#2 is definitely correct — sorry, I forgot that part of it :)
#1 is a little more nuanced. For the moment, it’s correct. The current line of
thinking (at least, what I’ve gathered) is that you only require trait bounds
on the methods that actually use that trait. For example, you could create a
Matrix struct, and you could call Matrix::new with any type, but then the Add
impl for Matrix would require T: Add<T, T>, the Mul would require T: Mul<T, T>,
etc. However, IIRC, there was a proposal (maybe a PR?) that would allow and
enforce trait bounds on generics in structs. I’m not sure what the opinion of
it was, though.
--
John Grosen
On Tuesday, February 4, 2014 at 1:31 PM, Marc Bowes wrote:
> I see, thanks. Just to be clear, is this correct then?
>
> ```
> trait MyTrait {
> fn be_traity(&self);
> }
>
> struct MyImpl {
> my_field: u32
> }
>
> impl MyTrait for MyImpl {
> fn be_traity(&self) {}
> }
>
> struct MyStruct<'a, T> {
> my_field: &'a T
> }
>
> impl<'a, T: MyTrait> MyStruct<'a, T> {
> fn new(my_field: &'a T) -> MyStruct<'a, T> {
> MyStruct {
> my_field: my_field
> }
> }
> }
>
> fn main() {
> let my_field = MyImpl { my_field: 0 };
> let my_struct = MyStruct::new(&my_field);
> }
>
> ```
>
> The main differences being:
>
> 1) The struct definition for MyStruct no longer gives any clue as to what
> my_field might be (this seems weird to me)
> 2) The impl now includes 'T: MyTrait' and returns the templated MyStruct
>
> How (if at all) will Niko's DST changes impact this? More broadly, what else
> might impact this in the (short/medium-term) future? #1 bothers me a bit
> (right now :-)).
>
> Thanks!
>
>
> On Tue, Feb 4, 2014 at 11:21 PM, John Grosen <[email protected]
> (mailto:[email protected])> wrote:
> > The problem here is that you are using a trait object in the struct
> > definition rather than a generic; at the moment, struct generics cannot
> > have trait bounds, though, so the code for the struct would be simply:
> >
> > ```
> > struct MyStruct<‘a, T> {
> > my_field: &’a T
> > }
> > ```
> >
> > Then the `impl` code should be exactly as you have now.
> >
> > --
> > John Grosen
> >
> >
> > On Tuesday, February 4, 2014 at 1:16 PM, Marc Bowes wrote:
> >
> >
> >
> > > Hello,
> > >
> > > I'm trying to implement a struct where one of the fields is a reference
> > > and therefore has bounded lifetime. The reason I would like it to be a
> > > reference is to encourage sharing of the value in question as setup of
> > > said value might be expensive. In my specific example, the value is a
> > > session manager and opening said session is expensive.
> > >
> > > I have come up with the following
> > >
> > > ```
> > > trait MyTrait {
> > > fn be_traity(&self);
> > > }
> > >
> > > struct MyImpl {
> > > my_field: u32
> > > }
> > >
> > > impl MyTrait for MyImpl {
> > > fn be_traity(&self) {}
> > > }
> > >
> > > struct MyStruct<'a> {
> > > my_field: &'a MyTrait
> > > }
> > >
> > > impl<'a> MyStruct<'a> {
> > > fn new<T: MyTrait>(my_field: &'a T) -> MyStruct {
> > > MyStruct {
> > > my_field: my_field
> > > }
> > > }
> > > }
> > >
> > > fn main() {
> > > let my_field = MyImpl { my_field: 0 };
> > > let my_struct = MyStruct::new(&my_field);
> > > }
> > >
> > > ```
> > >
> > > This fails to compile:
> > >
> > > rust-lifetimes-with-references.rs:20:23: 20:31 error: value may contain
> > > references; add `'static` bound
> > > rust-lifetimes-with-references.rs:20
> > > (http://rust-lifetimes-with-references.rs:20) my_field:
> > > my_field
> > > ^~~~~~~~
> > >
> > >
> > > This confuses me because "may contain references" is exactly what I want?
> > > I want to assign it as a ref.. If I slap on a & in the assignment (for no
> > > good reason other than being confused):
> > >
> > > MyStruct {
> > > my_field: &my_field
> > > }
> > >
> > >
> > > Then I get:
> > >
> > > rust-lifetimes-with-references.rs:20:23: 20:32 error: failed to find an
> > > implementation of trait MyTrait for &'a T
> > > rust-lifetimes-with-references.rs:20
> > > (http://rust-lifetimes-with-references.rs:20) my_field:
> > > &my_field
> > >
> > > ---
> > >
> > > I'm clearly doing something stupid but cannot find a reference example..
> > > _______________________________________________
> > > Rust-dev mailing list
> > > [email protected] (mailto:[email protected])
> > > https://mail.mozilla.org/listinfo/rust-dev
> > >
> > >
> > >
> >
> >
>
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