Question

When reading apple generated template code as well as in documentation, I see _ (underscore) and __ (double underscore). Example would be the code generated on checking off core data option:

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

What does each of this mean? I understand _ refers to a backing private variable. What does __ mean?

Answer 1

Apple’s documentation suggest NOT to use single underscores in your project (as it reserved by apple itself: best-known example is _cmd ). But underscore is traditional c-style modifier for privateness of identifier, so double underscore is a solution to make private identifier for your variable.

http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CodingGuidelines/Articles/NamingMethods.html

Names of most private methods in the Cocoa frameworks have an
underscore prefix (for example, _fooData ) to mark them as private.
From this fact follow two recommendations.

Don’t use the underscore character as a prefix for your private methods. Apple reserves this convention.