[swift-evolution] Nongeneric classes that inherit from generic classes not visible from objc

[Derrick Ho]

Swift generics are swift only

Objective-c lightweight generics are not true generics but rather just
annotation that helps with interoperability.

Even though you are subclassing a generic class into a supposed non-generic
type it is still a generic. Inheritance follows an is-a relationship and
the subclass gets its generic genes from the parent class.


On Wed, Dec 14, 2016 at 1:06 PM Davide Mendolia via swift-evolution <
swift-evolution at swift.org> wrote:

> Ok, I understand. But my use case is adding functionality to obj-c class
> provided by the ios sdk through generics. As I see it's not something
> possible, I will go for an implementation that's it's not checking the type
> through generic at compile time.
>
> On Wed, Dec 14, 2016 at 6:56 PM Jordan Rose <jordan_rose at apple.com> wrote:
>
> Right. Even though the class you want to expose isn't generic, its
> superclass is, and Objective-C needs to know the whole inheritance chain in
> order to use the class in any meaningful way.
>
> Depending on what your use case is, you may be able to work with a @objc
> protocol instead (either imported or defined in Swift).
>
> Jordan
>
>
> On Dec 14, 2016, at 08:47, Charlie Monroe via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Hi Davide,
>
> AFAIK this is not (easily) possible (please, correct me someone if I'm
> wrong) since Swift's generics aren't "lightweight" as ObjC generics are.
>
> In ObjC, no matter what you use for the generics, you still have just 1
> class that handles all call:
>
> @interface MyClass<T> : NSObject
> @end
>
> @class A, B;
>
> NSLog(@"%@", NSStringFromClass([MyClass<A *> self])); // MyClass
> NSLog(@"%@", NSStringFromClass([MyClass<B *> self])); // MyClass
>
> [MyClass<A *> self] == [MyClass<B *> self]; // YES
>
> In Swift, when you compile the generic class, a class is generated for
> each type you use - example:
>
> class MyClass<T> {}
> class A {}
> class B {}
>
> NSStringFromClass(MyClass<A>.self) // _TtGC14__lldb_expr_417MyClassCS_1*A*
> _
> NSStringFromClass(MyClass<B>.self) // _TtGC14__lldb_expr_417MyClassCS_1*B*
> _
>
> MyClass<A>.self == MyClass<B>.self // false
>
> This makes what you suggest very complicated.
>
> On Dec 14, 2016, at 2:47 PM, Davide Mendolia via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Hi,
>
> Maybe this has been asked before but I couldn't find it.
>
> I would like to be able to give visibility of non-generic subclass of a
> generic class to obj-c. Is there any limitation of the compiler knowing
> that the type of the non-generic type is closed, to generate a compatible
> version for obj-c?
>
> Code Example:
>
> class SwiftSuperType<T: NSObjectProtocol> : NSObject {
>
> }
>
> class NonGenericClass2: SwiftSuperType<NSObject> {
>
> }
>
> Or with a obj-c super class:
>
> @interface ObjcSuperType<T: id<NSObject>> : NSObject
>
> @end
>
> class NonGenericClass: ObjcSuperType<NSObject> {
>
> }
>
>
> Actual Behaviour:
>
> Non-generic classes are not visible in obj-c. If adding the @objc notation
> we get the following error.
>
> Actual error message:
> Generic subclasses of '@objc' classes cannot have an explicit '@objc'
> attribute because they are not directly visible from Objective-C
>
> regards,
>
> --
> Davide Mendolia
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