Kotlin: Generic Iterator over Enum: how to declare type variables

Question

My (non compiling) code for a generic iterator over enumerations is as follows:

class EnumIterator<T: Enum<T>>: Iterator<T> 
{
   private var cursor = 0

   override fun hasNext(): Boolean = cursor < enumValues<T>().size - 1
   override fun next(): T = enumValues<T>()[cursor++]
}

The IntelliJ IDE marks the T in both enumValues< T > red:

"Cannot use 'T' as reified type parameter. Use a class instead."

How to declare the type parameters to get rid of the error?

Answer 1

Without reified type

enum class Example {
    A, B, C, D
}

fun <T: Enum<T>> iterator(values:()->Array<T>):Iterator<T> = values()
    .asIterable()
    .iterator()

fun main(args: Array<String>) {

    val iterator = iterator(Example::values)

    iterator.forEach {
        println(it)
    }
}

This works by using a function instead of a wrapper class, depending on your use it may be a better option.

With reified type

enum class Example {
    A, B, C, D
}

inline fun <reified T: Enum<T>> iterator():Iterator<T> = enumValues<T>().iterator()

fun main(args: Array<String>) {

    val iterator = iterator<Example>()

    iterator.forEach {
        println(it)
    }
}

This requires the type to be explicitly set, but does not need a reference to the values method.