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objective-cgenericsnsarraynsdictionary

Specify value type of NSDictionary entries

For an NSArray, Objective-C allows us to specify the type of the values to be stored in the array. For example, I can declare an array of NSString's as follows:

NSArray<NSString*> *arrayOfStrings;

While it is not necessarily enforced by the compiler and runtime environment, such an indication can be handy both for readability and slightly simpler code syntax.

I was wondering if such a syntax existed for NSDictionary. For example, to specify the value type in the dictionary, does something like the following exist:

NSDictionary<UIImage*> *imageMappings;

where the dictionary's values are specified to be of type UIImage.

Additional questions:

Can key types be specified for NSDictionary?

If possible, how can I specify value type without key type?

Are there other common data structures in Foundation that have this generic parametrization?

Solution

  • For NSDictionary, you specify the type for both key and value:

    NSDictionary<NSString*, UIImage*> *imageMappings

    If you don't want a specific type for the key, use id<NSCopying> or even NSObject *.

    Only NSArray and NSDictionary support this notation.