Scratch that last message. Determining what's stored in `Any` was Charles's
original goal, and what I said made no sense. This should work:
func unbridgedCast<T, U>(_ x: T, to: U.Type) -> U? {
guard type(of: x) is U.Type else { return nil }
return x as! U
}
if let x = unbridgedCast(x, to: String.self) { ... }
> On Aug 24, 2016, at 6:06 PM, Jaden Geller <[email protected]> wrote:
>
> Actually, the code I proposed does not work. First of all, it doesn't work
> for subtype relations. This can easily be fixed though by changing `==
> U.self` to `is U`. More importantly though, it fails when `T` is an
> existential since `type(of:)` looks within the existential to determine the
> type. It would be useful if there were a way to determine the *actual*
> runtime representation of the value.
>
>> On Aug 24, 2016, at 6:00 PM, Jaden Geller <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> Taking that suggestion a step further, it's pretty easy to define a function
>> that performs this sort of casting without bridging.
>>
>> func unbridgedCast<T, U>(_ x: T, to: U.Type) -> U? {
>> guard type(of: x) == U.self else { return nil }
>> return unsafeBitCast(x, to: U.self)
>> }
>>
>>> On Aug 24, 2016, at 5:42 PM, Xiaodi Wu via swift-evolution
>>> <[email protected] <mailto:[email protected]>> wrote:
>>>
>>> import Foundation
>>>
>>> let foo: Any = "Hello"
>>> type(of: foo) == String.self // true
>>> type(of: foo) == NSString.self // false
>>>
>>> let bar: Any = "Hello" as NSString
>>> type(of: bar) == String.self // false
>>> type(of: bar) == NSString.self // true
>>>
>>> Why not this?
>>>
>>>
>>> On Wed, Aug 24, 2016 at 19:09 Charles Srstka via swift-evolution
>>> <[email protected] <mailto:[email protected]>> wrote:
>>> MOTIVATION:
>>>
>>> SE-0083 appears to be dead in the water, having been deferred until later
>>> in Swift 3 back in May and not having been heard from since then, with the
>>> Swift 3 release looming closer and closer. However, the predictability
>>> gains that would have been provided by this change remain desirable for
>>> cases where one needs to know the actual dynamic type of an entity before
>>> any bridging magic is involved. Additionally, performance-critical code may
>>> desire the ability to check something’s type quickly without incurring the
>>> overhead of Objective-C bridging code.
>>>
>>> PROPOSED SOLUTION:
>>>
>>> I propose the following operators: really_is, really_as, really_as?, and
>>> really_as!. These operators would only return a positive result if the type
>>> actually was what was being asked for, instead of something that might be
>>> able to bridge to that type.
>>>
>>> DETAILED DESIGN:
>>>
>>> let foo: Any = "Foo"
>>> let bar: Any = NSString(string: "Bar")
>>>
>>> let fooIsString = foo is String // true
>>> let fooReallyIsString = foo really_is String // true
>>>
>>> let fooIsNSString = foo is NSString // true
>>> let fooReallyIsNSString = foo really_is NSString // false
>>>
>>> let barIsString = bar is String // true
>>> let barReallyIsString = bar really_is String // false
>>>
>>> let barIsNSString = bar is NSString // true
>>> let barReallyIsNSString = bar really_is NSString // true
>>>
>>> ALTERNATIVES CONSIDERED:
>>>
>>> Stick with using an unholy combination of Mirror and unsafeBitCast when you
>>> need to know what you’ve actually got.
>>>
>>> Charles
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> [email protected] <mailto:[email protected]>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
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>>
>
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