Question

I have the following code:

NSUInteger one = 1;
CGPoint p = CGPointMake(-one, -one);
NSLog(@"%@", NSStringFromCGPoint(p));

Its output:

{4.29497e+09, 4.29497e+09}

On the other hand:

NSUInteger one = 1;
NSLog(@"%i", -one); // prints -1

I know there’s probably some kind of overflow going on, but why do the two cases differ and why doesn’t it work the way I want? Should I always remind myself of the particular numeric type of my variables and expressions even when doing trivial arithmetics?

P.S. Of course I could use unsigned int instead of NSUInteger, makes no difference.

Answer 1

When you apply the unary - to an unsigned value, the unsigned value is negated and then forced back into unsigned garb by having Utype_MAX + 1 repeatedly added to that value. When you pass that to CGPointMake(), that (very large) unsigned value is then assigned to a CGFloat.

You don’t see this in your NSLog() statement because you are logging it as a signed integer. Convert that back to a signed integer and you indeed get -1. Try using NSLog("%u", -one) and you’ll find you’re right back at 4294967295.

unsigned int versus NSUInteger DOES make a difference: unsigned int is half the size of NSUInteger under an LP64 architecture (x86_64, ppc64) or when you compile with NS_BUILD_32_LIKE_64 defined. NSUInteger happens to always be pointer-sized (but use uintptr_t if you really need an integer that’s the size of a pointer!); unsigned is not when you’re using the LP64 model.