Why can't we create generic wrapper classes, like `class Wrapper<T> (val value: T) : T by valu...

# announcements

[Maxim Kizub]

Why can't we create generic wrapper classes, like class Wrapper<T> (val value: T) : T by value ?

[diesieben07]

Generics are erased, at runtime T just becomes Any?.
And even if they weren't erased, the compiler would have to emit a new class for every T that's used, because superclasses cannot be dynamic on the JVM.

[Maxim Kizub]

Delegation (replacing access with finction call) is done at compile time, not at runtime.

[marstran]

But you can't replace a function call at compile time when you don't know which functions you are going to replace yet.

[Maxim Kizub]

? Sorry, I don't understand. Delegation just replaces instance with instance.value.

[marstran]

No, you are making Wrapper a subclass of T, which means that every public method in the class T will be inherited by Wrapper.

[marstran]

The delegation makes each of those overrided methods a call to the delegate.

[marstran]

So you can't do this when you don't know what the actual type T is.

[Maxim Kizub]

We do know T at compile time. wr: Wrapper<Int>(3); wr.coerceAtLeast(5) should be just replaced by compiler with wr.value.coerceAtLeast(5)

[diesieben07]

That's not how inheritance works.

[Maxim Kizub]

Ok, is there any description how delegation inheritance work?

[diesieben07]

It's syntax sugar.

interface Bar {
   fun barMethod(): String
}

class BarImpl : Bar {
   override fun barMethod() = "Hello from BarImpl"
}

class BarByDelegate(barImpl: BarImpl) : Bar by barImpl
// becomes:
class BarByDelegate(val barImpl: BarImpl) : Bar {
    override fun barMethod() = barImpl.barMethod()
}

[diesieben07]

BarByDelegate implements Bar normally.

[Maxim Kizub]

Ok, thanks. I missed (in documentation) that delegation is only for interfaces.

[streetsofboston]

No higher kind types in Kotlin. You can inherit only from types in Kotlin (classes, interfaces that may or may not have generic type parameters), but you can't inherit from a generic type parameter (they get erased at runtime).