Why again inline classes cannot inherit from super...

# language-proposals

[Aregev2]

Why again inline classes cannot inherit from super classes, what is the conflict?

[Andreas Sinz]

@Aregev2 Because at runtime there is no class

[Andreas Sinz]

inline class PositiveInteger(private val data: Int) is just a regular Int at runtime

[benleggiero]

I see where Alon's coming from tho. There's no reason the compiler can't pretend there is a type hierarchy, even if the runtime can't. This would be useful especially with inheriting functionality

[Andreas Sinz]

and that will get confusing especially for people new to kotlin:

interface Foo

inline class Bar(private val s: String) : Foo

fun doFoo(foo: Foo)

val bar = Bar("Hello World")

doFoo(bar) //Doesn't work, although Bar "inherits" from Foo

[benleggiero]

Yes, that looks annoying and confusing

[karelpeeters]

Isn't there still autoboxing for cases like this?

[ilya.gorbunov]

Inline classes can implement interfaces, but can't inherit from other classes. Because those classes can have state and inline class can only handle one piece of state — a property declared in its primary constructor.

[karelpeeters]

That reminds me, I wanted to ask whether the "one single property" limitation can/will be lifted in the future? Is it even possible to return multiple values on the JVM?

[Andreas Sinz]

hm, if only interfaces are allowed, my example would actually work with autoboxing

[Aregev2]

Its really disappointing that unsigned integers wont extend number, I mean Interfaces are kind of hierarchy as well no?


(also, my OCD is crashing badly ;P)

[benleggiero]

@Aregev2 see my mini-proposal from the other day

[Aregev2]

So in case of UInt for example, will we be able to acess `Number`'s functions and fields? as in runtime it is translated it an Int

[karelpeeters]

Why does everyone seem to like the Number interface class 😐

[Aregev2]

It's an abstract class, and I don't know 🤷

[karelpeeters]

Even worse, an abstract class that doesn't have any actual state.

[Aregev2]

Because it was the legacy (Integer)
🤷

[benleggiero]

I hate it, and prefer Swift’s setup. I still have qualms with theirs, but it’s much closer to my ideal and the qualms are minor

[Aregev2]

So will we be able to access Number functions anyways?

[Aregev2]

As in runtime UInt is Int basically

[karelpeeters]

As per https://github.com/Kotlin/KEEP/blob/master/proposals/inline-classes.md#motivation--use-cases, the inner values are going to be private.

[karelpeeters]

And regardless, the conversions would be wrong anyway.

[ilya.gorbunov]

@Aregev2 You won't be able to cast UInt as Number.
However there are conversion functions, like .toInt() or .toLong(), which return value that can be cast to Number