Are there strongly-typed collections in Objective-C?

> This answer is outdated but remains for historical value. As of Xcode 7, Connor's answer from Jun 8 '15 is more accurate.

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No, there are no generics in Objective-C unless you want to use C++ templates in your own custom collection classes (which I strongly discourage).

Objective-C has dynamic typing as a feature, which means that the runtime doesn't care about the type of an object since all objects can receive messages. When you add an object to a built-in collection, they are just treated as if they were type `id`. But don't worry, just send messages to those objects like normal; it will work fine *(unless of course one or more of the objects in the collection don't respond to the message you are sending)*.

Generics are needed in languages such as Java and C# because they are strong, statically typed languages. Totally different ballgame than Objective-C's dynamic typing feature.

In Xcode 7, Apple has introduced 'Lightweight Generics' to Objective-C. In Objective-C, they will generate compiler warnings if there is a type mismatch.

NSArray<NSString*>* arr = @[@"str"];

NSString* string = [arr objectAtIndex:0];
NSNumber* number = [arr objectAtIndex:0]; // Warning: Incompatible pointer types initializing 'NSNumber *' with an expression of type 'NSString *'

And in Swift code, they will produce a compiler error:

var str: String = arr[0]
var num: Int = arr[0] //Error 'String' is not convertible to 'Int'

Lightweight Generics are intended to be used with NSArray, NSDictionary and NSSet, but you can also add them to your own classes:

@interface GenericsTest<__covariant T> : NSObject

-(void)genericMethod:(T)object;

@end

@implementation GenericsTest

-(void)genericMethod:(id)object {}

@end

Objective-C will behave like it did before with compiler warnings.

GenericsTest<NSString*>* test = [GenericsTest new];

[test genericMethod:@"string"];
[test genericMethod:@1]; // Warning: Incompatible pointer types sending 'NSNumber *' to parameter of type 'NSString *'


but Swift will ignore the Generic information completely. (No longer true in Swift 3+.)

var test = GenericsTest<String>() //Error: Cannot specialize non-generic type 'GenericsTest'

> Aside from than these Foundation collection classes, Objective-C lightweight generics are ignored by Swift. Any other types using lightweight generics are imported into Swift as if they were unparameterized.

[Interacting with Objective-C APIs][1]


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