[swift-users] Intended behavior or bug in Dictionary's new subscript(_:default:)?

[Jens Persson]

https://bugs.swift.org/browse/SR-5250

On Sun, Jun 18, 2017 at 6:12 AM, Ben Cohen <ben_cohen at apple.com> wrote:

> 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 at 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
> >
> > _______________________________________________
> > swift-users mailing list
> > swift-users at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-users
>
>
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