Hi! I want to define an extension variable. However, as you all know, I don’t have a field because ...

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[t0yger]

Hi! I want to define an extension variable. However, as you all know, I don’t have a field because I can’t initialize it.
How do you implement it?

interface Test {
    val a: String
}

var Test.b: String
    get() = field
    set(value) = field = value

[Tobias Berger]

Well, basically you don't. Extensions don't modify the class, in Java terms they are just static methods (event if you define a property). Objects of your receiver class have do not "contain" extension properties.
In theory you could do something like this:

private val bValues = mutableMapOf<Test, String>()
var Test.b: String?
    get() = bValues[this]
    set(value) { bValues[this] = value }

but that has MEMORY LEAK written all over it, because the map will keep the Test instances in memory, even if you don't need them anywhere else.

[Tobias Berger]

The reason you don't have a field is not that you can't initialize it. You just don't have anywhere to put it.

[t0yger]

Oops. I still need to study more. thx!!

[Tobias Berger]

Let me clarify with a real world example:
Your interface/class (here Test) defines a ratchet box like this:

[Tobias Berger]

Now you're trying to create an extension property hammer that allows you to pull a hammer from that box or put it in. But there is no space for it in the actual box, because it wasn't designed to contain a hammer.

[t0yger]

I got it. it is a crazy way to break box.
Stop before the box owner gets angry.