What's an inline constructor in Kotlin?

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?

Solution

The ByteArray declaration you are looking at is not real, it’s a so called built-in type. This declaration exists for convenience, but is never truly compiled to a binary. (Indeed, on the JVM, arrays are special and don’t have corresponding class files anywhere.)

This constructor is marked inline because in practice the compiler emits the code corresponding to what would be its body at every call site. All the call-site checking is done accordingly (the lambda argument is treated in such a way that the compiler knows it’s going to be inclined).

Constructor inlining is not possible for user classes, so the inline modifier is prohibited in user code.