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 <jaden.gel...@gmail.com> 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
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> 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
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> 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
>>
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