I know you can use `obj::class` to check if objects are instances of a type ```abstract class Base {...

# getting-started

[Ray Rahke]

I know you can use obj::class to check if objects are instances of a type

abstract class Base { }
class SubA : Base()

fun fn(obj: Base, t: KClass<out Base>) {
    if (obj::class == t) {}
}
fun main() {
    val a = SubA()
    fn(a, SubA::class)

}Is there any possible way to mimic this behavior by using ::Foo instead

abstract class Base { }
class SubA : Base()
//                       v different
fun fn(obj: Base, t: KFunction<Base>) { 
    if (obj::class == t) {}
}
fun main() {
    val a = SubA()
    fn(a, ::SubA) // <- different
}

[Goetz Markgraf]

What do you want to achieve with that?

Foo is a type that is compared with is
::Foo is a reference to a function. Why would you want to compare in object with a function?

What you can do:

val x = ::Foo
if (x == ::Foo)

Here you are comparing two function references. Does that help?

[Ray Rahke]

@Goetz Markgraf I want to never have to write SubA::class

[Ray Rahke]

btw updated my question to be more clear

[Ray Rahke]

old

abstract class Base { }
class SubA : Base()

fun fn(obj: Base, t: KClass<out Base>) {
    if (obj::class == t) {}
}
fun main() {
    val a = SubA()
    fn(a, SubA::class)

desired

abstract class Base { }
class SubA : Base()
//                       v different
fun fn(obj: Base, t: KFunction<Base>) { 
    if (obj::class == t) {}
}
fun main() {
    val a = SubA()
    fn(a, ::SubA) // <- different
}

[Ray Rahke]

SubA::class is very verbose and ugly

[Ray Rahke]

and I my entire game framework is centered around this

[Ray Rahke]

so I type it hundreds of times

[Ray Rahke]

entity.hasComponents(::Physics, ::Graphics, ::Health)

is much cleaner than

entity.hasComponents(PHysics::class, Graphics::class, Health::class)

[Ray Rahke]

I get ::SubA refers to the constructor rather than the type

[Ray Rahke]

just wondering if there is a hack I can use to convert that internally

[Goetz Markgraf]

I don’t see, why ::class is ugly, but that depends on taste.

As I have said, ::something is a pointer to a function not to a type.

[Ray Rahke]

it's verbosity is clear though

[Ray Rahke]

in typescript you can just say the class

[Ray Rahke]

Physics

[Ray Rahke]

As I have said, ::something is a pointer to a function not to a type.

yeah so Is there a way to say "this object's types constructor is equivalent to this function"?

[Ray Rahke]

like if (::(obj::class) == fn) where fn is ::Foo

[Goetz Markgraf]

in typescript you can just say the class

Typescript does not really have classes, the concepts cannot be compared.

Maybe there is an alternative. Do all your type inherit from a single base type? Then you might add a function for comparison there taht “redirects” to an enum. Of course, you with have to manually sync you enum to your types.

If your types seldom change, that might be another way.

[Ray Rahke]

the design patterns I use involve you constantly passing around literal classes rather than instances. Hundreds of times. the ::class adds up

[Ray Rahke]

I just want to find a way to make that cleaner

[Ray Rahke]

so I don't have to live with it for years

[Goetz Markgraf]

🤷 Maybe someone else has an idea

[Ray Rahke]

they all share a base Component class

[Ray Rahke]

but yeah I'm not going to do 2x the work creating a corresponding enum for every single new component i add

[Riccardo Lippolis]

with reified generics you can 'fix' the specific example you mentioned:

abstract class Base
class SubA : Base()

inline fun <reified T : Base> fn(obj: Base) {
    if (obj is T) {}
}
fun main() {
    val a = SubA()
    fn<SubA>(a)
}

but that does mean inlining the function, and you'd really have to wonder if it's worth it all just because you find ::class ugly...

[Ray Rahke]

@Riccardo Lippolis Would that work for vararg functions? I need to be able to write stuff like

val Monitor = MonitorEntities(Physics::class, Gravity::class, Spatial::class, Material::class, Mesh::class)

[Ray Rahke]

with your example I would imagine it would be

val Monitor = MonitorEntities<Physics, Gravity, Spatial, Material, Mesh>()

but is that possible?

[Riccardo Lippolis]

as far as I know you can't define a function with a variable number of generic types, you'd need multiple functions

[Ray Rahke]

I have found out that you can pass literal classes like

fn(Physics)

if they have a companion object.

...but kotlin outlaws putting a single companion object on a base class to be inherited for all subclasses (arbitrarily, because they thought it was "too confusing"). So I would have to redundantly redefine the exact same companion object for every single subclass. (so, hundreds of times realistically)

[Ray Rahke]

https://github.com/Quillraven/Fleks/wiki/Component

[Ray Rahke]

this guy is managing to do some crazy syntax tricks or "templating" (whatever that means) to get it working.

[Ray Rahke]

i don't understand it though

[Tim McCormack]

Maybe do val CSubA = SubA::class as a global?