How to access the receiver of a method reference?

Question

I have a method reference (represented by the kotlin.reflect.KFunction0 interface). What I would like to do is to get the receiver object of the method reference.

For example:

data class MyClass(val name: String)

val john = MyClass("John")
val methodReference = john::toString

methodReference.receiver // doesn't work!

The receiver of methodReference is the object john. If I look into the IntelliJ debugger, methodReference has a receiver field which indeed points to john. But I cannot find a way to actually access it in my code.

Is there some sort of workaround for this?

It's probably worth asking why you want to get that; this feels like an XY problem. (Reflection is rarely a good solution in general-purpose code; it's slow, ugly, fragile, insecure, and turns compilation errors into runtime crashes. It's needed for frameworks, plugins, build tools, etc., but outside those there's usually a better approach.) — gidds
– gidds, Commented Jan 25, 2024 at 11:15
@gidds It is for a framework. The underlying API (which eventually binds to C++) requires a receiver object and a method name explicitly. I would like to be able to use method references as a single argument; but for that I need to be able to extract the receiver from it. — Martin Häusler
– Martin Häusler, Commented Jan 25, 2024 at 12:02

Sweeper · Accepted Answer · 2024-01-25 10:17:14Z

1

Assuming this is Kotlin/JVM, the receiver field you see is a protected field declared in CallableReference. You can get this using Java reflection.

val f = someINstance::foo
val receiverField = CallableReference::class.java.getDeclaredField("receiver")
receiverField.isAccessible = true
println(receiverField.get(f))

I'm not sure if this field is guaranteed to be called receiver, or whether it will exist at all, in future versions. I cannot find any official documentation on this.

Also note that not every KFunction0 has a receiver. When this is the case, receiverField.get(f) would return CallableReference.NO_RECEIVER.

edited Jan 25, 2024 at 10:17

answered Jan 25, 2024 at 9:26

Sweeper

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1 Comment

Martin Häusler Over a year ago

Thanks for your response! Yes it's Kotlin/JVM. Just like you, I had no luck finding any documentation on this. And yes, of course not all KFunction0 instances will have a receiver, it's perfectly fine if it's null. I'll give it a shot with your solution :)

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