47. Class Delegation

Kotlin supports the delegation pattern as a first-class language feature. With class Derived(b: Base) : Base by b, the compiler generates forwarding methods that delegate every member of Base to b — no hand-written boilerplate.

interface Base {
    fun greet()
    fun describe()
}

class BaseImpl(private val value: Int) : Base {
    override fun greet() = println("Hello from BaseImpl")
    override fun describe() = println("value is $value")
}

// `by b` forwards every Base method to b — no boilerplate.
// We override just greet(); describe() is still delegated.
class Derived(b: Base) : Base by b {
    override fun greet() = println("Hello from Derived")
}

fun main() {
    val base = BaseImpl(42)
    val derived = Derived(base)
    derived.greet()     // overridden
    derived.describe()  // delegated to BaseImpl
}

Any member you declare in Derived takes precedence over the generated delegate, so you override only what you want to change. This composes behavior without inheritance and keeps the wrapped object’s implementation intact.

Running it:

$ kotlin run
Hello from Derived
value is 42

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