Is there a way to only allow a single class `A` to instantiate a class `B`, while keeping `B` itself...

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[karelpeeters]

Is there a way to only allow a single class A to instantiate a class B, while keeping B itself public? Basically I want B to have a private constructor that can also be accessed by A but not by a random class C.
Putting A and B is the same file doesn't allow A to use the private constructor, and even making B a nested class of A also doesn't work. Is there a solution?

[Dias]

why making B nested doesn't work? It allows B to be instantiated by A only?

[karelpeeters]

How? This is the code:

class A {
    fun foo() = B()
    
    class B private constructor()
}

class C {
    fun bar() = A.B()
}

I want foo to compile and bar to not compile.

[Dias]

ah put the word 'nested' in front of B

[Dias]

and remove private modifier from the constructor

[karelpeeters]

There's no nested keyword in Kotlin. In Java there is "nothing" for inner, static for nested, in Kotlin it's inner for inner and "nothing" for nested.

[Dias]

sorry, I meant inner

[Dias]

class A {
    val b = B()
    inner class B {
        fun test(){

        }
    }
}
class C {
    val a = A()
    val b = A.B()

    fun test() {
        a.b.test()
    }
}

class C doesn't compile here

[karelpeeters]

That works, but I don't really want an inner class. It's a good workaround though, I didn't think of that, thanks!

[Dico]

Pretty sure you can do A().B() here no?

[Dico]

Still a good workaround indeed

[karelpeeters]

Ah you're right you can. That's too bad 😞, then I'm still looking for a real solution.

[Dico]

I agree this is a bit annoying, compared to java where this isn't a problem

[karelpeeters]

It'd be okay if there was any solution at all, but I can't find anything. I've come across this a couple of times already.

[Pablichjenkov]

Same thing here, I wish it had package private at the least.

[Dias]

spreading A + B and C between modules and using internal? 🙂

[Pablichjenkov]

There is this long thread on reasons why support/not support package private:
https://discuss.kotlinlang.org/t/kotlin-to-support-package-protected-visibility/1544/67

I really prefer something like friend classes in c++ over package private.