Is there a way to declare a definitely non-null pr...

# getting-started

[spleenjack]

Is there a way to declare a definitely non-null property by a delegate? So that such an expression would be impossible val property: Int? by delegate(…) but this should be ok val property: Int by delegate(…).

data class Holder<T : Any>(val value: T)

fun <T : Any> delegate(holder: Holder<T>) =
    ReadOnlyProperty<Any, T> { _, _ -> holder.value }

class Foo {
    // ERROR: Type argument is not within its bounds: should be subtype of 'kotlin. Any'.
    val holder = Holder<Int?>(1)

    // why OK?
    val property: Int? by delegate(Holder(1))
}

[Youssef Shoaib [MOD]]

Because it's a val property, the only check that kotlin does is to make sure that the property type is a supertype of the delegate's getValue type. In other words, this works for the same reason that:

val property: Int? = Holder(1).value

works.

[Sam]

👍 or val property: Int? = 1...

[spleenjack]

Thank you! Well, ok… It seems that above examples are correct and expected. But is it only me, or that doesn’t seem quite logical when such a restriction T : Any appears for the generic type? In my case a holder throws a runtime exception if the value is null, so it would return only non-nullable values. If I add a double-bang operator like this { _, _ -> holder.value!! }
there is a warning “Unnecessary non-null assertion (!!) on a non-null receiver of type ‘T’.” So I expect that compiler assumes that a nullable type Int? with such a restriction is not a valid case.

[Sam]

If you wrote out all the types in full, it'd be

val property: Int? by delegate<Int>(Holder<Int>(1))

You're certainly correct that delegate<Int?>(…) would not be valid. But delegate<Int>(…) is always going to be a valid delegate for any supertype of Int , including Int?

[ephemient]

if you think about how it desugars, it's basically

private val propertyDelegate = delegate(Holder(1))
val property: Int?
    get() = propertyDelegate.getValue()

which is not an error. the IDE does warn about that ? in similar circumstances though, so perhaps file a feature request in YouTrack

[spleenjack]

@Sam @ephemient thank you very much!