How to get a rust nested generic

Question

I'm trying to write a generic struct that will hold a collection and the sum of all the inserted values into that collection.

That is a generic type that keeps the sum of all added values to it.

pub struct SummedCollection<T>
where 
    T::Item: std::ops::Add<Output=T::Item> + std::ops::Div<Output=T::Item>
{
    sum: T::Item,
    values: T,
}

impl<T> SummedCollection<T> { 
    pub fn new() -> Self{
        SummedCollection {
            sum: T::Item::default(),
            values: T::new(),
        }
    }
    pub fn push(&mut self, value: T::Item) {
        self.values.push(value);
        self.sum = self.sum + value;
    }
    pub fn sum(&self) -> T::Item {
        self.sum
    }
}

The intended use would be:

let v: SummedCollection<Vec<i32>> = SummedCollection::new();
v.push(5);
v.push(10);

I would then expect: v.sum() == 15.

I get an error "^^^^ associated type `Item` not found" on each assurance of T::Item, what do I need to do to access the nested generic (the i32 in my example)?

score 4 · Accepted Answer · edited Mar 26 '22 at 19:45

The compiler doesn't know T::Item. You intended T to be some collection type, but didn't tell it to the compiler, so it doesn't know that.

You have to tell the compiler T implements some collection trait. A trait like that doesn't exist in the standard library, but you can easily write your own:

pub trait Collection {
    type Item;
    fn push(&mut self, value: Self::Item);
}

// Example implementation, you can implement the trait for any collection type you wish
impl<T> Collection for Vec<T> {
    type Item = T;
    fn push(&mut self, value: T) {
        self.push(value);
    }
}

pub struct SummedCollection<T: Collection> {
    sum: T::Item,
    values: T,
}

impl<T> SummedCollection<T>
where
    T: Collection + Default,
    T::Item: Default + Copy + std::ops::Add<Output = T::Item> + std::ops::Div<Output = T::Item>,
{
    pub fn new() -> Self {
        SummedCollection {
            sum: T::Item::default(),
            values: T::default(),
        }
    }
    pub fn add(&mut self, value: T::Item) {
        self.values.push(value);
        self.sum = self.sum + value;
    }
    pub fn sum(&self) -> T::Item {
        self.sum
    }
}

Note I made some additional changes:

Required T::Item to be Copy and Default. It may be possible to work around this need, but it's easy this way.
Required T: Default and changed T::new() to T::default(), since we already have a default-constructible trait - no need to reinvent the wheel.
Moved some of the bounds from the struct to the impl, since it is better this way.

Edit: Thanks for @mcarton that pointed out that we can use Extend for that:

#[derive(Debug)]
pub struct SummedCollection<Collection, T> {
    sum: T,
    values: Collection,
}

impl<Collection, T> SummedCollection<Collection, T>
where
    Collection: Extend<T> + Default,
    T: Default + Copy + std::ops::Add<Output = T> + std::ops::Div<Output = T>,
{
    pub fn new() -> Self {
        SummedCollection {
            sum: T::default(),
            values: Collection::default(),
        }
    }
    pub fn add(&mut self, value: T) {
        self.values.extend(std::iter::once(value));
        self.sum = self.sum + value;
    }
    pub fn sum(&self) -> T {
        self.sum
    }
}

But note that because it requires an additional generic parameter it will affect users: instead of

let v: SummedCollection<Vec<i32>> = SummedCollection::new();

You'll have to write

let v: SummedCollection<Vec<i32>, _> = SummedCollection::new();

Or explicitly, of course:

let v: SummedCollection<Vec<i32>, i32> = SummedCollection::new();

Your `Collection` trait could just as well be [`Extend`](https://doc.rust-lang.org/std/iter/trait.Extend.html). — mcarton, Mar 15 '22 at 20:59

How to get a rust nested generic

1 Answers1