# Using Imported Lightweight Generics in Swift Understand the constraints of imported Obj-C lightweight generic type declarations. ## Overview Objective-C type declarations that use lightweight generic parameterization are imported by Swift with information about the type of their contents preserved. For example, given the following Objective-C property declarations: ```occ @property NSArray *dates; @property NSCache> *cachedData; @property NSDictionary *> *supportedLocales; ``` Here’s the Swift version of those declarations when you import them: ```swift var dates: [Date] var cachedData: NSCache var supportedLocales: [String: [Locale]] ``` A parameterized class written in Objective-C is imported into Swift as a generic class with the same number of type parameters. All Objective-C generic type parameters imported by Swift have a type constraint that requires that type to be a class (`T: AnyObject`). If the Objective-C generic parameterization specifies class or protocols qualifications, the imported Swift declaration has a constraint that requires that type to be a subclass of the specified class or to conform to the specified protocol. For an unspecialized Objective-C type, Swift infers the generic parameterization for the imported class type constraints. For example, consider the following Objective-C class and category declarations: ```occ @interface List> : NSObject - (List *)listByAppendingItemsInList:(List *)otherList; @end @interface ListContainer : NSObject - (List *)listOfValues; @end @interface ListContainer (ObjectList) - (List *)listOfObjects; @end ``` When you import these declarations into Swift, the `NSCopying` protocol qualification of the `List` type and the `NSValue` class qualification of the `listOfValues` method are preserved. In addition, the unqualified `listOfObjects` method uses the `NSCopying` generic constraint inferred from the `List` type. ```swift class List : NSObject { func listByAppendingItemsInList(otherList: List) -> List } class ListContainer : NSObject { func listOfValues() -> List } extension ListContainer { func listOfObjects() -> List } ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)