> On Dec 29, 2016, at 2:03 PM, Guillaume Lessard via swift-users
> <swift-users@swift.org> wrote:
>
> Hi Etan,
>
> `withMemoryRebound` does not copy memory.
> The proposal for UnsafeRawPointer contains information about the memory model
> (as related to pointers):
> https://github.com/apple/swift-evolution/blob/master/proposals/0107-unsaferawpointer.md
>
> (also, the method is defined in the following file:
> https://github.com/apple/swift/blob/master/stdlib/public/core/UnsafePointer.swift.gyb)
>
> The capacity parameter of withMemoryRebound allows rebinding a contiguous
> buffer at once; it might be nice if it had a default value of 1.
>
> Cheers,
> Guillaume Lessard
Yep. To further reassure you, `withMemoryRebound` could not be implemented as a
copy without breaking various semantics.
Specifically, `forwardPointer` below is semantically equivalent to calling `f`
directly.
func forwardPointer(_ p: UnsafePointer<Int64>, to f: (UnsafePointer<Int64>) ->
()) {
p.withMemoryRebound(to: Int32.self, capacity: 1) {
$0.withMemoryRebound(to: Int64.self, capacity: 1) {
f($0)
}
}
}
[For an instant, the high bytes are initialized to untyped raw bits. The
documentation skirts around this case, but the model is consistent and
verifiable.]
The `capacity` label serves to distinguish this cast from the familiar C
pointer cast where it’s customary to dereference multiple elements from the
resulting typed pointer without specifying the array size. A default capacity=1
would be convenient but misleading.
-Andy
_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users
