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> 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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