Probably this should yield an error -- I tend to think we should only
permit moves that we cannot enforce from `*` pointers, just to add an
extra barrier.
Niko
On Tue, Jan 28, 2014 at 12:12:23PM -0800, Kevin Ballard wrote:
> Your code is moving the contents of Option<~MyStruct> into the match arm. It
> just so happens that this seems to be zeroing out the original pointer in
> memory, and that happens to be the same representation that None does for the
> type Option<~MyStruct> (since ~ pointers are non-nullable), so the act of
> moving the value just happens to be transforming it into a None.
>
> Normally you couldn't do this, but mutable statics are weird (which is why
> you need the unsafe block to access it).
>
> When you remove the ~, the lines end up printing the same because MyStruct is
> implicitly copyable, so your match arm is now copying instead of moving.
>
> The correct fix here is to use `Some(ref data)` instead of `Some(data)`. This
> will take a reference to the data instead of moving it, and the static will
> remain unchanged.
>
> -Kevin
>
> On Jan 28, 2014, at 11:48 AM, Alexander Stavonin <a.stavo...@gmail.com> wrote:
>
> > Hi all! I’m not sure is it an error or "static mut" variables
> > misunderstanding from my side. The source:
> >
> > struct MyStruct {
> > val: int
> > }
> >
> > static mut global_data: Option<~MyStruct> = None;
> >
> > fn test_call() {
> > unsafe {
> > match global_data {
> > Some(data) => { println!("We have data {:?}", data);}
> > None => { println!("We don't have data");}
> > }
> > }
> > }
> >
> > fn main() {
> >
> > unsafe {
> > global_data = Some(~MyStruct{val: 42});
> > }
> >
> > test_call();
> > test_call();
> > }
> >
> > and output:
> >
> > We have data ~MyStruct{val: 42}
> > We don't have data
> >
> > But if I’m changing global_data from Option<~MyStruct> to Option<MyStruct>
> > output is changed also:
> >
> > We have data ~MyStruct{val: 42}
> > We have data ~MyStruct{val: 42}
> >
> > Is it normal behaviour and owning pointers cannot be stored in global
> > variables or an error?
> > _______________________________________________
> > Rust-dev mailing list
> > Rust-dev@mozilla.org
> > https://mail.mozilla.org/listinfo/rust-dev
>
> _______________________________________________
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