Are there strongly-typed collections in Objective-C?

Question

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

This answer is outdated but remains for historical value.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.

No, but to make it clearer you can comment it with the type of object you want to store, I've seen this done a few times when you need to write something in Java 1.4 nowadays) e.g.:

NSMutableArray* /*<TypeA>*/ arrayName = ....

or

NSDictionary* /*<TypeA, TypeB>*/ dictionaryName = ...

This was released in Xcode 7 (finally!)

Note that in Objective C code, it's just a compile-time check; there will be no run-time error just for putting the wrong type into a collection or assigning to a typed property.

Declare:

@interface FooClass <T> : NSObject
@property (nonatomic) T prop;
@end

Use:

FooClass<NSString *> *foo = [[FooClass alloc] init];
NSArray<FooClass<NSString *> *> *fooAry = [NSArray array];

Be careful about those *s.

There are no generics in Objective-C.

From the Docs

Arrays are ordered collections of objects. Cocoa provides several array classes, NSArray, NSMutableArray (a subclass of NSArray), and NSPointerArray.