Friday, January 8, 2016 [Tweets] [Favorites]

Exploring Swift Array’s Implementation

Ankit Aggarwal:

Swift declares a C struct in its SwiftShim’s module inside GlobalObject.h called _SwiftArrayBodyStorage

[…]

  • count is the number of elements currently stored in the buffer
  • _capacityAndFlags is used to store two things:

    • first is the capacity of the buffer
    • second “Is the Element type bitwise-compatible with some Objective-C class?”

Via David Smith:

Apparently C-like performance, memory safety, resizing, copy-on-write, and ObjC bridging all together is hard

The subscript implementation in _ContiguousArrayBuffer has an interesting note:

Manually swap because it makes the ARC optimizer happy.

The method is also marked with a mysterious nonmutating keyword, which the Swift language guide mentions but does not explain. This could be an optimization to avoid unnecessary writebacks or a way to make the compiler enforce that the setter does not actually change any properties of the structure (just the contents of the memory they point to).

Update (2016-05-03): Jasdev Singh:

Stepping through Sidney’s example, we can see how nonmutating signals that a setter doesn’t modify the containing instance, but instead has global side effects.

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