[swift-evolution] deinit and failable initializers

Félix Cloutier felixcca at yahoo.ca
Tue Jan 26 11:58:18 CST 2016

A simple deinitOnNil/deinitOnError will be insufficient as soon as you have to allocate more than one resources with a failable point in between.

I think that it could best be solved with move semantics but that's far off Swift 3.


> Le 26 janv. 2016 à 12:15:46, Chris Eidhof via swift-evolution <swift-evolution at swift.org> a écrit :
> Now that we can return nil from a failable initializer without having initialized all the properties, it’s easier to make a mistake. For example, consider the following (artificial) code:
> class MyArray<T> {
>     var pointer: UnsafeMutablePointer<T>
>     var capacity: Int
>     init?(capacity: Int) {
>         pointer = UnsafeMutablePointer.alloc(capacity)
>         if capacity > 100 {
>             // Here we should also free the memory. In other words, duplicate the code from deinit.
>             return nil
>         }
>         self.capacity = capacity
>     }
>     deinit {
>         pointer.destroy(capacity)
>     }
> }
> In the `return nil` case, we should really free the memory allocated by the pointer. Or in other words, we need to duplicate the behavior from the deinit.
> Before Swift 2.2, this mistake wasn’t possible, because we knew that we could count on deinit being called, *always*. With the current behavior, return `nil` is easier, but it does come at the cost of accidentally introducing bugs. As Joe Groff pointed out, a solution would be to have something like “deferOnError” (or in this case, “deferOnNil”), but that feels a bit heavy-weight to me (and you still have to duplicate code).
> In any case, I think it’s nice that we can now return nil earlier. I don’t like that it goes at the cost of safety, but I realize it’s probably only making things less safe in a small amount of edge cases.
> Chris
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