[swift-users] Swift 4 "Cannot use mutating member on immutable value: 'self' is immutable"

[Jon Shier]

	I think there’s something strange with popFirst. It doesn’t show up in the autocomplete in Xcode, but it compiles, and popLast doesn’t throw the same error. removeFirst doesn’t either, though it’s unsafe. Weird.



Jon


> On Sep 13, 2017, at 9:47 PM, Zhao Xin via swift-users <swift-users at swift.org> wrote:
> 
> I begin to think it is related to how you `popFirst()` implemented. Check it or post it here.
> 
> Zhao Xin
> 
> On Thu, Sep 14, 2017 at 9:36 AM, Roderick Mann <rmann at latencyzero.com <mailto:rmann at latencyzero.com>> wrote:
> Yeah, that's not it. I made the change you suggested, I get the same error.
> 
> > On Sep 13, 2017, at 18:11 , Zhao Xin <owenzx at gmail.com <mailto:owenzx at gmail.com>> wrote:
> >
> > Change `self` to `ModelFetcher`. You are calling a class static property, not a class instance property.
> >
> > Zhao Xin
> >
> > On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> > Moving to Swift 4, I'm running into an issue for which I can't seem to find an answer in google:
> >
> > "Cannot use mutating member on immutable value: 'self' is immutable"
> >
> > The code looks like:
> >
> > class
> > ModelFetcher : NSObject, URLSessionDelegate
> > {
> >     ...
> >     static  let     managerDispatchQueue                    =   DispatchQueue(label: "Model Download Manager Queue")
> >     static  var     pendingFetchers                         =   [ModelFetcher]()
> >     static  var     currentFetcher:         ModelFetcher?
> >
> >     class
> >     func
> >     startNextFetcher()
> >     {
> >         self.managerDispatchQueue.async
> >         {
> >             guard
> >                 self.currentFetcher == nil,
> >                 let mf = self.pendingFetchers.popFirst()
> >                          ~~~~ ^                             error: cannot use mutating member on immutable value: 'self' is immutable
> >             else
> >             {
> >                 return
> >             }
> >
> >             self.currentFetcher = mf
> >             mf.start()
> >         }
> >     }
> >     ...
> > }
> >
> > This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a monolithic app (the error shows up when this code is factored into a framework). Other mutating references to self seem to compile okay (e.g. "self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)"). Not sure what's special about this one.
> >
> >
> > --
> > Rick Mann
> > rmann at latencyzero.com <mailto:rmann at latencyzero.com>
> >
> >
> > _______________________________________________
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> > swift-users at swift.org <mailto:swift-users at swift.org>
> > https://lists.swift.org/mailman/listinfo/swift-users <https://lists.swift.org/mailman/listinfo/swift-users>
> >
> 
> 
> --
> Rick Mann
> rmann at latencyzero.com <mailto:rmann at latencyzero.com>
> 
> 
> 
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