‘CGFloat’ is not convertible to ‘UInt8' and other CGFloat issues with Swift and Xcode 6 beta 4

问题

In case this illuminates the problem, here's the original Objective-C code.

int x = (arc4random()%(int)(self.gameView.bounds.size.width*5)) - (int)self.gameView.bounds.size.width*2;
int y = self.gameView.bounds.size.height;
drop.center = CGPointMake(x, -y);

I started out with this code. Lines 2 and 3 are fine, I'm presenting them for clarity later.

let x = CGFloat(arc4random_uniform(UInt32(self.gameView.bounds.size.width * 5))) - self.gameView.bounds.size.width * 2
let y = self.gameView.bounds.size.height
dropView.center = CGPointMake(x, -y)

In Xcode 6 beta 3, it was necessary to cast the arc4random_uniform UInt32 result to CGFloat in order for the minus and multiplication to work. This doesn't work anymore and the compiler shows an error:

‘CGFloat’ is not convertible to ‘UInt8’

The release notes state:

"CGFloat is now a distinct floating-point type that wraps either a Float on 32-bit architectures or a Double on 64-bit architectures. It provide all of the same comparison and arithmetic operations of Float and Double and may be created using numeric literals. Using CGFloat insulates your code from situations where your code would be !fine for 32-bit but fail when building for 64-bit or vice versa. (17224725)"

Am I just doing something wrong with types? I don't even know how to describe this problem better to submit a bug report to Apple for beta 4. Pretty much every single Swift project I have that does any kind of point or rect manipulation got hit by this issue, so I'm looking for some sanity.

回答1:

Since Swift doesn't have implicit type conversions, you must specify all the type conversions that take place. What makes this case particularly tedious, is that currently Swift seems to lack direct conversions between CGFloat and types such as UInt32, and you must go through an intermediate type as you've discovered.

In the end, two double conversions are needed for the arc4random_uniform:

let bounds = CGRectMake(0.0, 0.0, 500.0, 500.0)
var x = CGFloat(UInt(arc4random_uniform(UInt32(UInt(bounds.size.width) * 5))))
x -= bounds.size.width * 2
let center = CGPointMake(x, -bounds.size.height)

回答2:

Had the same problem ... try wrapping

arc4random_uniform 

with

Int()

like

Int(arc4random_uniform)

this worked for me ... don't know why Swift/Xcode hast problems converting unsigned INT's

回答3:

TL;DR simple shortcuts cause HCF: Halt and Catch Fire bugs

Note that there are some obvious work legal work arounds like implementing conversion to and from CGFloat:

Totally legal, but don't do this:

extension Float {
    func __conversion() -> CGFloat { return CGFloat(self) }
}

extension CGFloat {
    func __conversion() -> Float { return Float(self) }
    func __conversion() -> Double { return Double(self) }
}

extension Double {
    func __conversion() -> CGFloat { return CGFloat(self) }
}

I did not notice when typing, but later my machine kept overheating and hanging and SourceKit went to 300-500%, and the swift proceess + kernel_task took up 10+ gigs of RAM, consuming all that was left of my 16 gigs. It took a long time to trace it back to this - it wasn't swift.

回答4:

@Arkku provided the correct solution, so the one-liner for x is...

let x = CGFloat(UInt(arc4random_uniform(UInt32(UInt(self.gameView.bounds.size.width) * 5)))) - self.gameView.bounds.size.width * 2

As of Xcode 6 beta 5, you can still use an intermediate conversion if you want and your code will continue to work. However, it is no longer necessary, so the following now works as expected.

let x = CGFloat(arc4random_uniform(UInt32(self.gameView.bounds.size.width * 5))) - self.gameView.bounds.size.width * 2

Since the original question is only relevant to Xcode 6 beta 4, what is the proper way to handle the question? Is there a historical mark? Should it be deleted?

来源：https://stackoverflow.com/questions/24873219/cgfloat-is-not-convertible-to-uint8-and-other-cgfloat-issues-with-swift-and