First, big thanks to both of you, Ben and Clark, for helping out. :-)
Thanks for the hint on the lifetime syntax - it's a bit hard to grasp,
sometimes it's "Foo<'a>", sometimes it's "& 'a [Foo]", and I've seen
examples with "Foo+'a" too. But maybe this all makes sense when I'm more
used to the language.
As for how long the pointed-to data is alive for, I think it makes sense
to default to "the same lifetime as parent struct or enum", if the
lifetime is not explicitly specified.
Btw, for some reason this does not work:
let s = InBag(Bag { price: 10, weights: &[3, 5, 7] }); /* fails with
"borrowed value does not live long enough" and "consider using a `let`
binding to increase its lifetime" */
But the below does, and can be used as a workaround:
let b = Bag { price: 10, weights: &[3, 5, 7] };
let s = InBag(b);
I'm sure there's an explanation to why, but if it can be fixed so that
the compiler interprets the first as being equivalent to the second it
would be nice.
On 2014-10-17 21:01, Ben Foppa wrote:
Your struct has a fixed size - a reference is a pointer. Which is why
it requires a lifetime - how long is the pointed-to data alive for?
And so you need to tell it - in your enum example, you need to say
Bag<'a> (for some defined 'a). For example, Bag<'static> means the
pointed-to data lives as long as the program.
On Fri, Oct 17, 2014 at 11:50 AM, David Henningsson <[email protected]
<mailto:[email protected]>> wrote:
Thanks for the answer. Deref is a new one for me, looks
interesting by means of abstraction.
I'm also coming from the C camp but I'm not sure how to write the
code that I want in rust. Yet. :-)
E g, here's one, somewhat related, C example that I'm not sure how
to do in Rust:
struct bag {
int price;
int nitems;
int []items;
};
struct cart { /* ... */ };
struct shopping {
int kind; /* 0 means bag, 1 means cart */
union {
struct bag bag;
struct cart cart;
}
}
struct shopping* make_shopping_bag(int price, int nitems, int *items)
{
struct shopping* result =
malloc(sizeof(shopping)+nitems*sizeof(int));
result.kind = 0;
result.bag.price = price;
result.bag.nitems = nitems;
memcpy(result.bag.items, nitems*sizeof(int));
}
So, the bag struct would probably look like this in Rust:
struct Bag <'a> {
price: int,
weights: & 'a [int],
}
1) It feels like "weights: [int]" would be more like the way I
want it, and the declaration compiles, but i can't initialize the
struct or have it as a local variable because its size is now
unkown. Even though a static initialization like "let b = Bag {
price: 10, weights: [3,5,7] }" would be trivial to calculate the
total size of, the compiler seems not to be able to do this.
2) I'm not sure why this lifetime has to be explicit, should
default to "same lifetime as parent struct" IMO.
3) And when I try to do like:
enum Shopping {
InBag(Bag),
InCart( /* ... */ ),
}
I get an error: "error: wrong number of lifetime parameters:
expected 1, found 0". I've tried both "InBag('a Bag)" and
"InBag(Bag + 'a)" but that ends up with other errors instead...so
no idea on what to do about that?
On 2014-10-17 18:37, Clark Gaebel wrote:
Rust is not a replacement for java, it’s a replacement for C and C++.
To solve little “puzzles” like this, i tend to ask myself “how
would I do this in C”, and then write that code in rust. Building
inheritance trees is generally the wrong way of approaching
problems. In cases where it does apply, you can still do it, but
be gentle. Try not to lean on them as your primary means of
abstraction.
Anyhow, on to your actual problem.
Something which might be worth trying is implementing
`Deref<Circle>` and `DerefMut<Circle>` for your pancake, then
having a `DList<Box<Deref<Circle>>>` (or just use a normal &, if
you want that).
Then you can call all your circle traits after a quick call to
`.deref()`, AND your `DList` will free everything properly.
But again, there’s probably a simpler solution that doesn’t
involve “inheritance” that you should consider. Maybe a DList of
enums? Maybe just a Vec<uint> in this case? Think about how you’d
do it in C.
Regards,
- Clark
On Fri, Oct 17, 2014 at 4:27 AM, David Henningsson
<[email protected] <mailto:[email protected]>> wrote:
Hmm, right. The as_* could probably useful to write a macro for.
Coming from the C/Java side of things I have to figure out
how this
works in a bigger context, e g a DList or other structure
owning objects
implementing HasArea. This seems to compile, e g:
impl Pancake {
fn as_box_circle(&self) -> Box<Circle> { box self.circle }
}
fn make_pancake(dl: &mut DList<Box<HasArea>>) {
let p = Pancake { circle: Circle { x: 0f64, y: 0f64, radius:
1f64
}, is_tasty: true };
dl.push(p.as_box_circle());
}
But I'd assume that make_pancake would now make a copy of the
pancake's
circle, rather than taking ownership of the entire pancake,
right? The
pancake then gets dropped at function return.
In this simple example perhaps this does not make that much of a
difference though, but if you imagine a C struct like:
struct list {
list *next;
circle *data;
}
You can now put a pointer to a pancake as data, use it as a
circle, and
when you finally free the list and the data that goes with
it, the
entire pancake will be freed. This you cannot do in rust...or
can you?
On 2014-10-17 07:59, Clark Gaebel wrote:
> impl Pancake {
> fn as_circle(&self) -> &Circle { &self.circle }
> fn as_mut_circle(&mut self) -> &mut Circle { &mut
self.circle }
> }
>
> The compiler will optimize trivial functions, except
cross-crate. In
> those cases, use an #[inline] annotation.
>
>
>
> On Thu, Oct 16, 2014 at 10:57 PM, David Henningsson
<[email protected] <mailto:[email protected]>
> <mailto:[email protected]> <mailto:[email protected]>> wrote:
>
> This is probably a previously asked question, but I
couldn't find
> it on
> Google, so...
>
> Let's extend the Circle example from the guide a little:
>
> struct Circle {
> x:f64,
> y:f64,
> radius:f64,
> }
>
> trait HasArea {
> fn area(&self)-> f64;
> }
>
> impl HasArea for Circle {
> fn area(&self)-> f64 {
> std::f64::consts::PI * (self.radius * self.radius)
> }
> }
>
> struct Pancake {
> circle: Circle,
> is_tasty: bool,
> }
>
>
> ...now, what is the easiest way I can implement HasArea for
> Pancake? I
> could do this:
>
> impl HasArea for Pancake {
> fn area(&self) -> f64 { self.circle.area() }
> }
>
> ...but that means a lot of boiler-plate code, especially if
> HasArea has
> many methods. Hopefully rust will just inline/optimise the
> redirection
> away in most cases to avoid the runtime cost, but is there a
> smarter or
> more idiomatic way of doing this?
>
> // David
>
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> Rust-dev mailing list
> [email protected] <mailto:[email protected]>
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>
>
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