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As for (CGRect){0, 0, 320, 480}, it is indeed valid, but it's a lot nicer to have a consistent syntax instead of mixing C with Objective-C.

There is no mixing. Objective-C is a superset of C, therefore C syntax is part of the Objective-C language.

Mixing in the sense that ABAddressBook is a C based API, with no Cocoa wrapper, like how CGRect is a struct, not an class.