Is there a way to pass a generic parameter to a (secondary) constructor? If it was any other method,...

# announcements

[StavFX]

Is there a way to pass a generic parameter to a (secondary) constructor?
If it was any other method, I would do fun <T> foo(param: T), but with a constructor I can’t just do <T> constructor(param: T)
There are ways I can get around this limitation, but I’m just curious if it’s possible with slightly different syntax.
(also, I don’t want to make the entire class generic to T)

[streetsofboston]

Create a so-called smart constructor:

class MyClass private constructor(....) {
 
    companion object {
        operator fun <T> invoke(param:T , ...): MyClass = ...

        operator fun invoke(...): MyClass = ...
    }
}

[Nir]

damn

[Nir]

that's clever

[Nir]

shouldn't be necessary. but still clever.

[streetsofboston]

Smart-constructors also ‘allow’ you to validate the input and decide to return null or throw an exception or something similar….

[StavFX]

ooo that IS clever

[streetsofboston]

Or, MyClass can be an abstract class and the operator fun invoke calls will return sub-classes/implementations

[Nir]

what I really like about this, more generally

[Nir]

is that you can do work before calling the primary constructor

[Nir]

It's kind of annoying and seemingly pointlessly asymmetric that you can do work after, but not really before, the primary constructor call, in a secondary constructor

[ephemient]

other hacks to do work in secondary constructors before calling primary/super:

class Foo(x, y, z) {
    constructor(input) : this(computation(input))
    private constructor(result) : this(computed.x, computed.y, computed.z)
}

[StavFX]

Interesting that’s actually possible in Java (public <T> MyClass(T param) ) but not in kotlin without overriding invoke

[Nir]

@ephemient yeah i've used these hacks extensively in C++ 🙂 was hoping to never need to use them again