CODEWITHSUNDEEP
kotlin
Roger Lipscombe
Roger Lipscombe

Reputation: 91825

Default value for generic member

I'm trying this:

class Foo<T> {
    var member: T = T()
}

...but the Kotlin compiler gives me an error: Type parameter T cannot be called as function.

How do I default-construct a generic member variable?

Upvotes: 1

Views: 1728

Answers (2)

Михаил Нафталь
Михаил Нафталь

Reputation: 7882

It's implied that T has a public no-arg constructor, but in general case it may not be true. This code uses reflection to bypass compiler complains about it (which may end up with runtime error if you dissapoint the JVM expectations and indeed pass T without public no-arg constructor).

//Reified generics at class level are not yet supported in Kotlin (KT-33213), 
// so you have to pass instance of `KClass` manually as a consructor parameter
class Foo<T : Any>(clazz: KClass<T>) { 
    var member: T = clazz.createInstance() 
}

Upvotes: 0

Wang
Wang

Reputation: 1038

Well, to access the type information, we need to use the reified keyword on the type, but this is only applicable in inlined functions. So instead of relying on direct construction, a workaround can be to use a generator function wrapped in the companion object that immediately sets the member right after construction

// Test class to verify the implementation
class Yolo {
    override fun toString() = "Yolo swag"
}

class Foo<T : Any> {
    lateinit var member: T
    
    companion object {
        inline fun <reified T : Any> newInstance() = 
            T::class.java.newInstance().let { memberInstance ->
                Foo<T>().apply { member = memberInstance} 
            }
    }
}

fun main() {
    // generate a Foo<Yolo>
    val foo = Foo.newInstance<Yolo>()
    
    println(foo.member) // displays "Yolo swag"
}

Upvotes: 1

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