to re-export these operations,

Yep. That's because the original operation cannot be named. Otherwise, one
could just rename new "-" to the old one. As for attributes, it is another
case, they cannot be user-defined at all.

> Redefining "-" is an annoyance.

In general, the problem is that old-fashioned way emerged before Ada became
OO. Private_Distance_Type is a numeric type. That should be its *public*
contract. But in Ada it is impossible to specify things like this for
non-tagged types [outside generics, to be precise.] The compiler cannot
recognize the programmer's intent from a bunch of operations declared in
the public part. So it does not. Therefore when in the private part
Private_Distance_Type gets implemented as a clone of a numeric type, the
compiler treats public "-" as a different operation declared.

[ I'm not sure if interfaces of Ada 2005 might help here. Let's wait for a
working compiler. ]

Anyway, you could go this way:

   ...
private
   type Most_Private_Distance_Type is new Public_Distance_Type;
   function Sub (Left, Right : in Most_Private_Distance_Type)
      return Most_Private_Distance_Type renames "-";
       -- Renaming "as-declaration" creates a new primitive operation Sub

   type Private_Distance_Type is new Most_Private_Distance_Type;
       -- Both "-" and Sub are inherited, but "-" gets hidden by "-"
       -- declared in the public part

   function "-" (Left, Right : in Private_Distance_Type)
      return Private_Distance_Type renames Sub;
       -- Renaming "as-body" implements public "-" through private "-"
       -- via Sub

Warning, this may crash GNAT 3.15p which has many bugs in visibility rules
(let language lawyers correct me if I'm wrong and the above is illegal.) I
didn't try such things with GNAT GPL but it is even more buggy. (:-()

> I'm not even sure how to redefine
> attribute operations like 'min.  Suggestions?

They aren't "normal" operations, as they should be, so forget about them.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de