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From: mbk@caffeine.engr.utk.edu (Matt Kennel)
Subject: Re: Real OO
Date: 1996/05/01
Message-ID: <4m6l27$sa6@gaia.ns.utk.edu>#1/1
X-Deja-AN: 152339265
references: <JSA.96Apr19163742@organon.com> <DqAyLG.D47@assip.csasyd.oz>
 <JSA.96Apr29202942@organon.com>
followup-to: comp.lang.eiffel,comp.lang.ada,comp.object
organization: University of Tennessee,
 Knoxville and Oak Ridge National Laboratory
reply-to: kennel@msr.epm.ornl.gov
newsgroups: comp.lang.eiffel,comp.lang.ada,comp.object
Date: 1996-05-01T00:00:00+00:00
List-Id: <comp.lang.ada>


Jon S Anthony (jsa@organon.com) wrote:
: First, this is a really bad design (even for an example) as it
: violates any of the (several) reasonable definitions of is-a
: semantics.  Second, this example has nothing to do with "legacy" code
: or reuse (which is what you claimed was important and I was refering
: to).  But, let's take it at face value.  Neither the a) no b) case is
: a problem - not even an issue - in Ada.  This is mainly due to the
: separation of specific and classwide types.

: The example in Ada:

: package Drivers is
:     type Driver is tagged private;
: ...
: end Drivers;

: package Drivers.Cars is
:     type Car_Driver is new Driver with private;
: ...
: end Drivers.Cars;

: package Drivers.Trucks is
:     type Truck_Driver is new Driver with private;
: ...
: end Drivers.Trucks;



: with Drivers;  use Drivers;
: package Vehicles is
:     type Vehicle is tagged private;

:     procedure Register_Driver (V: Vehicle; D: Driver);
:     procedure Renew_Rego_By_Mail (V: Vehicle);
: ...
: end Vehicles;

: with Drivers.Cars; use Drivers.Cars;
: package Vehicles.Cars is
:     type Car is new Vehicle with private;

:     procedure Register_Driver (V: Vehicle; D: Car_Driver);
: ...
: end Vehicles.Cars;

: with Drivers.Trucks; use Drivers.Trucks;
: package Vehicles.Trucks is
:     type Truck is new Vehicle with private;

:     procedure Register_Driver (T: Truck; D: Truck_Driver);
:     procedure Renew_Rego_By_Inspection(T: Truck);
: ...
: end Vehicles.Trucks;


Sather:

abstract class $DRIVERS is

end; 

class CAR_DRIVER < $DRIVERS is

end; 

class TRUCK_DRIVER < $DRIVERS is

end; 


class VEHICLE is
    register_driver(d:$DRIVER);
end;

class CAR is
    include VEHICLE;
    register_driver(d:CAR_DRIVER); 
end;
    
class TRUCK is
    include VEHICLE;
    register_driver(d:TRUCK_DRIVER); 
end; 


Though I'd personally not rely on the punning, and would rather do it with
a distinct name.

class VEHICLE is
    register_driver_if_valid(d:$DRIVER):BOOL; -- return success
end; 

: jsa@organon.com