Instantiating generic array in Kotlin

Question

Why this doesn't compile? I get compile error in 3 line

Cannot use T as reified type parameter. Use class instead

class Matrix2d<T>(val rows: Int, val cols: Int, init: (Int, Int) -> T) {

   var data = Array(rows * cols, { i ->
      val r = Math.floor(i.toDouble() / cols).toInt()
      init(r, i - r * cols)
   })

   operator fun get(row: Int, col: Int): T = data[row * cols + col]

   operator fun set(row: Int, col: Int, v: T) = {
      data[row * cols + col] = v
   }
}

Solution

I added a factory function which looks like a second constructor but implemented in inline function

class Matrix2d<T>(val rows: Int, val cols: Int, private val data: Array<T>) {

   companion object {
      operator inline fun <reified T> invoke(rows: Int, cols: Int, init: (Int, Int) -> T): Matrix2d<T> {
         return Matrix2d(rows, cols, Array(rows * cols, { i ->
            val r = Math.floor(i.toDouble() / cols).toInt()
            init(r, i - r * cols)
         }))
      }
   }

   init {
      if (rows * cols != data.size) throw IllegalArgumentException("Illegal array size: ${data.size}")
   }

   operator fun get(row: Int, col: Int): T = data[row * cols + col]

   operator fun set(row: Int, col: Int, v: T) {
      data[row * cols + col] = v
   }
}

Possible duplicate of Kotlin NDArray with a lambda constructor with generic return type — mfulton26
– mfulton26, Commented Jan 30, 2017 at 17:31

Ilya · Accepted Answer · 2017-01-31 15:10:57Z

29

JVM arrays, on which Kotlin arrays are mapped to, require the element type to be known at compile time to create an instance of array.

So you can instantiate Array<String> or Array<Any>, but not Array<T> where T is a type parameter, representing the type that is erased at compile time and hence is unknown. To specify that a type parameter must be known at compile time it is marked with reified modifier.

There are several options, what you can do in this situation:

Use MutableList<T> for storing elements, which doesn't require reified T:

// MutableList function, available in Kotlin 1.1
val data = MutableList(rows * cols, { i ->
   val r = i / cols
   init(r, i % cols)
})
// or in Kotlin 1.0
val data = mutableListOf<T>().apply {
    repeat(rows * cols) { i ->
        val r = i / cols
        add(init(r, i % cols))
    }
}

Create an array from an inline function with reified type parameter:

inline fun <reified T> Matrix2d(val rows: Int, val cols: Int, init: (Int, Int) -> T) = 
    Matrix2d(rows, cols, Array(rows * cols, { .... })

class Matrix2d<T> 
    @PublishedApi internal constructor(
        val rows: Int, val cols: Int,
        private val data: Array<T>
    )

Use Array<Any?> as the storage, and cast its values to T in get function:

val data = Array<Any?>(rows * cols, { .... })

operator fun get(row: Int, col: Int): T = data[row * cols + col] as T

Pass a parameter of type Class<T> or KClass<T> to constructor and use java reflection to create an instance of array.

edited Jan 31, 2017 at 15:10

answered Jan 30, 2017 at 22:27

Ilya

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2 Comments

Lancaster Over a year ago

Thanks for explanation, I like the second way. I updated my post and code works properly, but now I have a public constructor in Matrix2d class which can be used by all, and it's confusing, because I have a third parameter as an array. And if I use that constructor I should pass the number of rows and columns. I wanted to specify contructor scope to private, but it's not available for inline function. How can I solve this problem in a different way?

Ilya Over a year ago

Use internal constructor, though to call it from public inline function, you have to annotate it with @PublishedApi annotation (available in Kotlin 1.1). I'll update the example.

Riki137 · Accepted Answer · 2018-05-12 08:38:39Z

5

Personally, the best workaround for me was:

@Suppress("UNCHECKED_CAST")
var pool: Array<T?> = arrayOfNulls<Any?>(initialCapacity) as Array<T?>

answered May 12, 2018 at 8:38

Riki137

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2 Comments

Bryan W. Wagner Over a year ago

This solution is just as viable as solution 3 in the selected answer; just probably should have private access.

Emanuel Moecklin Over a year ago

I don't think this works, it throws a ClassCastException with a misleading error message (see stackoverflow.com/questions/54536895/…) but the fact still remains that it cannot do the cast

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