On Wed, May 24, 2017 at 01:55 Martin R <martinr...@gmail.com> wrote: > > On 24. May 2017, at 04:34, Xiaodi Wu via swift-evolution < > swift-evolution@swift.org> wrote: > > Yes. First, the Foundation initializer converting from NSNumber has the > same behavior: > > ``` > import Foundation > let x = Int(42.5 as NSNumber) // x == 42 > ``` > > > As of SE-170 (which is implemented in Swift 4), the label "truncating" is > also used for conversions which loose precision: > > let x = Int(42.5 as NSNumber) // // warning: 'init' is deprecated > let y = Int(truncating: 42.5 as NSNumber) // 42 > let z = Int(exactly: 42.5 as NSNumber) // nil >
...it is true that SE-0170 gives you a labeled initializer called `truncating`, but in this case it really does mean truncating the bit pattern. If the argument is a floating point value, then rounding towards zero happens before truncating the bit pattern, but this is not why it's labeled as truncating: ``` import Foundation let x = Int8(truncating: 442.5 as NSNumber) // -70 let y = Int8(truncating: 442 as NSNumber) // -70 let z = Int8(extendingOrTruncating: 442 as Int) // -70 let a = Int8(442.5) // fatal error ``` This is very important because, if an NSNumber were converted by truncating the bit pattern if a whole number, but only by rounding if floating point, then two very close values could be converted very differently. Instead, this initializer does what it says on the tin: it always truncates the bit pattern of the value rounded toward zero in order to be representable. Again, illustrating the importance of consistent usage of this term; truncating here really does mean truncating the bit pattern.
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