After updating to the newest Version of Xcode 4.5 for iOS6 last night, i get Warnings and Errors like this
Property 'mapAnnotation' requires method 'mapAnnotation' to be defined - use @synthesize, @dynamic or provide a method implementation in this class implementation
because of missing @synthesize Statements, and even Errors about unknown iVars if i use them.
The thing is, i thought it was not necessary to write these @synthesize statements since the last Xcode Update to 4.5 that came out with Mountain Lion, AND all my projects worked without them before i've updated Xcode last night (i've deleted a whole bunch of @synthesize statements from my files back then) It's even still in the Release-Notes:
• Objective-C @synthesize command is generated by default when using properties.
So i'm confused, am i missing a new Project-Setting that turns automatic @synthesize generation on?
But it's not even working when i create a new Project and try it
I faced the same problem and found the reason and the solution.
If you look at the header file of NSManagedObject in iOS 6 SDK, you'll see "NS_REQUIRES_PROPERTY_DEFINITIONS" which forces classes to specify @dynamic or @synthesize for properties.
(You can see the NS_REQUIRES_PROPERTY_DEFINITIONS in the API diff between iOS 5.1 and iOS 6.0.)
This is because the compiler has to know if you want a property to be dynamic or synthesized especially in the implementation of a subclass of NSManagedObject class.
I could solve this problem simply by adding the @synthesize lines explicitly for the properties other than @dynamic in NSManagedObject subclasses.
来源：https://stackoverflow.com/questions/12510849/automatic-property-synthesize-not-working-on-nsmanagedobject-subclass