Override var requires initialization in assignment

Language Design
1

I don’t understand, why this code doesn’t compile with message Property must be initialized or be abstract:

interface A {
    var num: Int
}

class AImpl : A {
    override var num: Int
    
    init {
        // some calculations
        num = 123
    }
}

The same code works if num is declared as val, or if num is initialized in assignment. I can create some factory method or initialize it with kotlin.run { }, but both options are quite ugly. So why this code cannot be compiled?

2 Likes
2

Seems like a bug to me.

3

That’s not a bug.
You can read num in the “some calculations” part of the code. And num must have a value there.

With val it works because its know that there is a single initialization that has not been hit yet. (and then you get an error only if you actually try to read it).

4

This code compiles fine if you replace the init block by a secondary constructor, if this option is possible go with it.

5

If the above advice doesn’t work, you can make num a lateinit var.