In Swift you assign a large array to another variable with var b = a and then modify only b. Is what happens a shallow copy or a deep copy? Explain it together with Copy-on-Write.
At assignment only a shallow copy happens: both variables point at the same heap buffer. The first time b is modified, Swift checks whether the buffer is uniquely referenced and only then copies it in full before writing; that's Copy-on-Write (COW), and if the elements are classes the references in the copied buffer still point at the original instances, so it isn't a complete copy.
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