Did you guys think about “copy-constructors” for d...

# language-proposals

[noctarius]

Did you guys think about “copy-constructors” for data classes? 😉

[benleggiero]

Like this?

data class Foo(bar: Bar, baz: Baz) {
	fun copy (bar: Bar = this.bar.copy(), baz: Baz = this.baz.copy()) = Foo(bar, baz) 
}

[noctarius]

something like this, yeah but it be more like Foo(foo: Foo) => foo.copy()

[noctarius]

you pretty much create a new clone but instead of calling clone you use a copy constructor

[noctarius]

(especially in the future, with value types, that would be cool :D)

[benleggiero]

ah, I see. As a much more generally powerful feature, I'd like to see "factory" constructors, which you can call like a normal constructor but don't behave like one. For instance:

class Foo(bar: Bar) {
    private val shared = Foo(bar = Bar())
    
    factory constructor() = if (condition) shared else Foo(Bar())

[noctarius]

I think you could do this with companion object, can’t you?

[noctarius]

but I guess you want the opposite of https://kotlinlang.org/docs/reference/multi-declarations.html 😄

[noctarius]

constructing declarations 🙂

[benleggiero]

Mostly I want to change Foo.create() to Foo()

[noctarius]

ohh ok, sorry took a moment 😄

[miha-x64]

Are you guys looking for this? https://kotlinlang.org/docs/reference/data-classes.html#copying

[benleggiero]

@miha-x64 not me personally 😛

[Paul Woitaschek]

@benleggiero Then make it an operator fun called invoke

[Paul Woitaschek]

data class Sample(val string  : String) {
  
  companion object{
    
    operator fun invoke() = if(Random().nextBoolean()) Sample("bob") else Sample("alice")
  }
}

fun whatever(){
  val sample = Sample()
}

[benleggiero]

w-woah... I did not know this was possible. I'm gonna have to play with this.