How to convert Vec<Arc<Item>> to Vec<Item>?

help
1

I'm pretty new to Rust, how to make below code works:

struct Items {
    maps: HashMap<KeyType, Vec<Arc<ValueType>>>,
}

impl Items {
    pub fn get_value(&self, key: &KeyType) -> Option<&Vec<ValueType>> {
        // How?
    }
}

fn main() {
    let items = Items::new();

    // insert some key and value

    if let Some(results) = items.get_value(key) {
        // results is &Vec<ValueType>
    }
}

Or I'm on the wrong way, like Arc shouldn't in a Vec. Any suggestions are welcome :slight_smile:

2

As written, you can't make the get_value() function work. An Arc is a pointer, it represents indirection, so a Vec<Arc<ValueType>> stores pointers in the vector, whereas a Vec<ValueType> stores the values directly, in a contiguous buffer, without intermediary pointers.

Therefore, any conversion between Vec<Arc<ValueType>> and Vec<ValueType> will involve either the moving or copying of some objects – incurring a fresh allocation of a vector, which therefore isn't possible to return "by reference". So first of all, you'll have to change your function signature to return -> Option<Vec<ValueType>>.

If you know every value will be owned by exactly one Arc, you could steal each pointed object and return a Vec<ValueType>, using Arc::try_unwrap().

That would, however, require you to pass the (last and only) Arc instance by-value, of course, which you can't do if you are .get()ting it out of a HashMap. So you would either need to remove them from the HashMap or simply clone the inner value, which in turn requires impl Clone for ValueType, like this:

impl Items {
    pub fn get_value(&self, key: &KeyType) -> Option<Vec<ValueType>> {
        self.maps.get(key)
            .map(|arcs| {
                arcs.iter()
                    .map(|arc| ValueType::clone(&*arc))
                    .collect()
            })
    }
}

If you can't or don't want to clone the objects, another option would be to just expose the internal representation, and return an Option<&[Arc<ValueType>]>, and let the users of your function deal with the fact that they've got a bunch of pointers back.

(Also note that I changed the signature from &Vec<T> to &[T]. There's no value in returning or accepting an immutable Vec by reference, since anything an immutable reference-to-Vec can do, an immutable reference-to-slice can also do.)

5 Likes
3

Wah~ Thank you so much. I learned a lot from your post, not just the question, but also other knowledges.

4

If you want to hide the fact that you’re using Arc internally, you can also declare the return type as Option<&[impl Deref<Target = ValueType>]>. Callers won’t be allowed to use Arc-specific functionality, so you can later change to some other pointer type without breaking them.

4 Likes
5

Nice~ Another magic

6

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