[swift-evolution] Sketch: Teach init a 'defer'-like ability to deinit

[Jonathan Hull]

I really love this idea.  My mental model of it is that it is exactly like ‘defer’, except it works on the lifetime of the object instance instead of a function/method.  Same thing, different scope.

I like how the creation and destruction are right next to one another.  It also solves a lot of potential issues with partial initialization, I believe.

I might spell it ‘deferToDeinit’ or 'deferUntilDeinit'

The only issue I see is accidentally capturing self strongly.  Is there a way to mark a closure as implicitly unowned self so the end programmer doesn’t have to worry about it?

Thanks,
Jon
> Twitter tl;dr: 
> > Brent: So each instance must remember which init was used for it and then run the matching deinit code at deinit time?
> > Me: In my version, the constructive act and destructive act are always paired, even redundantly, using a stack if needed
> > Graham: so all your deferredDeinit blocks would run, no matter which init was invoked?
> > Brent: Closure stack in the worst case. Might be able to optimize to something cheaper if no captures.  Degenerate case: `for i in 0..<10 { deinit { print(i) } 
> 
> So continuing on from Twitter, assuming the compiler cannot optimize in the case of multiple inits, and init-redirections, how about allowing traditional deinit as well, and introduce compile-time optimization into traditional de-init if the compiler finds only one initialization path per class? We can also warn anyone using my version in a complicated degenerate way that it can be costly through education, manual, etc. It would also help if (especially in Cocoa), you could legally use shared initialization setup closures.
> 
> If I create an observer, I want to be able to handle its end-of-life at that point. If I allocate memory, ditto. Etc etc. Surely Swift should be able to support doing this.
> 
> -- E
> 
> > On Jun 8, 2016, at 3:43 PM, Erica Sadun via swift-evolution <swift-evolution at swift.org <https://lists.swift.org/mailman/listinfo/swift-evolution>> wrote:
> > 
> > I really like this idea. Spatially moving cleanup next to unsafe operations is good practice.
> > 
> > In normal code, I want my cleanup to follow as closely as possible to my unsafe act:
> > 
> > let buffer: UnsafeMutablePointer<CChar> = UnsafeMutablePointer(allocatingCapacity: chunkSize)
> >     defer { buffer.deallocateCapacity(chunkSize) }
> > 
> > (Sorry for the horrible example, but it's the best I could grep up with on a moment's notice)
> > 
> > I like your idea but what I want to see is not the deinit child closure in init you propose but a new keyword that means defer-on-deinit-cleanup
> > 
> > self.ptr = UnsafeMutablePointer<T>(allocatingCapacity: count)
> >     deferringDeInit { self.ptr.deallocateCapacity(count) }
> > 
> > Or something.
> > 
> > -- E
> > p.s. Normally I put them on the same line with a semicolon but dang these things can be long
> > 
> >> On Jun 8, 2016, at 10:54 AM, Graham Perks via swift-evolution <swift-evolution at swift.org <https://lists.swift.org/mailman/listinfo/swift-evolution> <mailto:swift-evolution at swift.org <https://lists.swift.org/mailman/listinfo/swift-evolution>>> wrote:
> >> 
> >> Teach init a 'defer'-like ability to deinit
> >> 
> >> 'defer' is a great way to ensure some clean up code is run; it's declaritive locality to the resource acquisition is a boon to clarity.
> >> 
> >> Swift offers no support for resources acquired during 'init'.
> >> 
> >> For an example, from https://www.mikeash.com/pyblog/friday-qa-2015-04-17-lets-build-swiftarray.html <https://www.mikeash.com/pyblog/friday-qa-2015-04-17-lets-build-swiftarray.html> <https://www.mikeash.com/pyblog/friday-qa-2015-04-17-lets-build-swiftarray.html <https://www.mikeash.com/pyblog/friday-qa-2015-04-17-lets-build-swiftarray.html>>
> >> 
> >> init(count: Int = 0, ptr: UnsafeMutablePointer<T> = nil) {
> >>     self.count = count
> >>     self.space = count
> >> 
> >>     self.ptr = UnsafeMutablePointer<T>.alloc(count)
> >>     self.ptr.initializeFrom(ptr, count: count)
> >> }
> >> 
> >> deinit {
> >>     ptr.destroy(...)
> >>     ptr.dealloc(...)
> >> }
> >> 
> >> Another 'resource' might be adding an NSNotificationCenter observer, and wanting to unobserve in deinit (no need in OS X 10.11, iOS 9, but for earlier releases this is a valid example).
> >> 
> >> Changing the above code to use a 'defer' style deinit block might look like:
> >> 
> >> init(count: Int = 0, ptr: UnsafeMutablePointer<T> = nil) {
> >>     self.count = count
> >>     self.space = count
> >> 
> >>     self.ptr = UnsafeMutablePointer<T>.alloc(count)
> >>     self.ptr.initializeFrom(ptr, count: count)
> >> 
> >>     deinit {
> >>         ptr.destroy(...)
> >>         ptr.dealloc(...)
> >>     }
> >> 
> >>     // NSNotificationCenter example too
> >>     NSNotificationCenter.defaultCenter().addObserver(...)
> >>     deinit { 
> >>         NSNotificationCenter.defaultCenter().removeObserver(...)
> >>     }
> >> }
> >> 
> >> The need to provide a separate implemention of deinit is gone. Reasoning for 'defer' applies here. There is good locality between what was initialized and what needs cleaning up.
> >> 
> >> Considerations:
> >> 1. Should deinit blocks be invoked before or after code in an explicit deinit method?
> >> 2. Should deinit blocks be allowed in other methods; e.g. viewDidLoad()?
> >> 3. How should deinit blocks be prevented from strongly capturing self (thus preventing themselves from ever running!)?
> > 

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