I kind of understand what is happening here, but h...

# getting-started

[Mark]

I kind of understand what is happening here, but how best to explain it?

fun <T> Foo<T>.getValue(): T

val foo: Foo<Boolean> = ...
println("no problem: " + foo.getValue())
val fooAnyValue: Any = foo.getValue()
println("also no problem: " + fooAnyValue)
val fooValue = foo.getValue()
println("class cast exception (fooValue is not a Boolean): " + fooValue)

[asdf asdf]

The type of fooValue is implicitly marked as a Boolean, as it is the expected return type of the getValue method

[asdf asdf]

However it seems like the value of foo is not actually a boolean (possibly from an unchecked cast), so it is fine when the variable type is Any, but not Boolean

[Mark]

Yes, it’s interesting that we can get access to such an illegal value by assigning to a property of type Any (or by passing it straight into some place that accepts Any) and so avoid the ClassCastException. However with val fooValue = foo.getValue() there is an implicit type Boolean of fooValue and so the class cast cannot be avoided. A part of me thinks that when getting the value there should be a check/exception regardless.

[Mark]

For context, I’m playing around with Android Jetpack DataStore which stores different types in a big map. So I’m checking what would happen if, say, a random String ended up in a value for what would normally be a Boolean

[Mark]

If I want to add my own check, is there a way (perhaps using an inline function with reified type) to ensure the value is the correct type?

Indeed, we can do this:

inline fun <reified T> Foo<T>.getValueSafely(): T? = getValue() as? T

Even though the compiler says “No cast needed” it does actually avoid the ClassCastException

[ilya.gorbunov]

This is tracked as https://youtrack.jetbrains.com/issue/KT-12451/checkcast-is-missing-when-passing-return-value-of-generic-function-into-another-generic-function
However Kotlin exhibits here the same behavior as Java, as I recall.