[swift-users] ARC // Precise Lifetime Semantics

[Daniel Eggert]

Thanks! That’s a very elegant solution.

/Daniel


> On 05 Jan 2016, at 20:48, Jacob Bandes-Storch <jtbandes at gmail.com> wrote:
> 
> This is what withExtendedLifetime is for: https://developer.apple.com/library/ios/documentation/Swift/Reference/Swift_StandardLibrary_Functions/index.html#//apple_ref/swift/func/s:FSs20withExtendedLifetimeu0_rFzTq_Fzq_q0__q0_
> 
> On Tue, Jan 5, 2016 at 9:57 AM, Kristof Liliom via swift-users <swift-users at swift.org> wrote:
> Hi,
> 
> I'm not a pro on Swift, but I would try to use the Unmanaged wrapper struct.
> 
> Sample (might not be correct, didn't try)
> 
> // Using Unmanaged
> func dummyfunc(backingData: dispatch_data_t) {
>     var base = UnsafePointer<Void>()
>     var size = Int()
>     let mapped = dispatch_data_create_map(backingData, &base, &size)
> 
>     // New code
>     let unmanaged = Unmanaged.passUnretained(mapped)
>     defer { unmanaged.release() }
> 
>     let buffer = UnsafeBufferPointer<Void>(start: UnsafePointer<Void>(base), count: size / sizeof(Void))
>     return someFunction(buffer)
> }
> 
> Best Regards,
> Chris
> 
> > On 05 Jan 2016, at 16:57, Daniel Eggert via swift-users <swift-users at swift.org> wrote:
> >
> > How do I extend the lifetime of a variable, i.e. make sure that ARC is less aggressive?
> >
> > clang has an attribute called objc_precise_lifetime — does Swift have something similar?
> >
> >
> > I have this code:
> >
> >  do {
> >    var base = UnsafePointer<Void>()
> >    var size = Int()
> >    let mapped = dispatch_data_create_map(backingData, &base, &size)
> >    let buffer = UnsafeBufferPointer<A>(start: UnsafePointer<A>(base), count: size / sizeof(A))
> >    return someFunction(buffer)
> >  }
> >
> > I need the ‘mapped’ variable to stay in scope (i.e. not be released) until the end of the scope, but ARC is free to release it immediately — in fact the compiler warns me that the immutable value is never used.
> >
> > But the API contract with dispatch_data_create_map(3) is that the values in ‘base’ and ‘size’ are only valid as long as ‘mapped’ is around. The above code is passing a buffer into ‘someFunction’ that’s no longer valid.
> >
> > How do I fix this?
> >
> > /Daniel
> >
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