Why does the ARC migrator say that NSInvocation's -setArgument: is not safe unless the argument is __unsafe_unretained?

Question

I was migrating a block of code to automatic reference counting (ARC), and had the ARC migrator throw the error

NSInvocation\'s setArgument is not sa

Answer 1

Why is it preventing you from doing this? It seems just as bad to use __unsafe_unretained objects as arguments.

The error message could be improved but the migrator is not saying that __unsafe_unretained objects are safe to be used with NSInvocation (there's nothing safe with __unsafe_unretained, it is in the name). The purpose of the error is to get your attention that passing strong/weak objects to that API is not safe, your code can blow up at runtime, and you should check the code to make sure it won't.

By using __unsafe_unretained you are basically introducing explicit unsafe points in your code where you are taking control and responsibility of what happens. It is good hygiene to make these unsafe points visible in the code when dealing with NSInvocation, instead of being under the illusion that ARC will correctly handle things with that API.