Question

NSArray and NSMutableArray’s +arrayWithArray: returns empty array instead of nil when argument is nil.

    NSLog(@"%@", [[NSArray arrayWithArray:nil] class]);
    NSLog(@"%@", [[NSMutableArray arrayWithArray:nil] class]);

output:

    __NSArrayI
    __NSArrayM

But this behavior is not documented on Apples documentation.

Is it safe to rely on the assumption that arrayWithArray:nil returns empty array?
Or should I assign empty array explicitly like this:

    NSDictionary *dic = [[NSDictionary alloc] init];
    NSMutableArray *arr = [dic objectForKey:@"a"];
    if (!arr) {
        arr = [[NSMutableArray alloc] init];
    }

Answer 1

The documentation of +arrayWithArray: says:

Creates and returns an array containing the objects in another given
array.

Of course, nil is not an array, but [nil count] is valid and returns 0, so it might be treated as an empty array here.

But I would not rely on that fact and create empty arrays with [[NSMutableArray alloc] init] or [NSMutableArray array].

ADDED:

If you call +arrayWithArray: with an invalid type, e.g. a NSString, then of course the program will throw an exception. But from the error message

-[__NSCFConstantString count]: unrecognized selector sent to instance 0x69e4

you can see that count is indeed the first method used to copy the array elements. That also explains why it works with nil.