Hi guys,
I’m having issues with Swift pointers. I feel like the Interactive With C APis
document only gets you half way there.
For example, look at this from the docs
If you have declared a function like this one:
func takesAMutablePointer(x: UnsafeMutablePointer<Float>) {
// ...
}
You can call it in any of the following ways:
var x: Float = 0.0
var p: UnsafeMutablePointer<Float> = nil
var a: [Float] = [1.0, 2.0, 3.0]
takesAMutablePointer(nil)
takesAMutablePointer(p)
takesAMutablePointer(&x)
takesAMutablePointer(&a)
Seem simple enough. But then I was trying to figure out Core Data validation,
which takes an AutoreleasingUnsafeMutablePointer<AnyObject?> and I can’t figure
out what to pass to it.
I tried to create a simple test in a Playground:
func takesAPointer(p: AutoreleasingUnsafeMutablePointer<AnyObject?>){
return
}
var myString = "Hello"
takesAPointer(p: &myString)
Then I get an error stating 'Cannot pass immutable type “AnyObject?” as inout
argument’.
Everything seems to match the example from the docs. I have a var (so it should
be mutable) and I’m using the ampersand, but still I’m getting an error.
Another problem. I have a specific byte pattern I want to create. For arguments
sake, lets call it 0x123ABC, and I have it as an Int. I want to access the
individual bytes (i.e. 12, 3A, BC).
The struct reference for UnsafePointer<T> doesn’t talk much about initializing
it. Most of the initializers take a pointer. I tried the init(_ bitPattern:)
initializer, and was able to create a pointer, but it seemed to point to the
address 0x123ABC rather than the address *of* 0x123ABC. I tried creating a
buffer with malloc, and it gives me an UnsafeMutablePointer but now I can’t
figure out how to copy my bytes to this buffer.
So clearly there’s something I’m just not grocking about Swift pointers. Does
anyone know of a more remedial tutorial that is updated for Swift 3? I’d like
to continue to work in pure Swift, but it just isn’t clicking.
--
Chris McIntyre
> On Jul 29, 2016, at 6:11 AM, James Campbell via swift-users
> <swift-users@swift.org> wrote:
>
> No I haven't thats a big help thank you !
>
> ___________________________________
>
> James⎥Head of Trolls
>
> ja...@supmenow.com <mailto:ja...@supmenow.com>⎥supmenow.com
> <http://supmenow.com/>
> Sup
>
> Runway East
>
>
> 10 Finsbury Square
>
> London
>
>
> EC2A 1AF
>
>
> On 29 July 2016 at 10:40, Zhao Xin <owe...@gmail.com
> <mailto:owe...@gmail.com>> wrote:
> Have you read
> https://developer.apple.com/library/tvos/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithCAPIs.html#//apple_ref/doc/uid/TP40014216-CH8-ID17
>
> <https://developer.apple.com/library/tvos/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithCAPIs.html#//apple_ref/doc/uid/TP40014216-CH8-ID17>
> ?
>
> Zhaoxin
>
> On Fri, Jul 29, 2016 at 4:55 PM, James Campbell via swift-users
> <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:
> Do you know of any resources to brush up on the pointer aspect of swift ?
>
> ___________________________________
>
> James⎥Head of Trolls
>
> ja...@supmenow.com <mailto:ja...@supmenow.com>⎥supmenow.com
> <http://supmenow.com/>
> Sup
>
> Runway East
>
>
> 10 Finsbury Square
>
> London
>
>
> EC2A 1AF
>
>
> On 29 July 2016 at 09:10, Dmitri Gribenko <griboz...@gmail.com
> <mailto:griboz...@gmail.com>> wrote:
> On Fri, Jul 29, 2016 at 12:55 AM, James Campbell <ja...@supmenow.com
> <mailto:ja...@supmenow.com>> wrote:
> > So this:
> >
> > if let data = someArrayGeneratingFunction() {
> > cFunction(UnsafeMutablePointer(data))
> > }
> >
> > Has issues with the array passed to c getting corrupted, but this doesn't:
> >
> > let data = someArrayGeneratingFunction()
> >
> > if let data = data {
> > cFunction(UnsafeMutablePointer(data))
> > }
>
> Neither piece of code is guaranteed to work. (You are just getting
> lucky that the second one happens to work.) Array-to-pointer
> conversion only extends the lifetime of the array until the immediate
> function call returns. So after UnsafeMutablePointer(data) returns,
> the array can be freed.
>
> Use someArrayGeneratingFunction.withUnsafeMutableBuffer { ... } instead.
>
> Dmitri
>
> --
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <griboz...@gmail.com
> <mailto:griboz...@gmail.com>>*/
>
>
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