In the Foundation framework for iOS and Mac the main container classes (NSString, NSArray, NSDictionary, and some others) have two states; Immutable and Mutable. If a class is immutable, it cannot be changed and is treated as ready only. When a class if mutable it can be modified after creation. It may seem that when passing one of these around it may be faster to retain it rather than copy it, because new memory does not need to be allocated. While this is true it can leave instabilities, see this example.