I am implementing a custom Array type for a distributed memory system. I have a DataView struct which represents the array to the user as a value type, and a backing store class object referenced by the DataView that does all the replication. Write accesses are mutating so they don't cause a problem.
However read only accesses are not mutating and there lies the problem. If the storage object is uniquely referenced, then I can do my business without taking a synchronization lock, otherwise I need to take a lock before syncing memory. I can "always" take a lock to work around this, but most of the time it isn't necessary and I care about performance. I think there should be a "read only pass by reference" I just pulled this comment from the source code. I was under the impression that isKnownUniquelyReferenced is thread safe, but the comments implies that it isn't?? ------------------------------ /// If the instance passed as `object` is being accessed by multiple threads /// simultaneously, this function may still return `true`. Therefore, you must /// only call this function from mutating methods with appropriate thread /// synchronization. That will ensure that `isKnownUniquelyReferenced(_:)` /// only returns `true` when there is really one accessor, or when there is a /// race condition, which is already undefined behavior. /// /// - Parameter object: An instance of a class. This function does *not* modify /// `object`; the use of `inout` is an implementation artifact. On Wed, Jan 25, 2017 at 10:31 AM, Joe Groff <jgr...@apple.com> wrote: > > > On Jan 25, 2017, at 10:20 AM, Edward Connell via swift-users < > swift-users@swift.org> wrote: > > > > I have a data structure that calls isKnownUniquelyReferenced on a > member. It forces everything to be marked as mutating because of the inout > parameter, however the parameter is never mutated, it is just read right?? > The reason it is inout is because a read only reference is required. > > > > If it is truly not mutating, is there some way around this so I don't > have to mark everything in the caller chain as mutating also? It's kind of > annoying... > > In Swift's current model, `isKnownUniquelyReferenced` needs to be inout > because that's currently the only way to assert unique access to an object > in memory, since read-only rvalues are otherwise freely copyable so the > result of the uniqueness check would be only momentarily valid at best. > What are you trying to do that requires using it on a nonmutable value? > There may be another way to go about it. > > -Joe > >
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