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