[swift-evolution] Proposal: [stdlib] Remove withUnsafe[Mutable]Pointer[s]()
kevin at sb.org
Wed Dec 16 16:40:40 CST 2015
On Wed, Dec 16, 2015, at 02:25 PM, Jacob Bandes-Storch via swift-evolution wrote:
> Is there a reason that this couldn't be made to work? let ptr:
> UnsafePointer<Void> = &x
Yes. That breaks several parts of the language:
1. Swift semantics allow you to use computed properties and stored
properties interchangeably. That expression there can't work with
computed properties unless Swift silently creates a temporary that
lives for the entire scope, which would be surprising behavior.
2. Swift also aggressively discards values that are no longer used even
if they're still in scope (at least under optimization; it likely
keeps them around in debug builds so they can be inspected). But
there's no way for Swift to know how long that pointer is going to be
used for (it could track that variable itself, but what about derived
values?). So any value that's used in that expression would also have
to have its lifetime extended for the entire scope, which is
3. The pointer definitely cannot be valid outside of the current scope,
but there's nothing in that statement to imply this.
withUnsafePointer() taking a scope tells the user that the pointer is
only valid for the scope (otherwise it would just return the
pointer), and similarly the current &x behavior can only be used as a
parameter to a function, which tells the user that it's only valid
for the duration of that function call.
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