Tips & Tricks: Overriding ‘toString’

JS++ has a default ‘toString’ method implementation but, sometimes, it is necessary to override this implementation. For example, when using Console.log, it may be desirable to be able to fully log and inspect a complex JS++ object.

In addition to the Unified External Type, there is also a “Unified Internal Type”: System.Object. All JS++ classes, including user-defined classes, inherit from System.Object. Due to auto-boxing, even primitive types such as int (wrapped by System.Integer32), inherit from System.Object.

Aside: Don’t worry about the performance implications of auto-boxing. JS++ is able to optimize auto-boxing to the point that toString is actually 7.2% faster in JS++ than JavaScript in the worst case (assuming the JavaScript variable is monomorphically-typed) and more than 50% faster for polymorphically-typed (and potentially type-unsafe) JavaScript variables as shown in benchmarks here.

System.Object has a toString method which is marked as virtual. In other words, this method can be overridden by derived classes – which are effectively all classes in JS++. Here’s an example of how to do it:

import System;

class Point
    int x;
    int y;

    Point(int x, int y) {
        this.x = x;
        this.y = y;

    override string toString() {
        return "(" + x.toString() + ", " + y.toString() + ")";

Point p = new Point(1,2);
Console.log(p); // "(1, 2)"

You’ll notice the Console.log statement doesn’t even make an explicit toString call. The reason is because passing any JS++ object to Console.log will call the toString method on the object for you.

Roger PoonRoger Poon
JS++ Designer and Project Lead. Follow me on Twitter or GitHub.