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Editorial Team
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Asked: 2026-06-01T03:53:04+00:00

In a recent Xcode 4.3 project template, some @synthesze are declared as: @synthesize window

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In a recent Xcode 4.3 project template, some @synthesze are declared as:

@synthesize window = _window;
@synthesize managedObjectContext = __managedObjectContext;
@synthesize managedObjectModel = __managedObjectModel;
@synthesize persistentStoreCoordinator = __persistentStoreCoordinator;
@synthesize navigationController = _navigationController;

Some come with a double underscore (__) as prefix. Why?

Anything to do with readonly attribute?

@property (strong, nonatomic) UIWindow *window;
@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;
@property (strong, nonatomic) UINavigationController *navigationController;
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1 Answer

  1. Editorial Team

    They probably shouldn’t use a double underscore, if they’re intended for use in your own program. I expect it’s just an oversight on the part of whoever wrote that template example. In practice, it’s unlikely that they’ll cause any problems.

    The C standard reserves all identifiers starting with a double underscore for the implementation’s own use. Since Objective-C is a superset of C, you shouldn’t be using those identifiers in Objective-C programs either. From the C spec, section 7.1.3 Reserved identifiers:

    All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.

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