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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