On Thu, 24 Sep 2009 09:06:41 -0400, Andrei Alexandrescu
<seewebsiteforem...@erdani.org> wrote:
Steven Schveighoffer wrote:
On Wed, 23 Sep 2009 11:13:26 -0400, Andrei Alexandrescu
<seewebsiteforem...@erdani.org> wrote:
class AutoVehicle { ... }
class Car : AutoVehicle { ... }
class Truck : AutoVehicle { ... }
class Driver {
// A driver is licensed to drive a car
void drive(Car c);
}
class TruckDriver : Driver {
// A truck driver is licensed to drive a car...
override void drive(Car c);
// ... and a truck
void drive(Truck c);
// No contravariance needed yet
}
class JamesBond : Driver {
// James Bond can drive any auto vehicle
// Contravariance needed here
override void drive(AutoVehicle c) { ... }
}
Now if what you have is a JamesBond and a Truck, you need
contravariance to have him drive it. (A HotGirl may or may not be
present in the scene.)
Your example just triggered a possible problem with contravariance.
Consider this class:
class Bad : TruckDriver {
override void drive(AutoVehicle c) { ...}
}
What does this override, drive(Truck) or drive(Car), or both? What if
you didn't want to override both? My instinct is that this should be
an error, to keep things simple. But it might be very annoying for
some designs...
It should override both. It is already the case that one method
overrides several others (e.g. with multiple inheritance of interfaces).
Yes, but interfaces have no implementation -- it's clear you want to
override both because there's no alternative.
Here is another example:
class RaceCar : Car {...}
class RaceCarDriver : Driver
{
alias Driver.drive drive;
void drive(RaceCar c) {...} // doesn't override Driver.drive(Car),
because he drives normal cars in a different way
}
class BadDriver : RaceCarDriver // drives all cars like a race car
{
override void drive(Car c) {...}
}
Should this override RaceCarDriver.drive(RaceCar)?
Currently, this is valid code (tested on 1.046) and results in different
behavior than what you propose. Although it's likely in my example you
*want* to override both, but there could be cases where you do not.
-Steve
