How does Kotlin's `lazy` implementation work without getValue and setValue?

Issue

In Kotlin properties with pre-set custom get/set behavior are implemented using delegated properties. According to the doc, a delegate for a property is just a class with getValue and setValue methods

But when I looked inside the implementation of lazy, I only found value property. So I tried to implement a delegate this way myself. It failed to compile, because getValue and setValue were not implemented explicitly. How does the official implementation of lazy work then?

Solution

getValue is declared as an extension function (yes that works too!) on all Lazy<T>s:

Source:

public inline operator fun <T> Lazy<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value

As you can see, it returns value, which is exactly what you would expect.

There is no setValue, and that is fine. That just means you can't use lazy { ... } as a property delegate for a var, and you indeed cannot:

var foo by lazy { 10 } // compiler error

After all, this doesn't make much sense anyway.

Answered By - Sweeper