Formatting NSUInteger with %i

Question

I am going through a former employees code and about 20 of these warnings show up:

Values of type 'NSUInteger' should not be used as format arguments; add an explicit cast to 'unsigned long' instead

one part of the code where this arises is:

NSUInteger Length;

With:

- (NSString *) description {

    // If no value was given, display type
    if([Value length] == 0)
    {
        NSString *type = @"";

        switch (Type) {
            case DTCompType_Year: type = @"year";
                break;
            case DTCompType_Month: type = @"month";
                break;            
            case DTCompType_Day: type = @"day";
                break;            
            case DTCompType_Hour: type = @"hour";
                break;            
            case DTCompType_Minute: type = @"minute";
                break;            
            case DTCompType_Second: type = @"second";
                break;            
            case DTCompType_Meridiem: type = @"meridiem";
                break;            
            case DTCompType_MonthName: type = @"month_name";
                break;
            case DTCompType_DayOfTheWeek: type = @"day_of_the_week";
                break;
            case DTCompType_Undefined: type = @"undefined";
                break;            
        }

        return Length == 0 ? [NSString stringWithFormat:@"[%@]", type] : 
        [NSString stringWithFormat:@"[%@:%i]", type, Length];
    }

No where in apples documentation can I find %i

Apple's Documentation

I have never worked with Objective-C before, and now I have to update this app. I understand that this needs to become an unsigned long, but I don't want to start changing things without knowing why. The app works just fine as is, so are there any inherent consequences for changing these to unsigned long? or even changing the format specifier from %i to %lu?

From what I've read, it could be a matter of the platform. (32-bit vs 64-bit)

This was developed for an iPad 2 in iOS7, and we just upgraded the SDK to iOS8.

I found this post: NSUInteger should not be used in format strings? which has given me some guidance, but I need more clarification.

Answer 1

%i is equivalent to %d. Technically, you should have been using %u anyway. The problem is, as you suspect, 32-bit vs 64-bit; NS[U]Integer is [unsigned] int on 32-bit builds, but [unsigned] long on 64-bit ones. Because the iPhone is little-endian, it will "work" as long as the %i/d/u is the last format specified, but it's still wrong. You should cast the argument to be the type the format specifier expects (int/long/unsigned/unsigned long), as the warning message tells you to.

From <objc/NSObjCRuntime.h>:

#if __LP64__ || (TARGET_OS_EMBEDDED && !TARGET_OS_IPHONE) || TARGET_OS_WIN32 || NS_BUILD_32_LIKE_64
typedef long NSInteger;
typedef unsigned long NSUInteger;
#else
typedef int NSInteger;
typedef unsigned int NSUInteger;
#endif

Answer 2

You can use a boxed literal to allow the compiler and the NSNumber class to handle the details of converting between the various numeric types and their string representations. For example, given the following variable definition...

NSUInteger foo = 42;

...you can create an instance of NSNumber as follows:

NSNumber *myNumber = @(foo);

You can then use the %@ format specifier whenever you need to format the value of myNumber. Of course it's easy enough to instead box the original numeric value right in line:

NSString *s = [NSString stringWithFormat:@"The answer is %@", @(foo)];