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?

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

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.

ah put the word 'nested' in front of B

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.

sorry, I meant inner

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

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

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

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

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.

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

spreading A + B and C between modules and using

internal

? 🙂

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.