What is the visibility of @synthesized instance variables?

by

A synthesized ivar is completely invisible to all code that cannot see the @synthesize line (which basically means anything outside of the .m file). It’s not @protected, it’s not @private, it’s simply unknown. With a @private ivar, other code trying to access it will be told that it’s private, but with a synthesized ivar, other code trying to access it will be told that the field simply doesn’t exist.

As a thought experiment, try imagining a situation where the ivar acted like it was @protected. You make a subclass, and you muck about with the ivar there. Now you go back to the superclass and change @synthesize myProp to @synthesize myProp=foo. What happens in the subclass? When the compiler processes the subclass, it cannot see the @synthesize line, so it would have no idea that you just changed the name of the ivar. In fact, it cannot even tell if the property is backed by an ivar at all, or if it’s implemented with custom-written accessor methods. I hope it’s obvious why this means that the subclass cannot possibly access the ivar, and neither can any other class.

That said, I’m not quite sure what the compiler does if you write code in the same .m file that tries to access the ivar. I expect it will treat the ivar as @private (since the compiler can, in fact, see that the ivar exists).

Also, none of this has any bearing on the runtime methods. Other classes can still use the obj-c runtime methods to dynamically look up your class’s ivar list and muck about with it.

More Related Contents:

  1. @synthesize vs @dynamic, what are the differences?
  2. How can I add properties to an object at runtime?
  3. Properties and Instance Variables in Objective-C
  4. @class vs. #import
  5. How do I use NSTimer?
  6. How to read data from NSFileHandle line by line?
  7. How to Check if an NSDate occurs between two other NSDates
  8. NSInvocation for Dummies?
  9. Number of occurrences of a substring in an NSString?
  10. Hidden Features of Xcode
  11. What exactly is a so called “Class Cluster” in Objective-C?
  12. Why are umbrella frameworks discouraged? [closed]
  13. Regular expressions in an Objective-C Cocoa application
  14. How to Write OS X Finder plugin
  15. View-based NSTableView with rows that have dynamic heights
  16. NSMutableArray – force the array to hold specific object type only
  17. In Objective-C on iOS, what is the (style) difference between “self.foo” and “foo” when using synthesized getters?
  18. Best way to remove from NSMutableArray while iterating?
  19. Difference between NSString literals
  20. Why is my transformable Core Data attribute not using my custom NSValueTransformer?
  21. How is release handled for @synthesized retain properties?
  22. Valid use of accessors in init and dealloc methods?
  23. Using Swift CFunctionPointer to pass a callback to CoreMIDI API
  24. using objc_msgSend to call a Objective C function with named arguments
  25. Call instance method from class method
  26. Mac Mountain Lion send notification from CLI app
  27. NSString – Convert to pure alphabet only (i.e. remove accents+punctuation)
  28. Getting array elements with valueForKeyPath
  29. Looping using NSRange
  30. NSNumber of seconds to Hours, minutes, seconds

Leave a Comment