Question

I saw an Apple sample code with a NSDictionary being initialized using @{ value:key } notation. I use to initialize a constant NSDictionary using +dictionaryWithObjectsAndKeys:

My question is:

Is there any difference between the two generated NSDictionaries? Do I need to worry about memory leaking? I’m using ARC.

All I found about it is this Apple doc, but it is related to Mac, not iOS. And the notation is @{ key = value } and not @{ value:key }.

http://developer.apple.com/legacy/mac/library/documentation/LegacyTechnologies/WebObjects/WebObjects_3.1/DevGuide/WebScript/CreatingObjects.html

A second question would be: is that safe to use this to submit the app to App Store, or would it be considered “undocumented API”?

Thanks!

Answer 1

I use… +dictionaryWithObjectsAndKeys: … Is there any difference between the two generated NSDictionaries?

The compiler uses +[NSDictionary dictionaryWithObjects:forKeys:count:]. So the ‘gotcha’ is that the parameters/values you use in these literal expression must not be nil. When you use +[NSDictionary dictionaryWithObjectsAndKeys:], the input stops when nil is encountered. It is an error to pass nil to a literal expression as a key or value to a literal expression. This may change your program (because semantics of dictionary creation through va_lists are different), but the stricter semantics would likely result in detecting bugs, more than anything.

Do I need to worry about memory leaking? I’m using ARC.

You might need an autorelease pool in some cases — that depends on the context you create it in.

Is that safe to use this to submit the app to App Store, or would it be considered “undocumented API”?

You will need Apple-Clang 4.0 (Xcode 4.4). It is compatible with all versions of OS X and iOS:

http://developer.apple.com/library/ios/#releasenotes/ObjectiveC/ObjCAvailabilityIndex/_index.html

More details here: http://clang.llvm.org/docs/ObjectiveCLiterals.html