Re: Mut. Recurs. in Ada9X w/o Breaking Encaps.? (LONG)

[stt@dsd.camb.inmet.com (Tucker Taft)]

In article <1994Sep29.021209.20769@swlvx2.msd.ray.com>,
John Volan <jgv@swl.msd.ray.com> wrote:

>NHC = ncohen@watson.ibm.com (Norman H. Cohen) writes:
>
>NHC>One variation on this is to declare recursive types meant to serve as the
>NHC>parents of types Employee and Office, but to provide no operations for
>NHC>these recursive types.  Then, in child packages, declare Employee and
>NHC>Office themselves as derived types and declare primitive operations in
>NHC>those child packages: 
>NHC>
>NHC>    with ... ; -- other stuff needed by Employee
>NHC>    with ... ; -- other stuff needed by Office
>NHC>
>NHC>    package Employees_And_Offices is
>NHC>
>NHC>      type Employee_Parent is tagged limited private;
>NHC>      type Employee_Pointer is access all Employee'Class;
>NHC>
>NHC>      type Office_Parent is tagged limited private;
>NHC>      type Office_Pointer is access all Office'Class;
>
>This solution suffers from the same problem as Mark Biggar's suggestion:
>These access types only give clients an "opaque" view of the designated objects.
>The useful primitive operations for these objects won't be declared until we 
>get to the concrete *subclasses* declared later.  So a client would have to
>resort to using a non-typesafe Unchecked_Conversion to "downcast" one of these
>pointers into designating the corresponding concrete subclass.

As mentioned in a previous post, there is no need to use
non-typesafe Unchecked_Conversion.  A safe, checked, explicit "downcast"
(or "narrowing") conversion is permitted in Ada 9X when the operand is 
of a class-wide type, or of an access-to-class-wide type.
See RM9X-4.6(15, 23, 42, 50);5.0.

>-- John Volan
>--                  Company             => "Raytheon Missile Systems Division",
>--                  E_Mail_Address      => "jgv@swl.msd.ray.com",

S. Tucker Taft   stt@inmet.com
Intermetrics, Inc.
Cambridge, MA  02138