I believe Ian is correct in that changing any struct's value in any way creates a whole new struct. See http://swiftstub.com/635749345 for proof (the didSet property observer fires every time a change happens to the struct).
> On Dec 23, 2015, at 11:00 AM, Lino Rosa via swift-evolution > <[email protected]> wrote: > > Reassigning structures will create new ones. Mutating functions and > reassigning its properties won't: they mutate it. > On Wed, Dec 23, 2015 at 10:51 AM Ian Ynda-Hummel <[email protected] > <mailto:[email protected]>> wrote: > I thought structures were immutable and mutating functions actually just > created new structures, thus maintaining the by value semantics. > On Wed, Dec 23, 2015 at 10:45 AM Lino Rosa via swift-evolution > <[email protected] <mailto:[email protected]>> wrote: > I believe the language would be improved by making structures immutable. > > 1) The choice between classes and structures isn’t clear right now. If > structures were immutable it would be natural to use them as value objects. > > 2) Refactoring a mutable structure into a class when it’s being passed around > multiple threads removes the by-value semantics seamlessly. The resulting > mutable class isn’t thread-safe. > > 2.1) Even when passed around a single thread, the resulting class would be > passed by reference, so any mutations would have unintended consequences. > > 3) We could probably remove some syntax: `mutating` keyword and variable > parameters. Also the `var` keyword before a structure could be used to denote > reassignment (not mutability), just as it does with classes. > > Of corse I might not be seeing the whole picture, so please weigh in. > _______________________________________________ > swift-evolution mailing list > [email protected] <mailto:[email protected]> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution> > _______________________________________________ > swift-evolution mailing list > [email protected] <mailto:[email protected]> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution>
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