strong / weak / retain / unsafe_unretained / assign

Question

properties for synthesizing the property : retain / assign

• retain - it is retained, old value is released and it is assigned
• assign -

Answer 1

There are a few classes in Cocoa and Cocoa Touch that don’t yet support weak references, which means you can’t declare a weak property or weak local variable to keep track of them. These classes include NSTextView, NSFont and NSColorSpace; for the full list, see Transitioning to ARC Release Notes.

If you need to use a weak reference to one of these classes, you must use an unsafe reference. For a property, this means using the unsafe_unretained attribute:

 @property (unsafe_unretained) NSObject *unsafeProperty;

For variables, you need to use __unsafe_unretained:

NSObject * __unsafe_unretained unsafeReference;

An unsafe reference is similar to a weak reference in that it doesn’t keep its related object alive, but it won’t be set to nil if the destination object is deallocated. This means that you’ll be left with a dangling pointer to the memory originally occupied by the now deallocated object, hence the term “unsafe.” Sending a message to a dangling pointer will result in a crash.

Courtesy:Apple (https://developer.apple.com/library/ios/documentation/cocoa/conceptual/ProgrammingWithObjectiveC/EncapsulatingData/EncapsulatingData.html).

Answer 2

if so, which one to prefer when in iOS4, or why there is (unsafe_unretained) if its exactly same as assign?

you should use unsafe_unretained. You want to show the reader of your code that you actually wanted to use weak but that this was not possible because weak is not available on the iOS version you want to deploy.

One day you will drop the support for iOS4. And then you could just search for unsafe_unretained and replace all of them with weak. This will be much easier than searching for assign and figuring out if you actually meant assign or weak

The use of unsafe_unretained creates more readable and understandable code where the intentions of the developer are easier to see. Basically the same reason we use YES instead of 1.