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

Davide Mendolia davide at karumi.com
Wed Dec 14 12:05:48 CST 2016

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).


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

@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:


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


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


Davide Mendolia
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swift-evolution at swift.org

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