> On Oct 12, 2015, at 2:00 PM, Rick Mann <[email protected]> wrote:
>
> In all this discussion, I forgot to bring up the containers: are they
> special? A "let" container can't be reassigned, nor can its contents be
> changed. How is this implemented?
No, they’re not special, but they’re structs (not classes). That means they’re
passed by value (copied), and their methods can be marked ‘mutable’ to indicate
that they modify the object.
Under the hood, as an optimization, the collection classes use copy-on-write
indirection — an Array struct just contains a pointer to an internal buffer (a
class instance) that has the actual data. That makes copying an Array
efficient. The implementation just has to be careful to allocate a new buffer
before modifying the contents, if the buffer is shared with any other Arrays.
(There are some low-level public Swift types that act as helpers for
implementing this pattern.)
If you want to know lots more, read Mike Ash’s article “Let’s Build Swift
Array”:
https://www.mikeash.com/pyblog/friday-qa-2015-04-17-lets-build-swiftarray.html
—Jens
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