What does `class C(val field: T): T by field` mean?

Question

I recently came across the following class declaration:

class C(val field: T): T by field

where T is an interface.

What is this technique called?

What is the effect of this declaration?

Answer 1

This is called delegation, specifically it is a type of delegation known as implementation by delegation.

By doing this, the class C implements the interface using the other object.

i.e. when you call the method (function) defined in the interface then the call is going to get transferred to the object you delegated in your case it is the field

interface T {
    fun print()
}
class TImpl: T {
    override fun print() = print("Hello World!")
}

class C(val field: T) : T by field {...}

fun main() {
    val c = C(TImpl())
    c.print()                 //Prints: Hello World!
}

Here the print() call gets transferred to TImpl because class C implements T by using field.