[rust-dev] Inheritance / delegation

David Henningsson diwic at ubuntu.com
Fri Oct 17 11:50:59 PDT 2014


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 <diwic at ubuntu.com 
> <mailto:diwic at ubuntu.com>> 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
>     <diwic at ubuntu.com
>     > <mailto:diwic at ubuntu.com>> 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
>     >
>     > _______________________________________________
>     > Rust-dev mailing list
>     > Rust-dev at mozilla.org
>     > https://mail.mozilla.org/listinfo/rust-dev
>     >
>     >
>
>

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