I often run into this delegation case that doesn't...

# language-proposals

[Derek Peirce]

I often run into this delegation case that doesn't compile:

class Foo: Bar by bar {
    val bar: Bar = ...
}

This would get Unresolved reference: bar. Is there any reasoning for this other than the order in which the compiler is evaluating symbols? Could the compiler be improved so that it understands that bar exists, and generates the proper delegation code?

override fun doBar() = bar.doBar()

I'd also like to see the ability to delegate by method instead of by entire interface, something like:

override fun doBar() by bar

with specifying the arguments being optional, otherwise all possible overloads are delegated.

[Rob Elliot]

I think you're looking for this:
https://youtrack.jetbrains.com/issue/KT-2747/Delegation-specifier-fails-to-refer-to-val-property-which-is-not-constructor-parameter

[Derek Peirce]

Indeed, thanks! I see you've had an even more pressing use case than mine! That it still exists after 11 years is a bit concerning, I guess it's a manner of patience at this point.

[uli]

what are the limitations of adding val bar as constructor parameter with default value?

interface Bar

object BarInstance: Bar

class Foo private constructor(val bar: Bar = BarInstance) : Bar by bar

Or to a private constructor making only a secondary constructor without `bar`public?

interface Bar

object BarInstance: Bar

class Foo private constructor(val bar: Bar) : Bar by bar {
    constructor() : this(bar = BarInstance)
}

[uli]

It is not beautiful and you’d have to add all vals that bar depends on in the same way.

[Rob Elliot]

In my use case, the instance to which the delegating class should delegate needs to be calculated every time it does the delegation.

[uli]

i see. that seems to be one more level of abstraction then the original feature provides for. You can have a var barFactory:: ()->Bar and delegate to barFactory(). But barFactory is only evaluated once at initialization time.

[Rob Elliot]

Yes, that’s why I pointed the OP at an open feature request on YouTrack.