generics - Are there strongly-typed collections in Objective-C?

Question

I'm new to Mac/iPhone programming and Objective-C. In C# and Java we have "generics", collection classes whose members can only be of the type declared. For example, in C#

Dictionary<int, MyCustomObject>

can only contain keys that are integers and values that are of type MyCustomObject. Does a similar mechanism exist in Objective-C?

Answer 1

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