[swift-users] How to check the type of a concrete class that inherits from a generic class?

[Slava Pestov]

Oh I see. I think the problem is that with Objective-C generics, you can always cast from Foo<A> to Foo<B>, because the type parameters do not really exist. Swift’s type checking logic for casts assumes Swift generic semantics, where in general Foo<A> and Foo<B> are unrelated types.

Do you mind filing a bug?

Slava

> On Oct 6, 2017, at 11:40 PM, Glen Huang <heyhgl at gmail.com> wrote:
> 
> NSFetchedResultsController is the class from Core Data: 
> 
> https://developer.apple.com/documentation/coredata/nsfetchedresultscontroller
> 
>> On 7 Oct 2017, at 2:38 PM, Slava Pestov <spestov at apple.com> wrote:
>> 
>> Can you post a self-contained example, including the declaration of NSFetchedResultsController?
>> 
>> Slava
>> 
>>> On Oct 6, 2017, at 11:28 PM, Glen Huang via swift-users <swift-users at swift.org> wrote:
>>> 
>>> Hi,
>>> 
>>> I defined some concrete classes inheriting from a generic class like this:
>>> 
>>> class Controller1: NSFetchedResultsController<NSManagedObject> {}
>>> class Controller2: NSFetchedResultsController<NSManagedObject> {}
>>> 
>>> And I assign them a shared delegate, and in the delegate method:
>>> 
>>> func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>)
>>> 
>>> I want to test the concrete type of controller, doing things differently for Controller1 and Controller2.
>>> 
>>> But doing the following gives me a warning: Cast from 'NSFetchedResultsController<NSFetchRequestResult>' to unrelated type 'Controller1’ always fails
>>> 
>>> switch controller {
>>> case is Controller1:
>>>      // ...
>>> default:
>>>      break
>>> }
>>> 
>>> I wonder what’s the correct way to check the concrete type?
>>> 
>>> Regards,
>>> Glen
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>>> swift-users at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-users
>> 
>