[swift-evolution] deinit and failable initializers

[Joe Groff]

> On Jan 26, 2016, at 9:58 AM, Félix Cloutier via swift-evolution <swift-evolution at swift.org> wrote:
> 
> A simple deinitOnNil/deinitOnError will be insufficient as soon as you have to allocate more than one resources with a failable point in between.

How so? In general, you want to clean up these resources unless you've already committed them to a fully-initialized object. This should do the right thing:

init() throws {
  self.foo = alloc(); deferOnError { dealloc(self.foo) }
  try something()
  self.bar = alloc(); deferOnError { dealloc(self.bar) }
  try something()
  super.init()
}

-Joe

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

You don't need move semantics; you could also factor out your resources into dedicated owner classes with non-failable initializers, which would allow for automatic memory management to do the right thing. The problem only arises with unsafe resources or other invariants that require manual management, and it's a general issue with implicit early exits.

-Joe

> 
> Félix
> 
>> Le 26 janv. 2016 à 12:15:46, Chris Eidhof via swift-evolution <swift-evolution at swift.org <mailto: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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