What's an inline constructor in Kotlin?

7

First of all I have to clarify I'm not asking what's an inline function is or what's an inline class is. There's nowhere in Kotlin Language documentation or specification any reference to an inline constructor but if you look in Arrays.kt source you see this class: ByteArray has an inline constructor:

/**
 * An array of bytes. When targeting the JVM, instances of this class are represented as `byte[]`.
 * @constructor Creates a new array of the specified [size], with all elements initialized to zero.
 */
public class ByteArray(size: Int) {
    /**
     * Creates a new array of the specified [size], where each element is calculated by calling the specified
     * [init] function.
     *
     * The function [init] is called for each array element sequentially starting from the first one.
     * It should return the value for an array element given its index.
     */
    public inline constructor(size: Int, init: (Int) -> Byte)

Let's consider we want to create a similar class, something like this:

    public class Student(name: String) {
        public inline constructor(name: String, age: Int) : this(name)
    }

If you try to create that class in Kotlin and write an inline constructor for it you see that it's impossible and IDE refers to this error:

Modifier 'inline' is not applicable to 'constructor'

So let's recap, how ByteArray definition is correct?

kotlin oop constructor inline

Share

edited 2 days ago

kometen

4,25533 gold badges3535 silver badges3939 bronze badges

asked 2 days ago

YaMiNYaMiN

87922 silver badges1717 bronze badges

7

• Im not sure, but I assume its due to the lambda (inlining is mostly used on things that take lambdas to avoid the use of funtion objects) – DownloadPizza 2 days ago
• It won't work if you have a lambda either, the error is Modifier 'inline' is not applicable to 'constructor' which is defined by the language spec (kotlinlang.org/spec). I don't know for sure, but at a guess, because these are built-in types they're "breaking the rules" and making an exception in this specific case, and allowing the init function to be inlined by the compiler, to make initialising arrays more efficient – cactustictacs 2 days ago
• 1
  Pretty sure this is something that is not available to use, and only appears in the psuedo-source code for built-in classes. – Tenfour04 2 days ago
• @cactustictacs breaking the rules? after all it's Kotlin code anyway so they have to use a different compiler to compile standard library then? – YaMiN 2 days ago
• 3
  It is pseudo-code. The standard library is not compiled from the code you're looking at there. That function doesn't even have a body. It's only there for documentation purposes. There's a comment at the top that it's auto-generated. I think it's marked inline to let you know that any lambda you pass won't generate a function object. – Tenfour04 2 days ago

 |  Show 2 more comments