Re: Real OO

[donh@syd.csa.com.au (Don Harrison)]

Not sure whether I put the right distribution on this, so here it is again.


Jon,

Stepping back to your original example, I'd like to clarify a few things before 
commenting on it (again).

: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);

I assume this syntax is correct. That is, you don't have to say Driver'Class?
The status of D would be clearer, IMO, if you had to write Driver'Class.

:    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);

Presumably, Register_Driver does not override the implementation for Vehicle
because of RM 6.3.1(15) and 8.3(8,9)?

:....
:end Vehicles.Trucks;
:
:
:-- In a client somewhere
:--
:    D: Driver;
:    Cd: Car_Driver;
:    V: Vehicle;
:    T: Truck;
:....
:    -- Option 1.
:    --
:    D := Cd; -- Illegal, compile time error
:    V := T;  -- Illegal, compile time error
:
:    -- Option 2.
:    --
:    D := Driver(Cd); -- OK, convert toward root
:    V := Vehicle(T); -- OK, convert toward root

I understand these are view conversions, but am wondering how this can be 
implemented without reference semantics?

:    Register_Driver(V, D); -- just fine and no dispatch needed

Is this this call statically bound to the implementation for Vehicle?


:    -- Option 3.
:    --
:    Register_Driver(Vehicle'Class(T),Cd); -- Compile error, no primitive ops

... for type Truck with formal parameter Car_Driver?

:    -- Option 4.
:    --
:    Register_Driver(Truck'Class(T), Cd);  -- Compile error, no primitive ops

... for type Truck with formal parameter Car_Driver?

:    -- Option 6.
:    --
:    Register_Driver(Vehicle'Class(V), D); -- Fine
:    Register_Driver(Vehicle'Class(T), D); -- Fine (calls inherited op!!)

Shouldn't the first one be:

    Register_Driver(Truck'Class(T), D);   -- Fine

:As you can see, you can't get the invalid system stuff in the Ada model.
:In particular, you should note that the Truck type has _two_ primitive
:operations Register_Driver:
:
:  procedure Register_Driver(V: Truck; D: Driver); -- Inherited from Vehicle
:  procedure Register_Driver(V: Truck; D: Truck_Driver); -- *New* primitive op

And, shouldn't these be:

:  procedure Register_Driver(V: Vehicle; D: Driver); -- Inherited from Vehicle
:  procedure Register_Driver(T: Truck; D: Truck_Driver); -- *New* primitive op

:
:/Jon
:-- 
:Jon Anthony
:Organon Motives, Inc.
:1 Williston Road, Suite 4
:Belmont, MA 02178
:
:617.484.3383
:jsa@organon.com
:


                    ///
Don.               (o o)
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