คีย์เวิร์ด by

interface Speaker {
    fun speak(): String
}

class LoudSpeaker : Speaker {
    override fun speak() = "HELLO!"
}

fun main() {
    println(LoudSpeaker().speak())
}

interface Speaker {
    fun speak(): String
}

class LoudSpeaker : Speaker {
    override fun speak() = "HELLO!"
}

class Announcer(s: Speaker) : Speaker by s

fun main() {
    val a = Announcer(LoudSpeaker())
    println(a.speak())
}

interface Repository {
    fun save(item: String): String
    fun count(): Int
}

class MemoryRepo : Repository {
    private val items = mutableListOf<String>()
    override fun save(item: String): String { items.add(item); return item }
    override fun count() = items.size
}

class LoggingRepo(r: Repository) : Repository by r

fun main() {
    val repo = LoggingRepo(MemoryRepo())
    repo.save("a")
    println(repo.count())
}

interface Repository {
    fun save(item: String): String
    fun count(): Int
}

class MemoryRepo : Repository {
    private val items = mutableListOf<String>()
    override fun save(item: String): String { items.add(item); return item }
    override fun count() = items.size
}

class LoggingRepo(private val r: Repository) : Repository by r {
    override fun save(item: String): String {
        println("saving " + item)
        return r.save(item)
    }
}

fun main() {
    val repo = LoggingRepo(MemoryRepo())
    repo.save("book")
    println(repo.count())
}

interface Clock { fun now(): Int }

class FixedClock : Clock { override fun now() = 12 }

class App(private val clock: Clock) : Clock by clock

fun main() {
    println(App(FixedClock()).now())
}

class TaggedList(private val inner: List<String>) : List<String> by inner {
    val tag = "v1"
}

fun main() {
    val tl = TaggedList(listOf("x", "y"))
    println(tl.size)
    println(tl[0])
    println(tl.tag)
}

interface Source { fun value(): Int }

class Const(val n: Int) : Source { override fun value() = n }

class Wrapper(s: Source) : Source by s

fun main() {
    val w = Wrapper(Const(42))
    println(w.value())
}