i've been playing with inline classes and noticed ...

# announcements

[bbaldino]

i've been playing with inline classes and noticed when doing some CPU profiling i was spending some time in what seems to be an init method of my inline class. i thought they'd just be compiled to the underlying type? RtpSequenceNumber is my inline class here, it wraps an Int

[hudsonb]

They are boxed when required, generics for example

[hudsonb]

https://typealias.com/guides/inline-classes-and-autoboxing/ gives a nice overview of when/why they are boxed

[bbaldino]

i'll take a look at that, thanks

[bbaldino]

so it looks like the inline class is getting boxed when doing !=, as in if (inlineClassInstance1 != inlineClassInstance2...but from that article i wouldn't think that would need boxing? maybe i misread. i do have a custom compareTo implemented for it, is that why?

[hudsonb]

Ya I believe so - it's like the Amount example in that article

[bbaldino]

i thought that was because of the inheritance bit (which i don't have)

[bbaldino]

also i'm not using it in any container here

[hudsonb]

Did some testing, they are always boxed when you use !=

[hudsonb]

So even this boxes them:

inline class Test(val n: Int)

fun main() {
    val t1 = Test(1)
    val t2 = Test(2)

    println(t1 != t2)
}

[hudsonb]

To avoid it compare the wrapped value instead:

inline class Test(val n: Int)

fun main() {
    val t1 = Test(1)
    val t2 = Test(2)

    println(t1.n != t2.n)
}

[hudsonb]

It's because equals takes an `Object`/`Any` so it has to be boxed

[bbaldino]

thanks brian! that's good to know. so maybe if i override equals as well in the inline class i can avoid it

[hudsonb]

That won't help unfortunately. equals is defined as fun equals(other: Any?): Boolean, since it takes Any? as a parameter, it'll be boxed.

[bbaldino]

damn

[hudsonb]

Using != on the wrapped Int is the only way to avoid it. That means it can't be private though which kinda sucks

[bbaldino]

yeah...or could use some custom equals fun i guess?

[hudsonb]

Yeah looks like that'd work. This doesn't box:

inline class Test(val value: Int) {
    infix fun eq(other: Test): Boolean = value == other.value
}

fun main() {
    val t1 = Test(1)
    val t2 = Test(2)
    
    println(t1 eq t2)
}

[bbaldino]

cool. i don't think the perf hit in this case feels like a dealbreaker yet, but maybe that's a decent workaround

[bbaldino]

though it would be really nice for inline classes to handle this case, as it seems like it'd be a common use

[hudsonb]

Yeah, it'd be nice. Maybe they'll support it in the future 🤞

[bbaldino]

hmm, on this page: https://github.com/Kotlin/KEEP/blob/master/proposals/inline-classes.md i see a reference to this:

// Reserved method for specialized equals to avoid boxing
    public final static equals-impl0(II)Z

[hudsonb]

Huh maybe it's a bug then. I see that they are generating a equals-impl and delegating to that:

public static boolean equals_impl(int var0, @Nullable Object var1)
public boolean equals(Object var1) {
      return equals-impl(this.value, var1);
   }

but main is clearly boxing:
boolean var2 = Intrinsics.areEqual(Test.box-impl(t1), Test.box-impl(t2));

[bbaldino]

cool, maybe it's just a bug then

[bbaldino]

was unable to find anything existing--some close, maybe i misunderstood them--but i filed https://youtrack.jetbrains.com/issue/KT-30785