Strange conversion from NSUInteger to floats.

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

OK without actually knowing, but reading around on the net about all of these datatypes, I'd say the issue was with the conversion from a NUSInteger (which resolves to either an int (x32) or a long (x64)) to a CGFloat (which resolves to either a float(x32) or double(x64)).

In your second example that same conversion is not happening. The other thing that may be effecting it is that from my reading, NSUinteger is not designed contain negative numbers, only positive ones. So that is likely to be where things start to go wrong.

Answer 2

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.