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

The important thing here is the standard behaviour of NSInvocation: By default, arguments are not retained and C string arguments are not being copied. Therefore under ARC your code can behave as follows:

// Creating the testNumber
NSDecimalNumber *testNumber1 = [[NSDecimalNumber alloc] initWithString:@"1.0"];

// Set the number as argument
[theInvocation setArgument:&testNumber1 atIndex:2];

// At this point ARC can/will deallocate testNumber1, 
// since NSInvocation does not retain the argument
// and we don't reference testNumber1 anymore

// Calling the retainArguments method happens too late.
[theInvocation retainArguments];

// This will most likely result in a bad access since the invocation references an invalid pointer or nil
[theInvocation invoke];

Therefore the migrator tells you: At this point you have to explicitly ensure that your object is being retained long enough. Therefore create an unsafe_unretained variable (where you have to keep in mind that ARC won't manage it for you).