Question

I have a hierarchy of classes, and I’d like that each object had an ID of the form Classname-integer (examples: Car-0, Car-1, Motorcycle-0, Truck-0, Truck-1, …)

The class hierarchy is

Vehicle
   Car
   Motorcycle
   Truck

The problem is: I want to write just once the code that manages IDs and I’m lost in costructors, prototypes, late binding, and so on.

Example of what I’d like to obtain in pseudocode:

car = new Car
anotherCar = new Car
car.id                  // "Car-0"
anotherCar.id           // "Car-1"
truck = new Truck
truck.id                // "Truck-0"

the Car constructor initializes the object id with the current Car available id, and then increments it so that the next new Car will have a different id. This must not affect other classes IDs.

Ideally, I’d like to write the code just in the base class Vehicle, but I don’t know if it is possible.

My current target language is Coffeescript / Javascript but other languages are welcome along with some reasoning on how it works.

How would you solve this?

Answer 1

You can use the Vehicle constructor to add a prototype function that will update the ID, like this: http://jsfiddle.net/ZDUEk/.

(function() {
    var ids = {}; // remember last id

    function Vehicle() {

    }

    Vehicle.prototype.applyVars = function() {
        var name = this.constructor.name, // Car or Truck
            id   = ids[name];

        if(!id) {
            id = ids[name] = 0; // init id
        }

        this.id = name + "-" + id; // set to form Type-X

        ids[name]++; // increment for next id
    };

    function Car() {
        this.applyVars(); // set id
    }

    function Truck() {
        this.applyVars(); // set id
    }

    Car.prototype = new Vehicle;
    Car.prototype.constructor = Car;

    Truck.prototype = new Vehicle;
    Truck.prototype.constructor = Truck;

    window.Vehicle = Vehicle;
    window.Car = Car;
    window.Truck = Truck;
})();

var car1   = new Car,
    car2   = new Car,
    truck1 = new Truck;

alert([car1.id, car2.id, truck1.id]);