I recently wasted about half an hour tracking down this odd behavior in NSLog(...):
NSString *text = @"abc";
long long num = 123;
NSLog(@"num=%lld, text=%@",num,text); //(A)
NSLog(@"num=%d, text=%@",num,text); //(B)
Line (A) prints the expected "num=123, text=abc", but line (B) prints "num=123, text=(null)".
Obviously, printing a long long with %d is a mistake, but can someone explain why it would cause text to be printed as null?
You just messed up memory alignment on your stack. I assume than you use newest Apple product with x86 processor. Taking these assumptions into account your stack looks like that in both situations:
| stack | first | second | +---------------------+-------+--------+ | 123 | | %d | +---------------------+ %lld +--------+ | 0 | | %@ | +---------------------+-------+--------+ | pointer to text | %@ |ignored | +---------------------+-------+--------+
In first situation you put on stack 8 bytes and then 4 bytes. And than NSLog is instructed to take back from stack 12 bytes (8 bytes for %lld and 4 bytes for %@).
In second situation you instruct NSLog to first take 4 bytes (%d). Since your variable is 8 bytes long and holds really small number its upper 4 bytes will be 0. Then when NSLog will try to print text it will take nil from stack.
Since sending message to nil is valid in Obj-C NSLog will just send description: to nil get probably nothing and then print (null).
In the end since Objective-C is just C with additions, caller cleans up whole this mess.