Overflow when converting from UInt32 to Int

Question

I'm having a really weird issue with Swift/Xcode (not really sure where the source lies, to be honest).

I have to following code:

extension Int {
    func random(min : Int = 0, max : Int = Int(UInt32.max - 1)) {
        return min + Int(arc4random_uniform(UInt32(max - min + 1)))
    }
}

When I build this code in Xcode, it works perfectly fine. When I try to build it using xcodebuild though, the compiler gives me the following error:

integer overflows when converted from 'UInt32' to 'Int'

    public static func random(min : Int = 0, max : Int = Int(UInt32.max - 1)) -> Int {

Which is weird, since the values of Int.max and UInt32.max are no where close.

I'm using Xcode 7.0 beta 5 for compilation if it is any help...'cause I'm absolutely stumped.

If you are going from Int to UInt32 there isn't a problem. But you are trying to go from UInt32 to Int will cause a issue. — Black Frog, Aug 18 '15 at 14:35
@BlackFrog: From Int to UInt32 can be a problem on 64-bit architectures. — Martin R, Aug 18 '15 at 15:05

score 3 · Accepted Answer · edited May 23 '17 at 12:29

That error occurs if you compile for a 32-bit device (e.g. iPhone 5), because Int is then a signed 32-bit integer, and UInt32.max - 1 outside of its range.

Another problem is the calculation of UInt32(max - min + 1), which can crash at runtime due to an overflow, e.g. if you call

random(min : Int.min, max : Int.max)

See How can I generate large, ranged random numbers in Swift? for a possible solution to avoid overflows when generating random numbers for arbitrary ranges.

Overflow when converting from UInt32 to Int

1 Answers1