[swift-users] Stored Property on Protocol Extension

[Zhao Xin]

​I don't know why the limit exists. Technical or on purpose. I hope someone
inside can answer this.

Zhaoxin​

On Mon, Sep 12, 2016 at 11:55 PM, Daniel Dunbar via swift-users <
swift-users at swift.org> wrote:

>
> > On Sep 11, 2016, at 6:12 PM, Tanner Nelson via swift-users <
> swift-users at swift.org> wrote:
> >
> > Hey Swift Users,
> >
> > I was wondering how you all work around not being able to add stored
> properties in extensions (especially protocol extensions).
> >
> > I ran into an issue recently where I needed an internal stored variable
> for a protocol, but I didn't want the conformer to worry about implementing
> the variable.
> >
> > I ended up using something like this to achieve the effect.
> >
> >    extension MyProtocol {
> >        private var address: String {
> >            mutating get {
> >                var id = ""
> >                withUnsafePointer(to: &self) { id = "\($0)"}
> >                return id
>
> BTW, you can write `return withUnsafePointer(to: &self) { "\($0)" }`
> instead.
>
> >            }
> >        }
> >
> >        var myStoredVar: Bool {
> >            mutating get {
> >                return _storage[address] ?? false
> >            }
> >            set {
> >                _storage[address] = newValue
> >            }
> >        }
> >    }
> >
> > Obviously not ideal, but I don't see another way to achieve this besides
> subclassing (which has its own problems for my situation).
> >
> > Wondering if anyone has run into this and come up with a better solution
> or other type of work around.
>
> I don't have a better answer to this part.
>
> This particular solution only works if it is ok for `_storage[address]` to
> potentially be reused by a new instance, though (if the old instance is
> deallocated and the new one is allocated in its place).
>
>  - Daniel
>
> >
> > Thanks!
> > Tanner
> > _______________________________________________
> > swift-users mailing list
> > swift-users at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-users
>
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-users/attachments/20160913/81ab7dfb/attachment.html>