Contravariant generics.md

Contravariant generics

Requires a type cast.

Explanations

In Kotlin, in T this is a contravariant generic.

open class SuperClass  
class Child : SuperClass()  
  
class InGenericItem<in T>
  
private fun example(example: InGenericItem<SuperClass>) {  
    val y: InGenericItem<Child> = example // Won't compile without in
}

In Objective-C in we generated a generic with an indication of its contravariant:

__attribute__((objc_subclassing_restricted))
__attribute__((swift_name("InGenericItem")))
@interface SharedInGenericItem<__contravariant T> : SharedBase
- (instancetype)init __attribute__((swift_name("init()"))) __attribute__((objc_designated_initializer));
+ (instancetype)new __attribute__((availability(swift, unavailable, message="use object initializers instead")));
@end

And in theory, similar code in Swift should work:

open class SuperClass {}
class ChildClass : SuperClass {}

private func inGenericUsage(generic: InGenericItem<SuperClass>) {
    let _: InGenericItem<ChildClass> = generic
}

But it doesn't compile, and gives the error:

Cannot assign value of type 'InGenericItem<SuperClass>' to type 'InGenericItem<ChildClass>'

Through type casting it works:

inGenericUsage(
    generic: InGenericItem<ChildClass>() as! InGenericItem<SuperClass>
)

private func inGenericUsage(generic: InGenericItem<SuperClass>) {
    let _: InGenericItem<ChildClass> = generic as! InGenericItem<ChildClass>
    print("inGenericUsage - ok")
}

---

Table of contents