Multiple Dispatch in practice

[Sergey Gromov]

Tue, 21 Oct 2008 22:01:15 -0400,
bearophile wrote:
> This is normal D1 code that shows a manual implementation of double dispatch:
> 
> import std.stdio: writefln;
> 
> /// ...
> template IsInstance(Class) {
>     static assert(is(Class == class), "IsInstance: Class must be a class");
> 
>     bool IsInstance(TyObj)(TyObj obj) {
>         return cast(Class)obj !is null;
>     }
> }
> 
> interface Vehicle {
>     void collide(Vehicle v);
> }
> 
> class Car : Vehicle {
>     void collide(Vehicle v) {
>         if (IsInstance!(Car)(v))
>             writefln("Car hits car");
>         else if (IsInstance!(Bike)(v))
>             writefln("Car hits bike");
>         else
>             throw new Exception("missing case: should not happen");
>     }
> }
> 
> class Bike : Vehicle {
>     void collide(Vehicle v) {
>         if (IsInstance!(Car)(v))
>             writefln("Bike hits car");
>         else if (IsInstance!(Bike)(v))
>             writefln("Bike hits bike");
>         else
>             throw new Exception("missing case: should not happen");
>     }
> }
> 
> void main() {
>     Vehicle car = new Car();
>     Vehicle bike = new Bike();
> 
>     car.collide(bike);  // Car hits bike
>     car.collide(car);   // Car hits car
>     bike.collide(car);  // Bike hits car
>     bike.collide(bike); // Bike hits bike
> }

You overcomplicate.

import std.stdio;

void main() {
    Vehicle car = new Car();
    Vehicle bike = new Bike();

    car.collide(bike);  // Car hits bike
    car.collide(car);   // Car hits car
    bike.collide(car);  // Bike hits car
    bike.collide(bike); // Bike hits bike
}

interface Vehicle {
    void collide(Vehicle v);
    void onCollision(Car c);
    void onCollision(Bike b);
}

class Car : Vehicle {
    override void collide(Vehicle v)     {
        v.onCollision(this);
    }
    override void onCollision(Car c) {
        writefln("Car hits car");
    }
    override void onCollision(Bike b) {
        writefln("Bike hits car");
    }
}

class Bike : Vehicle {
    override void collide(Vehicle v)     {
        v.onCollision(this);
    }
    override void onCollision(Car c) {
        writefln("Car hits bike");
    }
    override void onCollision(Bike b) {
        writefln("Bike hits bike");
    }
}