In order for this defaulting subscript technique to work as intended, the
subscript { get } needs to be called, then the mutation happens, then the
subscript { set } writes the mutated value back, including adding it for the
first time it the default was needed.
Reference types, not being value types, skip the write-back part, because they
shouldn’t need writing back – they should just get amended in place, because
they’re reference types.
Except this particular technique is relying on it.
This is probably worth a bug report, though I’m not sure if there’s an easy
fix. The alternative is that Dictionary.subscript(_: default:) be made a
mutating get that sets the default if not present, even without the set.
There’s downsides to this though: you would no longer be able to use this
defaulting subscript with immutable dictionaries, and getting a default value
would add it which might be very unexpected.
> On Jun 16, 2017, at 1:40 PM, Jens Persson via swift-users
> <swift-users@swift.org> wrote:
>
> // Swift 4, Xcode 9 beta 1, default toolchain
>
> import Foundation
>
> var d1 = [Int : String]()
> d1[1, default: .init()].append("a")
> d1[2, default: .init()].append("b")
> d1[3, default: .init()].append("c")
> d1[1, default: .init()].append("d")
> print(d1) // [2: "b", 3: "c", 1: "ad"] as expected.
>
> var d2 = [Int : NSMutableString]()
> d2[1, default: .init()].append("a")
> d2[2, default: .init()].append("b")
> d2[3, default: .init()].append("c")
> d2[1, default: .init()].append("d")
> print(d2) // [:] but why?
>
> I know that NSMutableString is a reference type and String is a value type
> and that the default argument is an @autoclosure. I also know that the newly
> created NSMutableString instance is just released immediately after the
> append call, without being stored and retained in the Dictionary's storage.
>
> Is this the intended behavior and if so, please let me better understand
> how/why.
>
> /Jens
>
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