var absences = [0, 2, 0, 4, 0, 3, 1, 0]
let midpoint = absences.count / 2
var firstHalf = absences.prefix(upTo: midpoint)
let secondHalf = absences.suffix(from: midpoint)
Quotation from Apple:
Neither the firstHalf nor secondHalf slices allocate any new storage of their own. Instead, each presents a view onto the storage of the absences array.
When I try to mutate firstHalf
as the following:
firstHalf[1] = 19
the values of firstHalf
changes but the original array absences
remains the same (firstHalf[1]
is equal to 19 while absences[1]
equals to 2)
So what happens in the background. Did I instantiate a new array by mutating the array slice?
Thanks in advance.
Yes, the standard library's collection types, including Array
and ArraySlice
, all have copy-on-write behaviour. This means that they can share storage of their elements with other collections until they are mutated, in which case they will take their own copy of them.
In your case, the underlying array buffer that the slice firstHalf
has a view onto is non-uniquely referenced (as both absences
& secondHalf
also have a view onto it). Therefore when you go to mutate firstHalf
, a copy is triggered – creating a new buffer containing the elements of the slice (but not necessarily the entire array).
firstHalf
now has a unique view onto this new buffer, with absences
& secondHalf
both sharing a view onto the old array buffer. Therefore firstHalf
can now mutate the elements of its buffer without affecting the elements of the original array, thus preserving value semantics.