Operator Overloading

data class Point(val x: Int, val y: Int) {
    // Overload the + operator
    operator fun plus(other: Point): Point = Point(x + other.x, y + other.y)

    // Overload the - operator
    operator fun minus(other: Point): Point = Point(x - other.x, y - other.y)

    // Unary minus
    operator fun unaryMinus(): Point = Point(-x, -y)
}

data class Vector(val x: Double, val y: Double) {
    operator fun plus(other: Vector) = Vector(x + other.x, y + other.y)
    operator fun minus(other: Vector) = Vector(x - other.x, y - other.y)
    operator fun times(scale: Double) = Vector(x * scale, y * scale)
    operator fun unaryMinus() = Vector(-x, -y)

    // Overloading invoke() lets you call the object like a function.
    operator fun invoke(): Double = Math.sqrt(x * x + y * y)  // magnitude

    override fun toString() = "($x, $y)"
}

class Matrix2x2(val a: Double, val b: Double, val c: Double, val d: Double) {
    // Access elements with [] (row and col are zero-based).
    operator fun get(row: Int, col: Int): Double = when {
        row == 0 && col == 0 -> a
        row == 0 && col == 1 -> b
        row == 1 && col == 0 -> c
        else -> d
    }
}

fun main() {
    val v1 = Vector(3.0, 4.0)
    val v2 = Vector(1.0, 2.0)

    println(v1 + v2)        // (4.0, 6.0)
    println(v1 - v2)        // (2.0, 2.0)
    println(v1 * 2.0)       // (6.0, 8.0)
    println(-v1)            // (-3.0, -4.0)
    println(v1())           // 5.0 (magnitude: √(9+16))

    val mat = Matrix2x2(1.0, 2.0, 3.0, 4.0)
    println(mat[0, 0])      // 1.0
    println(mat[1, 1])      // 4.0
}

Operator overloading is especially powerful for classes that represent mathematical concepts such as numeric calculations, coordinates, matrices, and complex numbers. However, using operators in non-obvious ways can cause confusion, so it is recommended to overload operators only when the meaning is immediately clear.

Overloading invoke() lets you call an object as if it were a function. This is useful in DSLs and builder patterns.

For data class features, see data class. For class inheritance, see open / override.